CIRIA C760 - Guidance On Embedded Retaining Wall Design [PDF]

Zitiervorschau

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

C760

This publication provides good practice guidance on the selection and design of vertical embedded retaining walls to satisfy the requirements of Eurocodes. It covers temporary and permanent cantilever, anchored, single and multi-propped retaining walls that are supported by embedment in soft soils, stiff clays, other competent soils and soft rocks.

9 780860 177647

CIRIA

C760

Guidance on embedded retaining wall design

The content addresses the technical and construction issues relating to the selection of appropriate wall types and construction sequence to achieve a satisfactory design. It clarifies areas of ambiguity and common misunderstandings when applying the Eurocodes to the design of embedded retaining walls and presents a clear, unambiguous method for the application of the Observational Method.

Guidance on embedded retaining wall design

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CIRIA C760

London, 2017

Guidance on embedded retaining wall design Asim Gaba Arup Stuart Hardy Arup Lauren Doughty Arup William Powrie University of Southampton Dimitrios Selemetas Cementation Skanska

Griffin Court, 15 Long Lane, London, EC1A 9PN Tel: 020 7549 3300

Fax: 020 7549 3349

Email: [email protected]

Website: www.ciria.org

Summary

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

This publication provides good practice guidance on the selection and design of vertical embedded retaining walls to satisfy the requirements of Eurocodes. It covers temporary and permanent cantilever, anchored, single and multi-propped retaining walls that are supported by embedment in soft soils, stiff clays, other competent soils and soft rocks. The content addresses the technical and construction issues relating to the selection of appropriate wall types and construction sequence to achieve a satisfactory design. It clarifies areas of ambiguity and common misunderstandings when applying the Eurocodes to the design of embedded retaining walls and presents a clear, unambiguous method for the application of the Observational Method (OM). Guidance on embedded retaining wall design Gaba, A, Hardy, S, Doughty, L, Powrie, W, Selemetas, D CIRIA C760

© CIRIA 2017

RP1010

ISBN: 978-0-86017-764-7

This publication supersedes C580 (2003) British Library Cataloguing in Publication Data A catalogue record is available for this publication from the British Library Keywords Ground engineering, ground investigation and characterisation, piling, soil-structure interaction, embedded retaining wall, ground movements, retaining walls Reader interest

Classification

Design, specification, construction, managers, clients and supervising engineers involved in civil and geotechnical works

Availability Unrestricted Content

Advice/guidance

Status

Committee-guided

User Civil, geotechnical and structural engineers, engineering geologists

Published by CIRIA, Griffin Court, 15 Long Lane, London, EC1A 9PN This publication is designed to provide accurate and authoritative information on the subject matter covered. It is sold and/or distributed with the understanding that neither the authors nor the publisher is thereby engaged in rendering a specific legal or any other professional service. While every effort has been made to ensure the accuracy and completeness of the publication, no warranty or fitness is provided or implied, and the authors and publisher shall have neither liability nor responsibility to any person or entity with respect to any loss or damage arising from its use. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the written permission of the copyright holder, application for which should be addressed to the publisher. Such written permission must also be obtained before any part of this publication is stored in a retrieval system of any nature. If you would like to reproduce any of the figures, text or technical information from this or any other CIRIA publication for use in other documents or publications, please contact the Publishing Department for more details on copyright terms and charges at: [email protected] or tel: 020 7549 3300.

Front cover: Main cover photo (bottom): Secant wall, Crossrail, Bond Street Station, London (courtesy Cementation Skanska) Top left (inside station): Diaphragm wall, Crossrail, Tottenham Court Road Station, London (courtesy BBMV) Top right (looking upwards inside shaft): Foxton Road wall, Birmingham (courtesy Bachy Soletanche)

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Acknowledgements The guidance given here is a result of Research Project (RP)1010 and has come from extensive consultation with clients, consultants, contractors and academic institutions. The work was undertaken by Arup with assistance from the University of Southampton and Cementation Skanska Limited.

Authors Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

Asim Gaba MA(Oxon) MSc DIC LLB(Hons) CEng FICE MASCE MHKIE FGS MCIArb Asim Gaba is a director and trustee of Arup and is the lead author of this publication. He has more than 35 years’ design and construction experience of major international multi-disciplinary projects involving embedded retaining walls, deep excavations and underground structures, particularly for mass rapid transit projects in the UK and across Europe, the Middle and Far East, Central Asia and North America. He is a Fellow of the Institution of Civil Engineers, Fellow of the Geological Society and a Member of the Chartered Institute of Arbitrators (MCIArb).

Stuart Hardy MEng ACGI PhD DIC CEng MICE Stuart Hardy is an associate director in the London geotechnics group of Arup. After completing an undergraduate degree in civil engineering and a PhD in geotechnical engineering at Imperial College, Stuart has worked on a number of challenging basement and foundation projects located mainly in the Middle East and London. These have included Wembley Stadium, the Exhibition Road Building for the Victoria and Albert Museum and the redevelopment of the Shell Centre building in Waterloo. Stuart is UK representative on the EC7 Evolution Group for the design of embedded retaining walls.

Lauren Doughty MEng MSc DIC MIAM Lauren Doughty is a geotechnical engineer based in the London office of Arup. She completed her undergraduate degree in engineering design at Bristol University and more recently has completed an MSc in soil mechanics at Imperial College. Since joining Arup, Lauren’s experience has been from both geotechnical and asset management projects. She has worked predominantly for large infrastructure clients and on a wide range of projects in London and the South East. Lauren is a previous winner of GE Magazine’s Young Geotechnical Engineer of the Year and has achieved the Certificate in Asset Management from the Institution of Asset Management.

William Powrie FREng MA MSc PhD CEng FICE William Powrie is Professor of geotechnical engineering and Dean of the faculty of engineering and the environment at the University of Southampton, and geotechnical consultant to WJ Groundwater. Since 1987 he has supervised research projects with a total value of nearly £50M and supervised over 50 research students to successful completion of their PhDs. He publishes widely on geotechnical transportation infrastructure, including embedded retaining walls, and awards include the Institution of Mechanical Engineers (IMechE) Thomas Hawksley Medal and the ICE Telford Medal. He was elected a Fellow of the Royal Academy of Engineering (RAEng) in 2009 for his work in the areas of infrastructure and waste management engineering. His textbook Soil mechanics concepts and applications published in 1997 is now in its third edition.

Guidance on embedded retaining wall design

iii

Dimitrios Selemetas BEng MSc PhD(Cantab) CEng MICE

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Dimitrios Selemetas is the chief geotechnical engineer of Cementation Skanska with over 15 years of practical experience on the design and construction of deep basements, piled foundations and instrumentation. Dimitrios was awarded a PhD in geotechnical engineering from Cambridge University on the effects of tunnelling on full-scale piled foundations and piled structures. He gained consultancy experience with Mott MacDonald and field instrumentation experience with CMCS at the Building Research Establishment (BRE) before joining the piling and foundations division of Skanska. At Skanska he has developed specialist skills in the structural design and detailing of embedded retaining walls and foundations based on constructability. Dimitrios has been the contractor design leader for some challenging basement and foundation works including the Nova Victoria basement in London and the AstraZeneca basement in Cambridge, UK. Dimitrios has been actively involved in the evolution of geotechnical and structural Eurocodes and has received numerous awards and prizes including the Cooling Prize of the British Geotechnical Association (BGA).

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Project steering group

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Following CIRIA’s established practice, the research project was guided by a steering group, which comprised: David Beadman

ByrneLooby

Malcolm Corlett

BAM Nuttall Ltd

Michael Gavins

Atkins

Tony Gould

Groundforce Shorco

Toby Hayward

Laing O’Rourke/Expanded Geotechnical

Brian McGinnity (chair)

London Underground Limited

Anthony O’Brien

Mott MacDonald

David Potts

Geotechnical Consulting Group/Imperial College London

David Preece

Laing O’Rourke (formerly Bachy Soletanche)

Colin Rawlings

High Speed Two (HS2) Ltd

Keith Reeves

High Speed Two (HS2) Ltd

Peter Scott

BuroHappold

Mark Shaw

Highways England

Tony Suckling

A Squared Studio (formerly Balfour Beatty Ground Engineering)

Andy Tan

Environment Agency

Graham White

ArcelorMittal

John Wilkinson

Kier

Other contributors The development of this publication also used contributions from: Wisam Al-Ani

Balfour Beatty Ground Engineering

Cedric Allenou

Atkins

Matthew Basterfield

Groundforce Shorco

Andrew Bond

Geocentrix Ltd

Daniel Borin

Geosolve

Jade Buckley-Jeffers

Arup

Rachel Burden

University of Southampton

Paul Chambers

Skanska Civil Engineering

Jim Cook

Geotechnical Services Bureau Ltd

David Cox

Independent Consultant

Steve Deeble

VINCI Construction UK/BAM Nuttal Ltd (joint venture)

Michael Eldred

Eldred Geotechnics Ltd

Imran Farooq

Mott MacDonald

Ian Feltham

Arup

Sophie Hamza

BSI Group

Paul Hodgson

Bachy Soletanche

Eloise Hollins

Arup

Will Howlett

Arup

Jason Hows

Cementation Skanska

Tony Jones

Arup

Ronnie Lancaster

LBH WEMBLEY Geotechnical & Environmental

Jimmy Lee

Cementation Skanska

Seamus Lefroy-Brooks

LBH WEMBLEY Geotechnical & Environmental

Christine Lozynskyj

University of Southampton

Bryan Marsh

Arup

Duncan McFadyean

High Speed Two (HS2) Ltd

Guidance on embedded retaining wall design

v

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David Milne

University of Southampton

Paul Morrison

Arup

Rob Moulds

BAM Nuttall Ltd

Andreea Nae

University of Southampton

Duncan Nicholson

Arup

Nick O’Riordan

Arup

Ian Palaczky

University of Southampton

Heleni Pantelidou

Arup

Bhavika Ramrakhyani

Costain

Brian Simpson

Arup

(Sakthi) S Srisakthivel

Laing O’Rourke

Joel Smethurst

University of Southampton

Stacey Taylor

Arup

Kieran Tully

CIRIA

Steve Wade

Cementation Skanska

Debbie Wheeler

Skanska UK

Scott White

Cementation Skanska

Project funders ArcelorMittal Bachy Soletanche Balfour Beatty Ground Engineering BAM Ritchies Cementation Skanska Environment Agency Groundforce Shorco High Speed 2 Ltd Highways England Institution of Civil Engineers Research and Development Enabling Fund Kier Laing O’Rourke London Underground CIRIA would also like to thank the authors for their substantial in-kind contribution in the production and dissemination of this publication

CIRIA project team Lee Kelly

Project manager

Kieran Tully

Project director

Clare Drake

Publishing manager

Supported by Chris Chiverrell and Victor Zasadzki. CIRIA, the funders, authors, PSG and other contributors gratefully acknowledge the work by Chris Chiverrell (former project director) in securing the funding for this project and for his technical advice and help given throughout the project. CIRIA would like to thank Lyn Nesbitt-Smith (LNS Indexing) for providing the index.

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Foreword

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Many forms of civil engineering infrastructure involve embedded retaining walls. Their design and performance in different ground conditions often involve complex issues of soil-structure interaction. They must be understood at all stages of the construction process, from feasibility and detailed design to completion of construction and long-term performance. The guidance and recommendations in this publication are a superb source of comprehensive information of enormous importance for practising geotechnical engineers. A wide range of wall types is covered: temporary and permanent cantilever, anchored, single and multipropped walls, all of which are supported by embedment in a wide variety of soil types ranging from soft soils to stiff clays, other competent soils and soft rocks. This publication covers a wider range of soil types than previous CIRIA publications on embedded retaining walls that principally focused on stiff clays. Comprehensive guidance is provided on the selection of appropriate wall types and construction sequences to achieve satisfactory designs. The clarification of potential ambiguities and common misunderstandings relating to the application of the Eurocodes to wall design is especially useful. The guide rightly emphasises the importance of the designer developing a suitable ground model for the proposed wall so that the ground engineering risks can be properly identified. There is no substitute for well-documented case studies – we learn most from detailed field measurements in ground engineering. Particularly welcome are the many case studies of embedded retaining wall performance, including associated ground movements, which have been compiled in this excellent publication. These are also highly relevant to the successful application of the Observational Method, which is well-suited to the design and construction of embedded retaining walls. Key parts of the publication are devoted to different methods of analysis, the determination and selection of appropriate geotechnical parameters for use in design calculations, the assessment of ground movements, and approaches to design of the wall and of temporary support systems. The appendices contain a wealth of information on wall types, geomechanics, ground movements and case study data, and there are also many worked examples, providing invaluable practical guidance to designers. The authors, the project steering group and CIRIA are to be congratulated on producing a comprehensive publication on an important topic that will undoubtedly be widely referred to nationally and internationally for many years.

Professor Lord Mair CBE FREng FICE FRS Sir Kirby Laing Professor of Civil Engineering, Cambridge University

Guidance on embedded retaining wall design

vii

Contents

Note to the reader The appendices in this guide relate to further supporting information given in the corresponding chapter, for example Appendix A2 relates to Chapter 2, Appendix A3 to Chapter 3 etc. Where there is no information in the appendix, this indicates no further details are required for the chapter.

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix Abbreviations and acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi

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Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiii 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 1.2 1.3 1.4 1.5 1.6 2

Design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1 2.2 2.3

2.4 2.5

2.6 3

Design communication during project life cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Risk assessment and management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Design concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.1 Design assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.2 Principles and application rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3.3 Geotechnical categorisation of retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3.4 Geotechnical design process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Design requirements and performance criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.5.1 Ultimate limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20 2.5.2 Serviceability limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Construction considerations and wall selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.1

3.2

3.3

3.4

viii

Background to the project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Readership . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Economic design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 How to use this guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Types of embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.1.1 Review of wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.1.2 Wall construction methods and tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.1.3 Relative construction cost data for various embedded wall types . . . . . . . . . . . . . . . . . . 34 Wall selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2.1 Ground conditions and obstructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2.2 Groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.2.3 Constructability requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.2.4 Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 3.2.5 Contaminated ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.2.6 Environmental issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Construction methods for soil support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3.1 Construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3.2 Temporary and permanent works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3.3 Construction requirements of temporary and permanent support system to retaining wall . . 45 3.3.4 Selection of appropriate construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

CIRIA, C760

4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.1

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4.2

4.3 4.4 5

Determination and selection of parameters for use in design calculations . . . . . . . . . . . . . . . . . . . . . . . . 91 5.1

5.2

5.3

5.4 5.5

5.6

5.7

5.8 5.9 5.10 5.11 5.12 5.13 6

Earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 4.1.1 In situ lateral stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.1.2 Effect of wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4.1.3 Limiting values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.1.4 Wall friction and adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.1.5 Determination of limiting lateral earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.1.6 Tension cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.1.7 Factors affecting limiting lateral earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.1.8 Factors potentially increasing earth pressures in serviceability limit state conditions . . 69 Methods of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.2.2 Limit equilibrium analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 4.2.3 SSI analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Effect of method of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Description and classification of soil and rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 5.1.1 Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93 5.1.2 Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 5.2.1 Fine-grained soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 5.2.2 Very weak rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Investigation of ground and groundwater conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 5.3.1 Site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 5.3.2 Ground stratigraphy and fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Assessment of drained/undrained soil conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Determination of soil and rock parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5.5.1 Classification properties and unit weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.5.2 Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 5.5.3 In situ stress conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 5.5.4 Shear strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 5.5.5 Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Determination of groundwater pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 5.6.1 Undrained conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 5.6.2 Drained conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 5.6.3 Mixed undrained and drained conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 5.7.1 Lateral loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 5.7.2 Vertical loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Unplanned excavation of formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Partial factors on soil strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Selection of design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Temporary works design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Permanent works design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

Ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 6.1

6.2

Sources of ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 6.1.1 Wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 6.1.2 Excavation in front of wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 6.1.3 Movements due to water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 Methods of estimating ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 6.2.1 Empirical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 6.2.2 Semi-empirical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

Guidance on embedded retaining wall design

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6.3 6.4 6.5 6.6 7

Design of wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

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7.1 7.2 7.3

7.4

7.5

7.6 8

8.2

8.3

8.4

Propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 8.1.1 Design responsibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 8.1.2 Prop stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 8.1.3 Prop selection considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 8.1.4 Design actions on props . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 8.1.5 Eurocode methodology for the structural design of props . . . . . . . . . . . . . . . . . . . . . . . . 235 8.1.6 Combination of actions for prop design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 8.1.7 Temperature effects on props . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 8.1.8 Sway effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 Berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 8.2.1 Modelling earth berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 8.2.2 Recommended method of modelling earth berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 8.2.3 Deflections of walls supported by berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 8.3.1 Deadman anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 8.3.2 Ground anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 8.3.3 Design and execution: wall and anchor designers’ responsibilities . . . . . . . . . . . . . . . . 249 8.3.4 Load transfer into supported wall structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 8.3.5 Ground anchor testing and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

Inspection, monitoring and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 9.1 9.2 9.3 9.4

x

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 Design philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 Geotechnical design of the wall by use of calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 7.3.1 ULS calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 7.3.2 SLS calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 7.3.3 Accidental design situation/progressive failure check . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 7.3.4 Summary of the design by calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 7.3.5 Outputs from embedded retaining wall calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 Geotechnical design of the wall by using the OM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 7.4.1 Definition of the OM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 7.4.2 EC7 requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 7.4.3 Application of the OM to embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 7.4.4 Roles and responsibilities of project team . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 7.4.5 Summary of OM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 Structural design of the wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 7.5.1 Steel sheet pile walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 7.5.2 Cast in situ concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

Design of temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 8.1

9

6.2.3 Numerical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 6.2.4 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Control of ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 Principles of building damage assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Protective measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 Key points and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 The role of the designer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Cost/benefit assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Design approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 9.4.1 Design strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 9.4.2 Whole life assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 9.4.3 Design life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

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9.5

9.6

9.7

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9.8 10

Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 9.5.1 Type and frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 9.5.2 Access/visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 9.5.3 Risk-based approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 9.5.4 During the inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 9.5.5 Post inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.6.1 Planned maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.6.2 Reactive maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.6.3 Choosing what to do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 9.6.4 Timing of maintenance activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 9.7.1 Purpose and extent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 9.7.2 Choice of instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 Recording information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

Areas of further work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Statutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 A1

(Intentionally left blank) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

A2

Example forms: CDM risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

A3

Wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 A3.1 A3.2 A3.3 A3.4 A3.5

Sheet pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 High modulus and combi walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 King post wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Contiguous pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 A3.5.1 Hard/soft secant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 A3.5.2 Hard/firm secant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 A3.5.3 Hard/hard secant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 A3.6 Diaphragm wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

A4 Geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 A4.1 Stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 A4.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 A4.1.2 Principal stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 A4.1.3 Plane strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 A4.1.4 Total and effective stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 A4.1.5 Mohr circle of stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 A4.2 Drained and undrained conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 A4.3 Stress history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 A4.4 Shear strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 A4.4.1 Effective stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 A4.4.2 Total stress (undrained shear strength) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 A4.4.3 Drained or undrained conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314 A4.5 Soil stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 A4.6 Effects of concrete diaphragm and bored pile wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 A4.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 A4.6.2 Key literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 A4.6.3 General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 A4.6.4 Summary of quantitative findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 A4.6.5 Summary of recommendations for simple representation of bored pile or diaphragm wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 A4.7 Earth pressure coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

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A4.7.1 Numerical procedure for calculating earth pressure coefficients . . . . . . . . . . . . . . . . . . 321 A4.8 Effect of Method of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 A4.8.1 Methods of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 A4.8.2 Software packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 A4.8.3 Problems analysed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 A4.8.4 Assumptions and analysis procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 A4.8.5 Finite element calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 A4.8.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 A4.8.7 Observations from Figures A4.28 to A4.31 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 A4.8.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 A5

Derivation of rock strength parameters for use in design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

A6

Ground movements and case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 A6.1 Ground movements due to wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 A6.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 A6.1.2 Case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 A6.2 Ground heave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 A6.2.1 Estimation of ground heave movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 A6.2.2 Case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 A6.3 Ground movements due to wall deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 A6.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351 A6.3.2 Case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 A6.4 Ground movement trends in relation to excavation depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 A6.5 Corner effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 A6.6 Movement at different excavation stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

A7

Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 A7.1

A7.2

A7.3

A7.4

A7.5

A7.6

A8

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Worked example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 A7.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 A7.1.2 Proposed basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 A7.1.3 Structural parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 A7.1.4 Analysis assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 Scenario 1: minimal site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 A7.2.1 Description of site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 A7.2.2 Determination of characteristic parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 A7.2.3 Site control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 Scenario 2: appropriate well considered site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 A7.3.1 Description of site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 A7.3.2 Determination of characteristic parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 A7.3.3 Site control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 Determination of design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 A7.4.1 Soil parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 A7.4.2 Design water pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .397 A7.4.3 Design actions (surcharges and loads) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 Set up of finite element model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 A7.5.1 DA1C2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 A7.5.2 DA1C1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 A7.5.3 SLS analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 A7.6.1 Scenario 1: minimal site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 A7.6.2 Scenario 2: appropriate well considered site investigation . . . . . . . . . . . . . . . . . . . . . . . 404 A7.6.3 Comparison of Scenario 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 A7.6.4 Discussion of predicted movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 A7.6.5 Comparison with results from Gaba et al study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409

Distributed prop load (DPL) method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412

CIRIA, C760

A8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 A8.2 DPL design method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 A8.3 Alternative methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416

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Boxes Box 4.1 Theoretical depths of tension cracks by the Rankine and Coulomb analyses . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Box 4.2 Steps involved in a typical limit equilibrium analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Box 4.3 Changing wall EI to allow for cracking, creep and relaxation of concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Box 6.1 Stress paths for soil elements near an excavation retained by a cast in situ embedded wall in overconsolidated clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Box 6.2 Typical movement profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 Box 6.3 Procedure for Stage 2 damage category assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Box 7.1 Indicative examples of scenarios for determining wall friction and adhesion for use in quasi-finite element SSI analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Box 8.1 Back analysis of temporary prop forces from a deep excavation in Paddington Station, London . . . . . . . . . 237 Box 8.2 Construction sequence analysed by Easton et al (1999) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Box 9.1 Prince of Wales pier, Dover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

Figures Figure 1.1 Wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 1.2 Eurocodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Figure 1.3 Principal design stages and publication layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Figure 1.4 Key issues considered in this publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 2.1 Decision paths for selection of appropriate wall type and construction sequence . . . . . . . . . . . . . . . . . . . . . . 10 Figure 2.2 CDM 2015 risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 2.3 Geotechnical categorisation of a structure or element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 2.4 Geotechnical categorisation of a project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 2.5 Ultimate limit state examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 3.1 Decision paths for selection of wall type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 3.2 Cross section through a secant hard/firm wall constructed with segmental cased rotary pile 1000 mm/ 900 mm [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Figure 3.3 Typical guide wall details for a piled secant wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Figure 3.4 Relative cost of different wall types with reference to typical wall depths [...] . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Figure 3.5 Example of estimated carbon embedded and emitted for a typical piled wall project . . . . . . . . . . . . . . . . . . . . 38 Figure 3.6 Geothermal loops in a diaphragm wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Figure 3.7 Temporary and permanent works at an underground car park. An example of the use of a sheet pile wall as the permanent wall, exposed and painted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Figure 3.8 Cantilever wall construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Figure 3.9 Top-down construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Figure 3.10 Top-down excavation at the Victoria and Albert Museum, London . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Figure 3.11 Bottom-up construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Figure 3.12 Bottom-up construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Figure 3.13 Bottom-up construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Figure 3.14 Circular shaft constructed with diaphragm wall panels, Hackney, East London . . . . . . . . . . . . . . . . . . . . . . . . . 45 Figure 3.15 Circular shaft constructed with secant piles at Eastney, Portsmouth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Figure 3.16 Various support systems to sheet pile walls at Thelwall Viaduct, Merseyside . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Figure 3.17 Temporary props spanning full width of excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Figure 3.18 The use of a berm and raking props . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Figure 3.19 The use of a berm and a prop to the permanent structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Figure 3.20 Anchored secant bored pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Figure 3.21 Typical connection detail at sheet pile wall/concrete slab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Figure 3.22 Sheet pile wall/concrete slab connection at Bristol underground car park . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Figure 3.23 Typical detail for couplers cast within a diaphragm wall panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Figure 3.24 Typical details of bent-out bars in diaphragm wall panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Figure 3.25 Possible connection details between secant piled wall and slab level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Figure 3.26 Hinged slab, A406 North Circular Road, London . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Figure 3.27 Hinged joint, A406 North Circular Road, London . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Figure 4.1 Schematic stress history of an overconsolidated clay deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Figure 4.2 Rankine plastic equilibrium for a frictionless wall or soil interface translating horizontally . . . . . . . . . . . . . . . 58 Figure 4.3 Coulomb’s method to calculate the limiting active force for a frictionless wall/soil interface translating horizontally . 58 Figure 4.4 Effect of wall friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Figure 4.5 Tension cracks: minimum total horizontal stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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Figure 4.6 Additional lateral effective stress acting on the back of a wall due to a strip load running parallel to it . . . . . 65 Figure 4.7 Pressure diagram for a line load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Figure 4.8 Concentrated and line load surcharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Figure 4.9 Enhancement factor on passive earth pressure coefficient for rough walls in close proximity . . . . . . . . . . . . . 68 Figure 4.10 Idealised stress distribution for an unpropped embedded cantilever wall at failure . . . . . . . . . . . . . . . . . . . . . 73 Figure 4.11 Approximate stress analysis for unpropped walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Figure 4.12 Normalised depth of embedment at failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Figure 4.13 Idealised stress distribution at failure for a stiff wall propped rigidly at the top . . . . . . . . . . . . . . . . . . . . . . . . . 76 Figure 4.14 Fixed earth support effective stress distributions and deformations for an embedded wall propped at the top . . 76 Figure 4.15 Stress analysis for an embedded wall propped at formation level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Figure 4.16 Forces acting on a stabilising base retaining wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Figure 4.17 Limiting at-depth lateral stress on a pile moving into a soil characterised by the effective stress failure criterion . . . 80 Figure 4.18 Reduction of lateral stress in the retained soil due to arching into rigid prop . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Figure 4.19 Components of wall displacement and definition of a ‘stiff’ wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Figure 4.20 Stress distributions behind and in front of stiff (a) and flexible (b) embedded walls . . . . . . . . . . . . . . . . . . . . . 82 Figure 4.21 Comparison of bending moment and prop loads from MSD and limit equilibrium analyses . . . . . . . . . . . . . . . 83 Figure 5.1 Determination and selection of parameters for use in design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Figure 5.2 Identification and description of soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Figure 5.3 Decision making process in site investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Figure 5.5 Correlation between the in situ coefficient of earth pressure and overconsolidated ratio for clays of various plasticity indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Figure 5.6 Influence of stress history on K0 and σh′ in a heavily overconsolidated clay . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Figure 5.7 Influence of the ratio of sample size to the fissure spacing on the strength measured in laboratory tests . . 118 Figure 5.8 Correlation between undrained shear strength and liquidity index for remoulded clays . . . . . . . . . . . . . . . . . 118 Figure 5.9 Correlation between N60 value and undrained shear strength and plasticity index for insensitive clays . . . . 119 Figure 5.10 Strength envelope for a given pre-consolidation stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Figure 5.11 Comparison of experimental data for ϕ′ vs plasticity index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Figure 5.12 Effect of overconsolidation on the relationship between (N1)60 and peak angle of friction φ′peak . . . . . . . . . . 123 Figure 5.13 Plot of differential stress (q) against mean effective stress (ρ′) for three different failure mechanisms . . . . 124 Figure 5.14 Stiffness – strain behaviour of soil with typical strain ranges for laboratory tests and structures . . . . . . . . . 126 Figure 5.15 Linear steady state seepage in uniform ground and the effect of excavation width . . . . . . . . . . . . . . . . . . . . 130 Figure 5.16 Various steady state seepage flownets for an impermeable wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Figure 5.17 Allowance for an unplanned excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Figure 5.18 Determination of characteristic values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Figure 5.19 Temporary works design assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Figure 6.1 Relationship between PSR and excavation geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Figure 6.2 Relationship between normalised lateral movement and FoS against basal heave for deep excavations in soft and firm clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Figure 6.3 Components of wall displacements and definition of a stiff wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Figure 6.4 Maximum lateral wall movements vs. system stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Figure 6.5 Movements potentially associated with water flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 Figure 6.6 Ground surface movements due to bored pile installation in stiff clay (non-normalised) . . . . . . . . . . . . . . . . 158 Figure 6.7 Ground surface movements due to diaphragm wall installation in stiff clay (non-normalised) . . . . . . . . . . . . 159 Figure 6.8 Ground surface movements due to bored pile installation in stiff clay (normalised) . . . . . . . . . . . . . . . . . . . . 160 Figure 6.9 Ground surface movements due to diaphragm wall installation in stiff clay (normalised) . . . . . . . . . . . . . . . 161 Figure 6.10 Typical ground movement pattern associated with excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Figure 6.11 Ground surface settlement due to excavation in soft and firm clay and normalised settlements behind the wall based on the same data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Figure 6.12 Relationship of normalised maximum lateral displacement with system stiffness . . . . . . . . . . . . . . . . . . . . . 166 Figure 6.13 Relationship between maximum lateral and vertical ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Figure 6.14 Normalised minimum wall deflection versus system stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Figure 6.15 Ground surface movements due to excavation in front of wall embedded in stiff clay . . . . . . . . . . . . . . . . . . 168 Figure 6.16 Ground surface settlement due to excavation in front of wall in sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Figure 6.17 Relationship between analysed lateral (propped) wall deflections and predicted ground surface settlements in stiff ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Figure 6.18 Analytically defined relationship between non-dimensional maximum lateral wall movement and FoS against basal heave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Figure 6.19 Influence factors on maximum lateral movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Figure 6.20 Simple field of plastic deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Figure 6.21 Idealised deformation pattern for a rotating bulkhead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Figure 6.22 Different rates of mobilisation of strength with shear strain in stress paths simulating those experienced by soil elements behind and in front of an embedded retaining wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Figure 6.23 Procedure for prediction of wall deflections and ground surface movements . . . . . . . . . . . . . . . . . . . . . . . . . 175 Figure 6.24 Cantilever movements as a proportion of maximum deflections for case studies in competent soil . . . . . . . 176 Figure 6.25 Sagging and hogging deformation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

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Figure 6.26 Procedure for building damage assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Figure 6.27 Relationship between damage category, deflection ratio and horizontal tensile strain . . . . . . . . . . . . . . . . . . 181 Figure 7.1 EC7 DA1 design method by use of calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 Figure 7.2 Extrapolating bending moment profile for DA1C1 limit equilibrium analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Figure 7.3 EC7 DA1C1 and DA1C2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Figure 7.4 Flow chart for determining groundwater levels during construction period . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Figure 7.5 Indicative movement for retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Figure 7.6 Illustration of potential advantages by using the OM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Figure 7.7 Ab initio OM applied to embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Figure 7.8 Definition of most probable parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 Figure 7.9 Trigger limits for multi-staged excavation – ab initio OM [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 Figure 7.10 Example of a traffic light system for a multi-staged excavation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 Figure 7.11 Ipso tempore OM applied to embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Figure 7.12 Identifying trigger limits for multi-staged excavation – ipso tempore OM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Figure 7.13 Design of sheet pile walls to EC3-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Figure 7.14 Corrosion zones for the assessment of crack width control criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Figure 8.1 Modular steel props supporting a diaphragm wall in a top-down sequence and a piled wall in a bottom-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Figure 8.2 Schematic diagram of actions to be considered for prop design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 Figure 8.3 Definition of sway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Figure 8.4 Definition of berm geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 Figure 8.5 Representation of a berm by means of a raised effective formation level . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 Figure 8.6 Normalised wall top displacement at the centre of the unsupported section against degree of discontinuity, β, for different excavated bay lengths, B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 Figure 8.7 3D finite element mesh, wall and excavation geometry and assumed soil parameters . . . . . . . . . . . . . . . . . 242 Figure 8.8 Relationship between berm height and effective uniform ground level, stiff to very stiff clay, ϕ = 22° (a), firm to stiff clay, ϕʹ = 28° (b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Figure 8.9 Typical intersection of anchors at re-entrant corners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Figure 8.10 Passive deadman anchor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Figure 8.11 Ground anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 Figure 8.12 Classification of ground anchor types A, B, C and D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Figure 8.13 Flow chart for the design and construction of ground anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 Figure 8.14 Typical use of steel walings with ground anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Figure 8.15 Typical use of reinforced concrete walings or load transfer assemblies with ground anchorages . . . . . . . . . 252 Figure 9.1 Whole life cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 Figure 9.2 Risk/cost/condition trade-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Figure 9.3 Optimum intervention timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Figure A2.1 Risk assessment – decision justification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Figure A2.2 Risk assessment – record of selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 Figure A2.3 Risk assessment register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 Figure A3.1 Typical steel sheet pile shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 Figure A3.2 Sheet pile wall for an underground car park in Belgium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Figure A3.3 Examples of high modulus (a) and combi steel walls (b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Figure A3.4 King post wall in City Park, Aberdeen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Figure A3.5 Contiguous bored pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Figure A3.6 Contiguous bored pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Figure A3.7 Secant wall Crossrail, Bond Street Station, London . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Figure A3.8 Hard/soft secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 Figure A3.9 Hard/soft secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 Figure A3.10 Hard/firm secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 Figure A3.11 Hard/firm secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 Figure A3.12 Determining minimum cut and spacing required for hard/firm piles constructed with CFA piling rigs . . . . . . 297 Figure A3.13 Determining minimum cut and spacing for hard/firm piles constructed with segmental cased rotary piling rigs . . . 298 Figure A3.14 Corner detail for hard/firm secant wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Figure A3.15 Hard/hard secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Figure A3.16 Hard/hard secant pile wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 Figure A3.17 Typical diaphragm wall panels and joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 Figure A3.18 Typical diaphragm wall construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Figure A3.19 Diaphragm wall attendances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Figure A3.20 Circular diaphragm wall shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 Figure A3.21 Peanut-shaped diaphragm wall shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 Figure A4.1 Normal and shear stresses acting on an imaginary plane within the cross-section plane . . . . . . . . . . . . . . . 305 Figure A4.2 Mohr circles of stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 Figure A4.3 Schematic stress history of an overconsolidated clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 Figure A4.4 Typical stress-strain data for a loose [...] soil and for dense [...] soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

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Figure A4.5 Critical state line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Figure A4.6 Undrained state paths for clay specimens having the same specific volume . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Figure A4.7 Ring shear test data for undisturbed London Clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 Figure A4.8 Normalised failure behaviour regimes for soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Figure A4.9 Normalised failure behavioural regimes for rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 Figure A4.10 Mohr circles representation of undrained shear strength failure criterion in terms of total stresses for shearing at constant specific volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Figure A4.11 Failure after dissipation of negative excess pore water pressures induced on excavation . . . . . . . . . . . . . . . 314 Figure A4.12 Definitions of soil stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 Figure A4.13 Earth pressure coefficient profiles one metre behind the centre of the primary panel during construction of the wall: 3D analysis with five metre panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 Figure A4.14 Earth pressure coefficient profiles normal to the centre of the primary panel following completion of the wall: 3D analysis with five metre panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Figure A4.15 Active and passive earth pressure coefficients, (β/φ) = -1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 Figure A4.16 Active and passive earth pressures coefficients, (β/φ) = -0.75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 Figure A4.17 Active and passive earth pressure coefficients, (β/φ) = -0.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324 Figure A4.18 Active and passive earth pressure coefficients, (β/φ) = -0.25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 Figure A4.19 Active and passive earth pressure coefficients, (β/φ) = 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326 Figure A4.20 Active and passive earth pressure coefficients, (β/φ) = 0.25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Figure A4.21 Active and passive earth pressure coefficients, (β/φ) = +0.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328 Figure A4.22 Active and passive earth pressure coefficients, (β/φ) = 0.75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 Figure A4.23 Active and passive earth pressure coefficients, (β/φ) = 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 Figure A4.24 Example 1 – cantilever wall: effective stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 Figure A4.25 Example 2 – propped wall: effective stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 Figure A4.26 Example 3 – cantilever wall: total stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Figure A4.27 Example 4 – propped wall: total stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Figure A4.28 Results for Example 1 – cantilever wall: effective stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 Figure A4.29 Results for Example 2 – propped wall: effective stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 Figure A4.30 Results for Example 3 – cantilever wall: total stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 Figure A4.31 Results for Example 3 – propped wall: total stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 Figure A5.1 Comparison of failure criteria for the both sides of the wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 Figure A6.1 Observed maximum lateral wall deflections for excavations in London Clay . . . . . . . . . . . . . . . . . . . . . . . . . . 352 Figure A6.2 Normalised maximum wall deflection versus system stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 Figure A6.3 Horizontal movements with excavation depth [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Figure A6.4 Horizontal movements with excavation depth [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Figure A6.5 Vertical movements with excavation depth taken from within 0.75 excavation depths from the wall . . . . . . 377 Figure A6.6 Vertical movements with excavation depth taken from within 0.75 excavation depths from the wall . . . . . . 377 Figure A6.7 Ground surface settlement contours at New Palace Yard, London . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 Figure A6.8 Plan of the Hai-Hua building, Taipei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378 Figure A6.9 Relationship between cantilever height and initial cantilever movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379 Figure A7.1 Geometry of proposed basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Figure A7.2 Proposed construction sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Figure A7.3 Interlock of segmental cased piled walls [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 Figure A7.4 900 mm diameter secondary pile through casing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 Figure A7.5 900 mm primary pile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 Figure A7.6 750 mm diameter secondary pile below casing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 Figure A7.7 Finite element mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 Figure A7.8 SPT N60 values in gravel layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 Figure A7.9 SPT N values in clay stratum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 Figure A7.10 SPT N60 values in gravel layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 Figure A7.11 Undrained shear strength profile with depth from triaxial testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Figure A7.12 SPT N60 values in clay stratum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Figure A7.13 Undrained shear strengths from triaxial testing and factored SPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 Figure A7.14 Water level recorded in gravel layer over 14 day period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 Figure A7.15 Scenario 1 – DA1C1 effects of actions [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 Figure A7.16 Scenario 1 – DA1C2 effects of actions [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 Figure A7.17 Scenario 1 – Design effects of actions [...] from ULS and SLS analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 Figure A7.18 Scenario 1 – SLS wall deflections and ground surface settlement behind retaining wall . . . . . . . . . . . . . . . . 404 Figure A7.19 Scenario 2 – DA1C1 effects of actions [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Figure A7.20 Scenario 2 – DA1C2 effects of actions [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Figure A7.21 Scenario 2 – Design effects of actions [...] from ULS and SLS analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 Figure A7.22 Scenario 2 – SLS wall deflections and ground surface settlement behind retaining wall . . . . . . . . . . . . . . . . 407 Figure A7.23 Comparison of wall movements from serviceability analyses from Gaba et al (2003) and Scenarios 1 and 2 . . . . . 411 Figure A7.24 Comparison of settlement behind wall from serviceability analyses from Gaba et al (2003) and Scenarios 1 and 2 and case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

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Figure A8.1 Figure A8.2 Figure A8.3 Figure A8.4

Method for calculating the DPL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristic DPL diagrams for Class A, Class B and Class C soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geometry of example modelled in stiff clay [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of maximum prop forces for the geometry shown in Figure A8.1 . . . . . . . . . . . . . . . . . . . . . . . . .

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Tables Table 1.1 Relationship between EC7, CEN/TC 288 execution standards and equivalent BS CoP . . . . . . . . . . . . . . . . . . . . 4 Table 2.1 Key considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 3.1 Wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Table 3.2 Typical applications of embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Table 3.3 Range of typical wall sizes for piled embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Table 3.4 Measures for dealing with obstructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Table 3.5 Environmental issues throughout the life cycle of the wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Table 3.6 Advantages and limitations of adopting a cantilever wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Table 3.7 Top-down construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Table 3.8 Tolerances for top-down construction [...] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Table 3.9 Bottom-up construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Table 3.10 Advantages and limitations of ground anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Table 3.11 Wall/slab connection types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Table 4.1 Advantages and limitations of common methods of retaining wall analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Table 4.2 Values of Young’s modulus E and second moment of cross-sectional area I for various embedded retaining wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Table 5.1 Particle size fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Table 5.2 Consistency index Ic of silts and clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Undrained shear strength of fine soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Table 5.3 Table 5.4 Correlations to classify density of coarse-grained soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Table 5.5 Generalised SPT energy ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Table 5.6 Rock identification for engineering purposes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Table 5.7 Field identification of unconfined compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Table 5.8 Soil and rock parameters required for various calculations/analysis methods . . . . . . . . . . . . . . . . . . . . . . . . 100 Table 5.9 Effect of fabric on ground properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Table 5.10 Index properties to be determined for fine-grained and coarse-grained soils . . . . . . . . . . . . . . . . . . . . . . . . . 110 Table 5.11 Suggested values for the characteristic weight density of fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Table 5.12 Comparison of common methods of permeability determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Table 5.13 φ′cv,k for clay soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Table 5.14 Values for φ′PSD and φ′dil to obtain values of φ′cv , and φ′pk , for siliceous sands and gravels . . . . . . . . . . . . . . 122 Table 5.15 Value of mi for intact rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Table 6.1 Ground surface movements due to bored pile and diaphragm wall installation in stiff clay . . . . . . . . . . . . . . 162 Table 6.2 Support stiffness categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Table 6.3 Ground surface movements due to excavation in front of bored pile, diaphragm wall and sheet pile walls wholly embedded in competent ground (stiff clays) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Table 6.4 Classification of visible damage to walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 Table 7.1 Partial factors for DA1C1 and DA1C2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Table 7.2 Summary of OM approaches to the design of embedded retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205 Table 7.3 Identification of trigger limits at each construction stage – ab initio OM (Approach A and B) . . . . . . . . . . . . . 210 Table 7.4 Summary of most commonly adopted monitoring systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Table 7.5 Identification of trigger limits at each construction stage – ipso tempore OM (Approach C) . . . . . . . . . . . . . 215 Table 7.6 Recommended value for the loss of thickness [mm] due to corrosion for piles and sheet piles in soils, with or without groundwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Table 7.7 Exposure classes related to environmental conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Table 7.8 Crack width control criteria as a function of water tightness class for liquid retaining and containment structures . 228 Table 8.1 Types of ground anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Wall and anchors designers’ relative responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 Table 8.2 Table 9.1 Suggested design lives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 Table 9.2 Points for inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 Table 9.3 Some instruments to consider for monitoring propped/anchored embedded walls retaining excavations . . 261 Table A3.1 Manufacturing tolerances for steel sheet piles to BS EN 10248-1:1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Table A4.1 Key assumptions and values of partial factors used in calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 Table A5.1 Equivalent Mohr-Coulomb parameters for both sides of the wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344 Table A6.1 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 Table A6.2 Table A6.3 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348 Table A6.4 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 Table A6.5 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349 Table A6.6 Relevant ground heave case studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351

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Table A6.7 Average δhmax values due to excavation in front of walls embedded in stiff soil for data where δhmax < 0.3%H . . . . 354 Table A6.8 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Table A6.9 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358 Table A6.10 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360 Table A6.11 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Table A6.12 Wall deflection effects – case study data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Table A6.13 Case studies with available displacement data for cantilever stages and final excavation stages . . . . . . . . 380 Table A7.1 Summary of flexural stiffness values for retaining wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385 Table A7.2 Summary of prop stiffness for design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 Table A7.3 Tension pile structural properties for analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 Table A7.4 Material partial factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 Table A7.5 Summary of design parameters adopted in calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 Table A7.6 Partial factors applied to unfavourable actions (surcharges) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 Table A7.7 Stages for DA1C2 PLAXIS analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 Table A7.8 Stages for DA1C1 PLAXIS analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 Table A7.9 Stages for serviceability PLAXIS analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 Table A7.10 Scenario 1 – summary of effects of actions (prop/slab forces) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 Table A7.11 Scenario 2 – summary of effects of actions (prop/slab forces) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 Table A7.12 Comparison of results from Scenario 1 and 2 analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 Table A7.13 Comparison of characteristic parameters used in current study and parameters used in Gaba et al (2003) design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 Table A7.14 Comparison of structural design values with those reported in Gaba et al (2003) . . . . . . . . . . . . . . . . . . . . . 410 Table A8.1 Classification of ground types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

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Glossary Ab initio  “ The intended use of the Observational Method from the inception of the construction phase” (Peck, 1969a). Analysis The process of breaking down a design into its constituent parts and of calculating the behaviour of each part.

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Client An organisation or individual using the services of construction professionals in order to invest in new building or construction work. Conceptual design The identification of an appropriate design solution by qualitatively assessing the strengths and weaknesses of a range of possible design variants, without recourse to detailed analyses. Desk study An examination of all existing information concerning a site (eg geological maps, previous borehole records, aerial photographs) to determine ground conditions and previous land use. Engineering judgement

A feel for the appropriateness of a situation, from the narrowest technical details to the broadest concepts of planning.

Geotechnical adviser  A n individual who has attained chartered membership through ICE, GSL or IoM3 (2011) and by means of qualification, training and experience meets the competence requirements to obtain registered ground engineering advisor. Geotechnical engineer

An individual who has attained chartership membership through ICE, IoM3 or GSL and by means of training and experience meets the competence requirements to obtain registered ground engineering professional as defined by ICE et al (2011).

Geotechnical risk

The risk posed to construction by the ground or groundwater conditions at a site.

Ground investigation The sub-surface field investigation, with the associated sampling, testing and factual reporting. See Site investigation. Ground model A conceptual model based on the geology and morphology of the site, and used to speculate on likely ground and groundwater conditions and their variability. Hazard An event, process or mechanism that could affect the performance of an embedded retaining wall and prevent performance objectives from being met. Ipso tempore

The application of the OM after the construction phase of a project has started.

Likelihood

The probability that an event will occur.

Most probable A set of parameters that represents the probabilistic mean of all possible sets of conditions. It represents, in general terms, the design condition most likely to occur in practice. Mitigation

The limitation of the undesirable effects of a particular event.

Moderately A cautious estimate of soil parameters, loads and geometry – worse than the conservative probabilistic mean, but not as severe as a worst credible parameter value. Sometimes termed a conservative best estimate. Project manager

The individual or organisation responsible for managing a project.

Observational The OM in ground engineering is a continuous, managed, integrated process of Method (OM) design, construction control, monitoring and review that enables previously defined modifications to be incorporated during or after construction as appropriate.

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Risk

The combination of the probability and consequences of a hazard occurring.

Risk assessment A structured process of identifying hazards, their probability and consequence of occurring, and their likely impact can be anything that will affect the project and should be considered.

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Risk mitigation Measures taken to either remove a hazard or to minimize the likelihood or consequences of it occurring to an acceptable level, including monitoring, and remedial action. Risk register

A list of risks arising from relevant hazards and the benefits of mitigating them.

Rupture surface

The detachment surface on which differential movement occurs.

Serviceability limit state

State of deformation of a retaining wall such that its use is affected, its durability is impaired or its maintenance requirements are substantially increased. Alternatively, such movement that may affect any supported or adjacent infrastructure, eg track, road or canal. See Ultimate limit state.

Site investigation The assessment of the site, including desk study, planning and directing the ground investigation, and interpretation of the factual report. Ultimate limit state State of collapse, instability or forms of failure that may endanger property or people or cause major economic loss. See Serviceability limit state. Worst credible The worst value of soil parameters, loads and geometry that the designer realistically believes might occur.

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Abbreviations and acronyms ALARP

As low as reasonably practicable

BGA

British Geotechnical Association

BIM

Building Information Modelling

BRE

Building Research Establishment

BS

British Standards

BTS

British Tunnelling Society

CDG

Completely decomposed granite

CFA

Continuous flight auger

CGS

Canadian Geotechnical Society

CIArb

Chartered Institute of Arbitrators

CoP

Codes of practice

DBFO

Design, build, finance, operate

DGGT

German Geotechnical Society

DPL

Distributed prop load

EC

Eurocode

EE

Embodied energy

EN

Euronorm

FoS

Factor of safety

FPS

Federation of Piling Specialists

GA

Geotechnical advisor

GC

Geotechnical contractor

GSI

Geological Strength Index

GSL

Geological Society of London

HS1

High Speed 1

HS2

High Speed 2

ICE

Institution of Civil Engineers

IMechE

Institution of Mechanical Engineers

IoM3

Institute of Material, Minerals and Mining

IStructE

Institution of Structural Engineers

MEFP

Minimum equivalent fluid pressure

MEMS Microelectromechanical MSD

Mobilised strength design

NA

National Annex

NCCI

Non-Contradictory Complementary Information

NDP

Nationally Determined Parameters

OCR

Overconsolidation ratio

OM

Observational Method

PD

Principal designer

PSR

Plane strain ratio

PTA

Principal technical advisor

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RAEng

Royal Academy of Engineering

RMR

Rock mass rating system

RQD

Rock quality designation

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SAA ShapeAccelArray SCR

Solid core recovery

SFARP

So far as reasonably practicable

SLS

Serviceability limit state

SPT

Standard penetration test

SSI

Soil-structure interaction

SSSI

Sites of Special Scientific Interest

TCR

Total core recovery

TPO

Tree Preservation Order

TRL

Transport Research Laboratory

UB

Universal beams

UCS

Uniaxial compressional stress

UDL

Uniformly distributed load

ULS

Ultimate limit state

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Notation

Note to the reader While the authors have endeavoured to include most of the common notation in this section, not all notation has been provided especially partial factors because of the extensive use of them throughout the guide.

Δa

Allowance for unplanned excavation

a

Parameter in Hoek-Brown failure criterion that depends on rock mass characterisation

A

Cross sectional area

b

Soil specific parameters for non-linear failure criterion

B

Excavation width

cu

Undrained shear strength of the soil

cv

Coefficient of consolidation

cw

Undrained soil/wall adhesion

cw,d

Limiting value of design wall adhesion

c′

Effective cohesion

c′cv,k

Characteristic value of the constant volume effective cohesion

c′w,d

Design effective wall adhesion

CN

Overburden pressure correction

CU

Uniformity coefficient

d

Depth of embedment required to prevent collapse

D

Pile diameter

D

Particle size

D

Disturbance factor due to blasting and stress relaxation

Dprime

Diameter of primary pile

Dr

Relative density

Dsec

Diameter of secondary pile

Dw

Depth to firm layer

e

Eccentricity of load application

e

Void ratio

emax

Void ratio corresponding to the minimum density

emin

Void ratio corresponding to the maximum density

E

Young’s modulus of elasticity

Ed

Design effects of actions (wall bending moments, shear or prop/anchor forces)

Ek

Characteristic value of bending moments, shear forces or prop/anchor forces

E0

Uncracked short-term Young’s Modulus of concrete

Es

Representative soil stiffness

Eu

Young’s modulus of elasticity of the ground under short term undrained conditions

E′

Young’s modulus of elasticity of the ground under long term drained conditions

EI

Young’s modulus multiplied by the second moment of area of the wall section per metre length (flexural rigidity)

ERr

Rod energy ratio

(ERr )/60

Rod energy ratio normalised to 60 per cent

fck

Factored design stress of reinforced concrete in flexure

fcu

Ultimate design stress of reinforced concrete in flexure

f1

Empirical correlation factor relating SPT ‘N’ value to undrained shear strength

F

Prop or anchor force

G

Shear modulus of the ground

Gk

Characteristic prop self-weight

Gk,GEO

Characteristic geotechnical loading

*

G

Rate of increase in soil shear modulus with depth

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h

Retained height or average vertical prop spacing of a multi-propped support system

H

Maximum excavation depth

I

Second moment of cross sectional area

Ic

Consistency index

ID

Density index

Ip

Plasticity index

Iprime

Second moment of cross sectional area of primary piles

Isec

Second moment of cross sectional area of secondary piles

k

Effective normal stiffness

k

Coefficient of permeability

k soil

Soil permeability

K

Lateral earth pressure coefficient

Ka

Active earth pressure coefficient

K0nc

Value of K0 for soil in a normally consolidated state

Kp

Passive earth pressure coefficient

Kw

Stiffness of a simple supported waling beam

K0

In situ earth pressure coefficient

leff

Effective length of prop

L

‘Free length’ of the prop, ie the distance between the wall and the point at which the prop is rigidly supported

LA

Length of anchor

Le

External length of tendon measured from the tendon anchorage in the anchor head to the anchorage point in the stressing jack

Lfixed

Fixed anchor length

Lfree

Free anchor length

LL

Liquid limit

Ltb

Tendon bond length

Ltf

Tendon free length

mb

Modified Hoek-Brown constant (mi)

mi

Hoek-Brown constant

M

Modulus multiplier

Mc,Rd

Design moment resistance

M0

One-dimensional modulus (constrained modulus) in the direction of compression or swelling

MEFP

Minimum equivalent fluid pressure

N

SPT N value (blows per 300 mm penetration)

Nc

Bearing capacity factor

Nk

Cone factor

N60

SPT N value normalised for energy ratio of 60 per cent

N1(60)

SPT N value normalised for energy ratio of 60 per cent and normalised for effective overburden pressure of 96 kPa

OCR

Over consolidation ratio

p

Mean total stress

p′

Mean effective stress

p′v

Effective overburden pressure

Pc

Pre-consolidation pressure

PSLS

SLS action derived from either soil structure interaction analysis or other methods

PULS,d

Maximum factored design effect of actions

P′u

Ultimate net effective resisting force

q

Any uniform surcharge at the ground surface

q

Deviator stress

qc

Static penetrometer cone resistance

qk

Uniform variable surcharge load

qk,G

Characteristic value of permanent unfavourable surcharge

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qk,G

Characteristic value of variable unfavourable surcharge

qud

Unconfined compressive strength (rocks)

Qk,accidental

Characteristic accidental point load

Qk,temp

Characteristic thermal loading

QL

Line load

R

Dimensionless flexibility number

Rc

Critical flexibility number

s

Parameter in Hoek-Brown failure criterion that depends on rock mass characterisation

s

Spacing between props or king posts

s′

Average effective stress

S

Equivalent surcharge acting at the revised formation level

Sk

Strut stiffness per horizontal unit of length

t

Elapsed time

T0

Tensile strength

u

Pore water pressure

w

Water content

wL

Liquid limit

Wel

Elastic section modulus for a continuous wall

Wpl

Plastic section modulus for a continuous wall

Xd

Design strength

Xk

Characteristic strength

z

Depth below ground surface or top of wall

zp

Depth of pivot point (about which the wall can be imagined to rotate) below formation level

ztc

Depth of (dry) tension cracks

α

Wall adhesion factor

α

Thermal coefficient of expansion for the prop material

β

Slope angle (to the horizontal) of the soil surface

β

Factor allowing for the redistribution of earth pressure from the waling beam to the prop locations

βB

Reduction factor taking into account a possible lack of shear force transmission in the interlocks

γ

Unit weight of the soil

γ

Shear strain

γb

Bulk unit weight

γc

Unit weight of concrete

γcu

Partial factor applied to undrained shear strength

γc′

Partial factor applied to effective cohesion

γf,G

Partial factor of permanent unfavourable surcharge

γf,Q

Partial factor of variable unfavourable surcharge

γG

Partial factor for permanent unfavourable actions

γM

Partial factor

γqu

Partial factor applied to unconfined compressive strength

γSd

Model factor for stress redistribution effects

γw

Unit weight of water

δ

Angle of friction between soil and wall

δe

Deflection at excavated soil surface

δhmax

Maximum lateral wall deflections

δmax

Maximum friction angle

δt

Deflection at toe

δV

Settlement

δvmax

Maximum ground surface settlements

δ 2D

Movement assuming two dimensional behaviour

Guidance on embedded retaining wall design

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δ3D

Movement assuming three dimensional behaviour

δγ

Increment of shear strain

∆

Displacement flexibility

εh

Horizontal strain

εlim

Limiting tensile strain

εs

Shear strain

η2

Coefficient related to the bond condition

θ

Change in gradient of a line joining two reference points

θc

Angle of the critical passive Coulomb wedge to the horizontal

λ

Parameter relating to in situ stress for over consolidated soils

ν′

Value of Poisson’s ratio under drained conditions

νu

Value of Poisson’s ratio under undrained conditions

ρs

System stiffness

ρ*

Bending stiffness

σc

Aggregate crushing stress

σci

Unconfined compressive strength

σe

Equivalent consolidation pressure

σf

Mean effective stress in the soil at peak strength

σh

Horizontal total stress

σv

Vertical total stress

σ′

Effective stress

σ′a

Effective active pressure acting at a depth in the soil

σ′h

Horizontal (lateral) effective stress at the same point within the soil mass

σ′p

Effective passive pressure acting at a depth in the soil

σ′v

Vertical effective stress

σ′1

Major principal effective stresses at failure

σ′3

Minor principal effective stresses at failure

τ

Shear stress

τ max

Maximum shear stress within the specimen

ϕpl

Plastic rotation angle

ϕ′

Angle of shearing resistance

ϕ′ang

Contribution to ϕ′cv,k from angularity of the particles

ϕ′cv , ϕ′crit

Constant volume (critical state) angle of shearing resistance of the soil

ϕ′cv,k

Characteristic value of the constant volume (critical state) effective angle of shearing resistance

ϕ′dil

Contribution to ϕ′cv,k from soil dilatency

ϕ′mob

Mobilised strength

ϕ′peak

Peak angle of shearing resistance

ϕ′pk,k

Characteristic value of the peak angle of shearing resistance

ϕ′ps

Peak value of the plane strain angle of shearing resistance

ϕ′PSD

Contribution to ϕ′cv,k from the soil’s particle size distribution

ϕ′residual

Residual angle of shearing resistance

ϕ′tgt

Slope of the failure envelope

ϕ′ult

Ultimate or critical state angle of effective shearing resistance

ψ

Angle of dilation

xxvi

CIRIA, C760

1 Introduction

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An earth retaining wall is required to withstand forces exerted by a vertical or near-vertical ground surface. An embedded retaining wall is one that penetrates and obtains some lateral support from the ground at its base. The bending capacity of such a wall typically plays a significant role in the support of the retained material, particularly if it is freestanding. The wall may also be supported by structural members such as props, berms, ground anchors and slabs and it may form part of a larger structure. This publication provides guidance on the selection and design of vertical embedded retaining walls in compliance with the principles embodied in the Eurocodes. It covers all types of embedded walls, as shown in Figure 1.1. Retaining walls

Gravity retaining walls

Hybrid, eg gravity wall supported partially by piles

Sheet pile wall

Embedded retaining walls

King post wall

Contiguous bored pile wall

Secant bored pile wall

Diaphragm wall

Wall types included in this publication Figure 1.1

Wall types

It is possible to make economies in embedded retaining walls by selecting an appropriate wall type and support system for the envisaged construction method and sequence and long-term use. It is important to adopt a clear, unambiguous design method and to follow appropriate good practice in ground investigation, laboratory and field testing, design analysis and the use of good quality case study data. This publication provides guidance on all these points.

1.1

BACKGROUND TO THE PROJECT

In 1984 CIRIA published R104 (Padfield and Mair, 1984). This was quickly adopted by practitioners as the most authoritative document on the subject and was hugely influential. Although strictly applicable to the design of singly-propped or cantilever walls embedded in stiff overconsolidated clay, the principles presented in that guide were applied to a wide range of wall types and soils in the UK and overseas, including multi-propped embedded walls and even nonembedded walls. Some 20 years later, Padfield and Mair (1984) was updated and extended by CIRIA C580 (Gaba et al, 2003). Since then, this guide has been among the best-selling and most downloaded guides published by CIRIA. It has been used nationally and internationally as a design standard for many major projects, past and current. Gaba et al (2003) applies to the design of temporary and permanent cantilever, anchored, single and multi-propped retaining walls supported by embedment in stiff clay and other

Guidance on embedded retaining wall design

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competent soils. Although that publication explicitly excludes retaining walls embedded in soft clay or weak rock, its principles were also widely applied internationally to walls embedded in such ground conditions. A revision to Gaba et al (2003) was required to broaden its scope and application to retaining walls partly embedded in less competent soils and weak rocks. The most significant contribution made by Gaba et al (2003) was the articulation and application of the limit state design method for embedded retaining walls. This further developed the principles that were then under consideration in the development of the Eurocode standards, particularly Eurocode 7 (EC7) (BS EN 1997-1:2004 and BS EN 1997-2:2007). Such principles are now mature in their application and a revision to Gaba et al (2003) was also required to draw together the key lessons learnt from this experience over recent years. Since publication of Gaba et al (2003), all 10 Euronorm (EN) design standards (Eurocodes) have been published and they have been adopted for implementation within the UK and across all European Union (EU) countries for publicly-funded projects. There are also firm steps toward their adoption internationally, beyond the EU. The Eurocodes recognise the principle that the level of safety remains a country’s prerogative. Consequently, some safety factors and a number of other parameters are left open in the Eurocodes for selection at national level. These are termed nationally determined parameters (NDPs) which are made available in a national annex (NA) to each Eurocode. The NA also includes reference to Non-Contradictory Complementary Information (NCCI). Currently, Gaba et al (2003) is referenced in the UK NA to EN 1997 as a NCCI (NA+A1:2014 to BS EN 1997-1:2004+A1:2013). The Eurocode family of design standards are listed in full in the reference sections of this guide, and illustrated in Figure 1.2: zz

BS EN 1990 Eurocode 0 (EC0) Basis of structural design

zz

BS EN 1991 Eurocode 1 (EC1) Actions on structures

zz

BS EN 1992 Eurocode 2 (EC2) Design of concrete structures

zz

BS EN 1993 Eurocode 3 (EC3) Design of steel structures

zz

BS EN 1994 Eurocode 4 (EC4) Design of composite steel and concrete structures

zz

BS EN 1995 Eurocode 5 (EC5) Design of timber structures

zz

BS EN 1996 Eurocode 6 (EC6) Design of masonry structures

zz

BS EN 1997 Eurocode 7 (EC7) Geotechnical design

zz

BS EN 1998 Eurocode 8 (EC8) Design of structures for earthquake resistance

zz

BS EN 1999 Eurocode 9 (EC9) Design of aluminium structures. Principles of structural safety, serviceability and durability

EN 1990 Eurocode 0

Actions on structures

EN 1991 Eurocode 1

EN 1992

EN 1993

EN 1994

Eurocode 2

Eurocode 3

Eurocode 4

EN 1995

EN 1996

EN 1999

Eurocode 5

Eurocode 6

Eurocode 9

Figure 1.2

2

EN 1997

EN 1998

Eurocode 7

Eurocode 8

Structural design and detailing

Geotechnical and seismic design

Eurocodes

CIRIA, C760

In the UK, the British Standards Institution (BSI) has published European standards as national standards. The technical content of each Euronorm has been retained entirely with only the addition of a national title page, a national foreword and a national annex to each constituent component of each Eurocode part. For example, in the UK, EN 1997 has become BS EN 1997.

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In this publication, for convenience, each Eurocode will be referred to in a short hand manner, for example, Eurocode 7 (BS EN 1997-1:2004 and BS EN 1997-2:2007) will be referred to as EC7 and Part 1 of Eurocode 7 (BS EN 1997-1:2004) as EC7-1. Similarly, the National Annex to BS EN 1997-1:2004 will be referred to as the NA to EC7-1. In geotechnical engineering, design and construction are intertwined – knowledge of the ground is not only acquired during the design phase, but also emerges during construction. EC7-1 states little about construction other than “execution is covered to the extent that is necessary to comply with the assumptions of the design rules” (Clause 1.1.1(6)) and that “separate European standards are intended to be used to treat matters of execution and workmanship” (Clause 1.1.1(5)). This is a departure from the geotechnical British Standards (BS) codes of practice (CoP), which provided a combination of design guidance and pragmatic construction advice relating to specific geotechnical elements. Instead, EC7-1 requires the designer to consult execution standards published by the European Committee for Standardization (CEN/TC 288, 2016), which were developed by a separate group of European engineers with the objective of meeting EC7-1’s design requirements. Table 1.1 shows the relationship between these execution standards and the BS CoP for the various sections of EC7-1 and EC7-2. Not all of the good practice pragmatic practical guidance embodied in the BS CoP has been incorporated into the execution standards. There is a need for a coherent and authoritative publication that collates the best ideas and experiences available in British practice and provides clear design guidance compliant with the requirements of the Eurocodes that also capitalises on where the Eurocodes allow the designer to exercise innovation. That is the aim of this publication.

1.2 OBJECTIVES The guidance given here should enable users to achieve compliance with the requirements of Design Approach 1 (DA1) of the Eurocodes, and to achieve economy in the resulting retaining structure and its support system while maintaining simplicity, but also facilitating more complex approaches that give an advantage. In addition, it: zz

discusses available wall types and construction methods

zz

provides a comprehensive update of the ground movements database presented in Gaba et al (2003)

zz

offers good practice guidance consistent with recent research and current analytical techniques.

It is intended that the guidance and recommendations provided in this publication will inform the debate and considerations currently underway for future revisions and updating of the Eurocodes, particularly EC7. This publication supersedes Gaba et al (2003).

Guidance on embedded retaining wall design

3

BS 6031:2009

BS 6349-1-3:2012

12 Embankments

11 Overall stability

10 Hydraulic failure

9 Retaining structures

8 Anchorages

BS 8081:2015

BS 8002:2015

7 Pile foundations

6 Spread foundations

BS 8008:1996 +A1:2008

BS 8004:2015

BS 8004:2015

(BS 8006:1995)

5 Fill, dewatering, ground improvement and reinforcement (note BS EN 1997-1 does not cover the design of reinforced soils or ground strengthened by nailing etc)

4 Supervision of construction, monitoring and maintenance

Some of those listed here:

BS 6031:2009

3 Geotechnical data

BS 5930:2015

1 General 2 Basis of geotechnical design

Section – title

Eurocode 7

Design of specific elements

Design aspects of construction activities

Ground investigation

Overall approach

General issues covered

BS EN 1997-1:2004+A1:2013 Eurocode 7 Geotechnical design. General rules Section – title

EN 1997-2:2007 Geotechnical design Part 2: Ground investigation and testing

1 General 2 Planning of ground investigations 3 Soil and rock sampling and groundwater measurement 4 Field tests in soils and rocks 5 Laboratory tests and soils and rocks 6 Ground investigation report Annex B – Planning strategies for geotechnical investigations

Relationship between EC7, CEN/TC 288 execution standards and equivalent BS CoP (after DCLG, 2007)

BS CoP equivalent

Table 1.1

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4

CIRIA, C760

BS EN 15237:2007

BS EN 14731:2005

BS EN 14679:2005

BS EN 14490:2010

BS EN 14475:2006

BS EN 12716:2001

BS EN 12715:2000

BS EN 12063:1999

BS EN 1538:2010+A1:2015

BS EN 1537:2013

BS EN 14199:2015

BS EN 12699:2015

BS EN 1536:2010+A1:2015

BS EN 12063:1999

BS EN 14490:2010

BS EN 14475:2006

CEN/TC 288 Execution of special geotechnical works

1.3 READERSHIP This publication is intended for use by those concerned with the selection, design and construction of embedded retaining walls. In addition to providing guidance to designers, it also: zz

Gives background information on wall selection, construction methods and associated ground movements for clients and owners and their technical advisers.

zz

Presents geotechnical principles and guidance to structural and geotechnical engineers and to students who wish to gain an appreciation of the issues relevant to the selection, design and construction of an embedded retaining wall.

zz

Acts as a reference for more experienced geotechnical engineers.

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1.4 APPLICABILITY This publication covers the design of temporary and permanent cantilever, anchored, single- and multipropped retaining walls embedded in a wide range of fine-grained and coarse-grained competent soils (as classified and described in BS EN ISO 14688-1:2002+A1:2013) and very weak rocks (as classified and described in BS EN ISO 14688-2:2004+A1:2013). However, few walls are constructed entirely in such ground conditions. It is likely that the walls will also retain other less competent soils and be partly embedded in those soils, such as made ground, river gravels and other alluvial deposits. The principles presented in this publication apply to this common situation.

Competent soils In the context of the design guidance provided in this publication, typical British soils that fall into the category of competent soils include London Clay, Oxford Clay, Gault Clay, Lias Clay, Atherfield Clay, Weald Clay, Barton Clay, Kimmeridge Clay, Lambeth Beds, and glacial tills. These soils have experienced high overburden pressures in their geological history, which have caused them to consolidate to a dense state. Further erosion of the upper soil horizons, or the removal of Quaternary ice cover, has resulted in significant unloading and swelling. These soils typically exhibit in situ moisture contents that are lower than they would have been if no over-consolidation had occurred. This geological history results in soils that: zz

may be fissured

zz

have an in situ earth pressure coefficient, K0, which is greater than unity

zz

have an undrained shear strength that is significantly greater than that of a normally consolidated soil at similar depth

zz

exhibit a peak shear strength at low strains and reduced shear strength at high strains.

Very weak rocks In this publication, reference to very weak rocks relates to weathered rocks, typically with unconfined compressive strengths of less than 5 MPa, where the engineering behaviour of the mass can be characterised by failure through the material itself and not along pre-existing discontinuities/joints. Such materials can be represented by the Mohr-Coulomb failure criteria with carefully considered parameters or the Hoek-Brown model.

Less competent soil Less competent soils are assumed to comprise principally inorganic, siliceous, fluvial and marine deposits, typically fine-grained soils with undrained shear strengths of less than 40 kPa.

Guidance on embedded retaining wall design

5

Geotechnical categories The geotechnical categorisation set out in EC7-1 (Clauses 2.1(10)-2.1(21)) has been adopted in this guide as a means of classifying risk. The guidance applies to the design of embedded retaining walls for Geotechnical Category 1 (small and relatively simple structure with negligible risk) and Geotechnical Category 2 (conventional types of structure and foundation with no exceptional risk or difficult ground or loading conditions). It does not specifically address the design requirements of walls for Geotechnical Category 3 (structures or part of structures that fall outside the limits of Geotechnical Categories 1 and 2. For example very large or unusual structures, structures involving abnormal risks, or unusual or exceptionally difficult ground or loading conditions, structures in highly seismic areas, and structures in areas of probable site instability or persistent ground movements that require separate investigation or special measures), although the principles presented here will, in general, apply to these structures.

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According to EC7-1 (Clause 3.2.1(2)P), geotechnical investigations must reflect the Geotechnical Category: zz

Geotechnical Category 1 – qualitative geotechnical investigations based on local experience should suffice.

zz

Geotechnical Category 2 – the provisions of EC7-2 apply.

zz

Geotechnical Category 3 – requires careful thought because of the exceptional nature of the issues to be addressed.

The particular circumstances that place a structure or project in Geotechnical Category 3 may necessitate additional investigations and more advanced tests compared to what may be required for Category 2 (Bond and Harris, 2008). Geotechnical categories are defined and discussed further in Section 2.3.3. Ground engineering requires a thorough knowledge and understanding of basic principles and the application of sound engineering judgement based on experience. This guide is not a substitute for professional knowledge. If in doubt, seek appropriate advice.

1.5

ECONOMIC DESIGN

Economy can be achieved by: zz

ensuring ease of construction and minimising construction duration

zz

optimising the use of materials

zz

applying appropriate design effort.

The biggest economies will be available at the start of a project during the selection of an appropriate method and sequence of construction, wall type and the optimisation of the temporary and permanent use of the retaining structure. Achieving economy requires commitment and adherence to an approach that takes a holistic view of project requirements. Whole-life costs should be considered. A robust design that minimises long-term maintenance requirements may be appropriate in some circumstances. A design that minimises wall dimensions and material use, but increases construction duration because it is difficult to build may not result in overall economy. The designer cannot achieve the most cost-effective solution in isolation from the client and the constructor. The client, designer and constructor and, where appropriate, the architect and the quantity surveyor, should be involved as early as possible to: zz

Optimise the temporary and permanent use of the retaining structure (such as adopting one wall instead of two to serve both the temporary and permanent requirements), which is also compatible with long-term maintenance requirements.

zz

Establish appropriate design and performance criteria for the retaining structure, such as acceptable limits for wall deflection and associated ground movements, and crack width criteria.

6

CIRIA, C760

zz

Consider appropriate wall type.

zz

Consider an appropriate method and sequence of construction to ensure buildability with minimum construction duration.

Initial ideas should be reviewed and alternatives explored before agreeing the preferred solution. It is important to involve individuals with appropriate qualifications and experience at all stages of the project and to maintain adequate continuity and communication between the staff involved in data collection, design and construction. The involvement of the constructor at an early stage should minimise wasteful abortive work arising from design changes.

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Where the design of a wall is governed by temporary works considerations, a risk-based approach to design may lead to significant savings. A clear, unambiguous design method that eliminates confusion will avoid unnecessary design effort. The choice of analysis method can result in significant savings in the wall structure. For example, propped or anchored walls designed using soil-structure interaction (SSI) methods will be shorter and computed wall bending moments will be smaller than those designed using limit equilibrium methods (Section 4.3). Also, savings in material use of about 15 to 25 per cent are possible if plastic design (adopting plastic section properties) is applied to sheet pile walls (Section 7.5.1). When driving in competent ground, it may not be possible to realise such savings, as it is often necessary to adopt sheet pile sections that are of greater thickness than those determined from the analysis of the section in service in order to withstand the driving forces. However, lighter sections may be possible by adopting higher grades of steel, which can enhance driveability in more difficult ground conditions. Savings can also result from the extraction and reuse of steel sheet piling for temporary works. Procedures for higher categories of wall can be used to justify more economic design. For example, the design of walls to Geotechnical Category 2 can be used to justify a more economic design for structures that would otherwise be classified as Geotechnical Category 1. The higher investigation and design costs of a Geotechnical Category 2 wall should be balanced against the potential savings in materials and construction over a Geotechnical Category 1 design.

1.6

HOW TO USE THIS GUIDE

Figure 1.3 shows the principal design stages and the corresponding sections of this publication. For readers needing guidance on specific issues, Figure 1.4 provides a map of the sections discussing key issues. Details are presented in boxes, separately from the main text. A comprehensive index is provided at the end of this publication. This publication is organised into 10 largely self-contained chapters. In view of the anticipated wide readership (Section 1.3), the beginning of each chapter outlines the target readership likely to gain most from the content of that chapter. In addition, cross-references between related subjects in different sections of the guide are given to help the reader. Chapter 1 sets out the objectives of this guide and its applicability. Chapters 2 and 3 help the reader select the appropriate wall type and construction sequence. Some of the issues covered in these chapters are interrelated. Chapter 2 provides guidance on the determination of the key wall design criteria, health and safety issues, risk assessment and management, site-specific constraints and projectspecific requirements. Chapter 3 reviews wall types and available construction sequences. The advantages and limitations of different construction sequences and wall types are compared and guidance is provided on the selection of the most appropriate sequence and wall type to satisfy particular site and project requirements. This chapter also presents construction cost data relating to wall types and construction methods. Chapter 4 presents key principles of soil and weak rock behaviour relevant to embedded retaining walls and the determination of earth pressures. SSI and methods for its analytical modelling are also discussed.

Guidance on embedded retaining wall design

7

Chapter 5 provides guidance on the determination and selection of parameters for use in design calculations. Advice on the assessment of drained or undrained ground behaviour is given together with guidance on the selection of parameters appropriate for temporary works and permanent works design. Chapter 6 provides guidance on the estimation and effects of ground movements associated with wall installation and wall deflection performance, including principles of building damage assessment. Chapter 7 offers good practice guidance on the geotechnical and structural design of the wall in compliance with DA1 requirements of the Eurocodes. Chapter 8 presents good practice guidance on the design of propping systems, berms and anchors for lateral support to the wall.

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Chapter 9 provides guidance on maintenance, inspection and monitoring. Chapter 10 identifies areas of further work and research. References and further reading are included following the main chapters for the readers’ use. Appendices provide background information, a commentary on current practices and a list of case studies. The site Is an embedded retaining wall necessary?

No

Consider alternative methods, eg open-cut excavation, gravity retaining wall etc

Yes

Chapter 2

Design considerations

Interactive

Chapter 3 Construction considerations

Select wall type and construction sequence

Chapter 4 Analysis

Chapter 5 Determination and selection of parameters for use in design calculations

Chapter 6 Ground movement considerations due to wall installation and excavation and building damage assessment

Chapter 7 Design of wall

Chapter 8 Design of support systems

Chapter 9 Guidance on maintenance, inspection and monitoring of embedded retaining walls

Figure 1.3 Principal design stages and publication layout

8

Chapter 10 Areas of further work and research

Key Information only Key design activity

CIRIA, C760

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Figure 1.4

Key issues considered in this publication

Guidance on embedded retaining wall design

9

2 Design considerations

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This chapter is intended for the general reader including designers, constructors, clients and owners and their technical representatives. It assumes that the reader has some experience and knowledge of engineering, of design and construction, and of risk assessment and risk management. This is the first of two chapters that consider general issues of design and construction. It gives guidance on selecting the appropriate wall type and construction sequence for a project involving an embedded retaining wall. This chapter deals with design issues while Chapter 3 deals with construction considerations. Because some of the issues are interrelated, this chapter should be read in conjunction with Chapter 3. See also Figure 2.1. The site Is an embedded retaining wall necessary? Yes

Chapter 3

Chapter 2

Construction considerations

Design considerations understand role and responsibility of designer within the project team zzUnderstand health and safety obligations zzEstablish geotechnical category zzEstablish wall design requirements and performance criteria zzEstablish ultimate and serviceability states zz

Consider alternative wall types and construction methods through risk assessment and mitigation

Establish construction requirements for temporary and permanent support, eg one wall or two, constructor preference

zz

Consider alternative construction sequences and wall types zzConsider relative costs of alternative wall types zz

zz

Interactive

Select wall type and construction sequence

zz

Is the selected wall type appropriate for the site and available working space?

zz

No

Yes Determine wall depth, structural capacity and estimate wall deflections and ground movements and their effects from Chapters 6 and 7

zz

No

Does wall satisfy design requirements and performance criteria?

zz

Yes Key

Topics covered in Chapter 3 Figure 2.1

10

Accept design

Decision paths for selection of appropriate wall type and construction sequence

CIRIA, C760

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The cost of constructing a retaining wall is usually high compared with the cost of forming a battered slope. The need for a retaining wall should be assessed carefully during design and efforts should be made to keep the retained height as low as possible. Construction methods should be fully considered at the start of the design stage, because different construction methods may require different detailed design considerations. Early contractor involvement is recommended. An excavation cannot be made without causing ground movements. The chosen wall type and construction sequence should ensure that these movements, and their effects, remain within pre-defined limits. Such limits should not be unduly severe (Sections 2.4 and 2.5). In addition to any technical considerations, it is important to ensure that the design and construction procedures are safely undertaken and result in overall economy. It is inappropriate to adopt advanced technological solutions that minimise the dimensions and material costs of the retaining structure while prolonging the design and construction periods and long-term maintenance requirements resulting in increased overall costs. Similarly, it is inappropriate to take undue risks during construction to minimise construction duration to reduce costs. Balance is required. Whole-life costs should be considered. A robust design that minimises long-term maintenance requirements may be appropriate in some circumstances (Section 1.5). This chapter: zz

Emphasises the importance of continuity of design communication at all stages of the project life cycle to ensure health and safety considerations and the client’s requirements are satisfied.

zz

Highlights the requirement to adopt a risk-based approach to design and construction management and provides guidance on the assessment of such risk.

zz

Defines geotechnical characterisation of retaining walls and identifies the issues relevant to the establishment of design requirements and performance criteria for the wall.

zz

Describes limit state design principles in accordance with the philosophy of Eurocodes.

2.1

DESIGN COMMUNICATION DURING PROJECT LIFE CYCLE

Design and construction typically involves a multitude of interacting roles, including: zz

client

zz

project/construction manager

zz

designers and checking engineers

zz

architect

zz

constructors

zz

third party owners and their representatives, including party wall surveyors.

There is no clear and unambiguous relationship between these roles. The processes of design and construction are fragmented as design consultants and contractors are typically employed at different times under various forms of contractual arrangement and construction management. For example, a contractor undertaking a design, build, finance, operate (DBFO) contract can reasonably be described as the client for the work. This entity may also undertake some or all of the design, possibly in association with specialist subcontractors and subconsultants. To a subcontractor, the main contractor is the client. A designer or checking engineer may be employed by the client, a contractor, or an engineering consulting practice etc. The project manager’s function may be fulfilled directly by the client, the designer or the constructor. To ensure certainty of outcome in an increasingly fragmented construction environment, the following are essential: zz

good communication between all parties

zz

a team approach to problem-solving

Guidance on embedded retaining wall design

11

zz

an integrated total project process

zz

risk-based approach to design and construction management.

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Design is a continuous process, requiring regular reviews to ensure that the client’s requirements are being met over the project life cycle. The importance of maintaining effective continuity of design communication between all of the mentioned parties engaged on a project regarding risks, design and construction assumptions etc cannot be over-emphasised. Clear allocation of design responsibility between the various parties is essential and necessary, but not sufficient to maintain continuity of design communication over the full project life cycle. To do this effectively, one party should maintain an overview over the full project cycle – the lead designer. The assumptions, data, methods of calculation and results of verification of safety and serviceability that comprise the design of the embedded retaining wall should be recorded in a geotechnical design report, which complies with the requirements of Section 2.8(1) of EC7-1. The lead designer should be responsible for compiling the inputs if more than one designer is involved. The concept of a lead designer maintaining an overview of all design aspects over the project life cycle was recommended in Gaba et al (2003) with only limited take up. It is recommended that a lead designer is appointed for all projects where more than one designer is engaged. With regard to health and safety considerations, the Construction (Design and Management) Regulations (CDM 2015) are intended to ensure that health and safety issues are properly considered during a project’s life cycle. The CDM Regulations were introduced in 1994 following publication of Directive 92/57/EEC on minimum health and safety standards for temporary or mobile construction sites. The Regulations were revised in 2007 and a further revision came into effect in 2015. These latest revisions identify the roles of principal designer (replacing the CDM co-ordinator) and principal contractor during the pre-construction and construction phases for all projects where there is, or will be, more than one contractor on site, and where site construction work is expected to involve more than 20 persons working simultaneously at any point in the project or will exceed 500 person days/ shifts in aggregate. These latest requirements envisage that the principal designer (PD) role would be fulfilled from within the project team appointed by the client for the design of the project, which gives greater control and influence over design compared to the CDM co-ordinator. The key function of the PD is to plan, manage, monitor and co-ordinate the pre-construction phase to ensure that the project is carried out without undue risks to health or safety. In addition, the PD should identify, eliminate or control (mitigate), so far as is reasonably practicable, foreseeable risks to the health and safety of those undertaking the construction works or liable to be affected by the construction works or maintaining the works. The PD’s duties also include: zz

ensuring all designers comply with their respective duties under CDM 2015

zz

co-operating with all those working on the project

zz

preparing pre-construction information and providing this to the design team and contractors appointed by the client to work on the project

zz

preparing and updating as necessary a health and safety file, containing information relating to the project which is likely to be needed as pre-construction information for any future construction work

zz

liaising with the principal contractor (as appropriate) for the duration of the project. In particular, any information that they may need to prepare the construction phase plan or other information that could affect the planning and management of the construction work.

The principal contractor has a reciprocal duty to plan, manage, monitor and co-ordinate the construction phase of the project. The role of the PD should not be confused with that of the lead designer. Within the context of embedded retaining wall design, the entity acting as the lead designer could take on the role of the PD for compliance with CDM 2015 if it is sufficiently conversant with the design of other aspects of the project (for example, building the superstructure). In addition to health and safety considerations, the lead designer should also ensure continuity of design communication between all parties engaged on the project to ensure that the client’s requirements are being satisfied at all stages.

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2.2

RISK ASSESSMENT AND MANAGEMENT

Risks associated with the ground typically fall into two broad categories, those relating to safety, and those relating to geotechnical and financial matters.

Safety Risk assessment and management are statutory safety obligations requiring compliance with CDM 2015 (Section 2.1). This is to ensure that safety is not degraded and that risks are maintained as low as reasonably practicable (ALARP) (Perry et al, 2001).

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Geotechnical and financial Clayton (2001) defines geotechnical risk as “the risk to building and construction work created by the site ground conditions” and is deemed to include geo-environmental considerations. This is relevant to embedded retaining walls. Ground-related problems can adversely affect project cost, completion times, profitability and quality and can lead to environmental damage. For more information and advice on general risk management see Clayton (2001), Godfrey (1996), the RAMP report (ICE and IFOA, 1998) and the PRAM report (Simon et al, 1997). There are four stages of risk management (Nicholson et al, 1999): 1

Hazard identification: hazards are identified and documented in a register using experience of similar projects, brainstorming etc. Hazards should be identified from the findings of a comprehensive desk study (Section 5.3.1) and from experienced practitioners brainstorming in a small group.

2

Risk assessment: the likelihood and consequence of each hazard are evaluated and combined to estimate the risk corresponding to each hazard.

3

Risk reduction: hazards are eliminated, if possible, and the risks are reduced by a combination of design changes, procedural changes etc.

4

Risk control: risks are monitored, mitigated and managed throughout the project.

Figure 2.2 shows the typical process of incorporating CDM 2015 into the design process. Hazard identification, risk assessment and management techniques range from the qualitative to the relatively complex quantitative. Any method can prove useful provided it is appropriate and its limitations are recognised. In design and construction, common use of the term ‘risk assessment’ refers to a written description of hazard identification, risk assessment and intended controls, and is used loosely. The designer should identify the project-specific risks that a competent contractor would not be expected to routinely anticipate and control. Risk assessment serves two main functions: zz

Comparative risk assessment: to assist and document decision making. This is used to indicate the relative potential effect of design options on health and safety. Forms of the type presented in Appendix A2 (Figures A2.1 and A2.2) provide qualitative and simple quantitative example formats for recording such assessments. Subsequent events may cause decision making to be questioned, but the designer will have recorded the basis of their decision on information available at that time. It is good practice for the designer to complete either Figure A2.1 or A2.2 (or similar).

zz

Task risk assessment: to allocate resources, justify and record measures to be taken. This demonstrates an awareness of hazards and the intended application of controls to minimise risk so far as reasonably practicable (SFARP) and to communicate residual risk to third parties where relevant. Figure A2.3 in Appendix A2 presents an example for this purpose.

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Element Embedded retaining wall

Next element

Hazard identification

No

Are significant hazards associated with the construction/repair/maintenance/ demolition of this element? Yes

Risk assessment For each hazard: Next hazard

Next hazard

How likely is the hazard (A)? What are the consequences (B)?

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Level of risk: A x B

Yes

Is the risk tolerable? No

Prevention and protection Can the risk be adequately controlled through the design process? (Figure A2.1 or A2.2) No

Yes Modify design

Would a competent contractor be expected to routinely anticipate and control the risk?

Yes

No Provide details of risk to the principal designer Inclusion in the health and safety file (Figure A2.3) Figure 2.2

CDM 2015 risk assessment

2.3

DESIGN CONCEPTS

2.3.1 Design assumptions EC7-1 (Clause 1.3(2)) lists a number of important assumptions about the way structures are to be designed and executed and upon which the standard is based: zz

Data required for design are collected, recorded and interpreted by appropriately qualified personnel.

zz

Structures are designed by appropriately qualified and experienced personnel.

zz

Adequate continuity and communication exist between the personnel involved in data collection, design and construction.

zz

Adequate supervision and quality control are provided in factories, in plants, and on site.

zz

Execution according to the relevant standards and specifications by personnel having the appropriate skill and experience.

zz

Construction materials and products are used as specified in this standard or in the relevant material or product specifications.

zz

The structure will be adequately maintained to ensure its safety and serviceability for the designed service life.

zz

The structure will be used for the purpose defined for the design.

EC7-1 requires the designer and the client to consider these assumptions and to document compliance with them.

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2.3.2 Principles and application rules EC7-1 (Clauses 1.4(1)-(6)) makes a distinction between Principles and Application rules: zz

Principles (which are preceded by the letter P in EC7-1) are statements, definitions, requirements and analytical models for which no alternative is permitted (unless specifically stated), ie they must be followed and adhered to.

zz

Application rules are those that comply with the principles and satisfy their requirements. Alternatives are permitted to the application rules provided the alternatives accord with the relevant principles and are at least equivalent with regard to structural safety, serviceability and durability, which would be expected when using the Eurocodes.

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2.3.3 Geotechnical categorisation of retaining walls EC7-1 sets out application rules that classify risks in terms of geotechnical categorisation of structures. This publication sets out the design requirements for retaining walls falling within Geotechnical Categories 1 and 2. It does not specifically address the design requirements of walls for Geotechnical Category 3, although the general principles presented in this guide will also apply to these structures (see Section 1.4). Before the geotechnical investigations, the designer should assign to the retaining wall structure a Geotechnical Category. The category indicates the degree of effort required for site investigation and design, and should be reviewed and changed (if necessary) at each stage of the design and construction process. Figure 2.3 shows the decisions needed when assigning a category. As stated in Section 1.4, geotechnical categories are defined in EC7-1 (Clause 2.1(10)-(21)) and interpreted in this publication in relation to embedded retaining walls as follows.

Category 1 Category 1 walls are small and relatively simple structures with the following characteristics: zz

There is no excavation below the water table or comparable local experience indicates that a proposed excavation (below the water table) will be straightforward.

zz

The ground conditions are known from comparable local experience to be straightforward enough to allow routine methods of design and construction to be used.

zz

Previous experience indicates that a site-specific geotechnical investigation will not be required.

zz

There is negligible risk to property or life in terms of overall stability or ground movements.

Comparable experience is defined in EC7-1 (Clause 1.5.2.2) as: “documented or other clearly established information related to the ground being considered in design, involving the same types of soil and rock and for which similar geotechnical behaviour is expected, and involving similar structures. Information gained locally is considered to be particularly relevant.” While there are no examples given in EC7-1 of structures that fall within Geotechnical Category 1, in terms of embedded retaining walls, it is considered that provided the requirements are fully satisfied, walls with a retained height of less than two metres would typically fall into this category.

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Is the structure ‘small and relatively simple’?

No

Yes Are the ground conditions known from comparable experience to be sufficiently straightforward that routine methods may be used for design and construction?

No

No

No No

No

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No

No No

Is the site free from exceptional risks such as unusual loading or seismic risk?

Category 1

Category 2

Small and relatively simple structures

Conventional structures

Retaining wall and excavation support where the difference in ground levels does not exceed 2 m

zz

No

Yes

Yes

Figure 2.3

Yes

Are the loading conditions unusual or exceptional?

Yes Is there negligible risk to property and life?

Yes

Does it involve abnormal risks?

Yes Is the site free from exceptional risks such as unusual loading or seismic risk?

Yes

Does it involve unusual or exceptionally difficult ground?

Yes If excavation below the water table is involved, does comparable experience indicate that it will be straightforward?

Yes

Is the structure very large and unusual?

Wall and other structures retaining or supporting soil or water zzExcavations zzBridge piers and abutments zzGround anchors and other tie-back systems zz

Category 3 Structures or parts of structures that do not fall within the limits of Geotechnical Categories 1 and 2

zz

Geotechnical categorisation of a structure or element (after Simpson and Driscoll, 1998)

Category 2 Category 2 walls comprise conventional structures with no exceptional risk or difficult ground or loading conditions. These walls require site-specific quantitative geotechnical data (eg a desk study and routine ground investigation and field and laboratory testing data complying with the requirements of EC7-2) to be obtained and analysis to be carried out to ensure that the fundamental requirements are satisfied. The majority of embedded retaining walls fall into Geotechnical Category 2.

Category 3 Category 3 walls are structures or parts of structures that do not fall within the limits of Geotechnical Categories 1 and 2. These include: zz

very large or unusual structures

zz

structures involving abnormal risks, or unusual or exceptionally difficult ground or loading conditions

zz

structures in highly seismic areas

zz

structures in areas of probable site instability or persistent ground movements that require separate investigation or special measures.

The general advice and principles contained in this publication are applicable to Geotechnical Category 3 walls, but specialist advice should be sought to ensure that the particular circumstances are adequately dealt with to satisfy the principles in EC7-1.

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A typical project may comprise elements with a mixture of Geotechnical Categories 1 and 2 and even 3, as illustrated in Figure 2.4.

Figure 2.4

Geotechnical categorisation of a project (after Bond and Harris, 2008)

2.3.4

Geotechnical design process

Key elements of geotechnical design within a good understanding of project constraints include: zz

Understanding the geological and hydrogeological setting of the site and its environs and the historical development of the site to determine ground stratigraphy and groundwater conditions.

zz

Understanding ground behaviour.

zz

Undertaking design by using calculations, adopting prescriptive measures, experimental models and load tests, or by using the OM.

zz

Applying empiricism based on sound judgement and experience.

Each of these involves a distinct and rigorous activity. It is important to distinguish clearly between them. All should be kept in balance – all are important.

Ground stratigraphy and groundwater conditions In some instances, site-specific data are not available and there is insufficient information to define ground stratigraphy and groundwater conditions accurately. In cases where a limited number of boreholes exist, without surveyed borehole ground levels, this information can be similarly inadequate in establishing a ground model, especially on sloping sites. This is not acceptable for the design of walls other than those that fall into Geotechnical Category 1. Walls in Geotechnical Category 2 require quantitative geotechnical data, but routine procedures for field and laboratory testing and for design and construction. For such walls, no amount of field and laboratory testing or analysis will compensate for a lack of knowledge about the ground stratigraphy and groundwater conditions. The designer should obtain appropriate sound geotechnical advice at an early stage.

Ground behaviour Knowledge of the principles of soil and rock mechanics (as appropriate), ground fabric, permeability, stress history, and in situ strength and stiffness is essential to understand ground behaviour. Of particular importance is the assessment of whether drained or undrained ground conditions will apply over the lifetime of a temporary structure or during the construction stages of a permanent wall in the short term. This is discussed further in Section 5.4.

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Undertaking design EC7-1 provides a framework for undertaking design to ensure that the structure does not reach a ‘limiting condition’ in prescribed ‘design situations’. Such limiting conditions or ‘limit states’ are defined and discussed in Section 2.5. EC0 Clause 3.2(1)-(3)P) classifies design situations as: zz

persistent: conditions of normal use

zz

transient: temporary conditions, eg during construction or repair

zz

accidental: exceptional conditions applicable to the structure or its exposure, eg to fire, explosion, impact or the consequences of localised failure

zz

seismic: conditions applicable to seismic events.

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EC7-1 (Clause 2.1(4)) permits design to be undertaken by one or a combination of the following: zz

By calculation: this approach is the most common for Geotechnical Category 2 walls. It requires explicit identification of limit states and calculation of the destabilising (unfavourable) actions (or their effects) and the stabilising (favourable) actions and resistances. It requires demonstration by calculation that these limits are not exceeded. The calculations and analyses undertaken should be appropriate for the geotechnical category of the wall. As a minimum, these calculations and analyses should demonstrate that equilibrium is achieved without overstressing materials. Analytical methods and the computer software and hardware necessary for them to be carried out have developed rapidly. It is now possible to obtain solutions to many complex problems. The great advantage of these methods is that they can help the designer gain a better understanding of the behaviour of a ground-structure system such as an embedded wall. However, they should be used with discernment and scepticism. The designer should understand the idealisations and assumptions made in the numerical modelling. Carrying out such analysis demands specialist knowledge and experience in the use of the particular software application. Also, numerical analysis requires appropriate highquality input data. If such data are not available, the results of the analysis should be treated with caution. Software and hardware improvements will continue, which will enable larger, more complex problems to be analysed in the future. However, even with unlimited analytical power, the inherent uncertainties in the ground, the structure and the construction procedure are so significant that accuracy in the prediction of expected behaviour will seldom be achieved. This continues to present a significant challenge to the designer. Detailed guidance on the design of the wall by calculation is provided in Sections 7.3 and 7.5.

zz

By prescriptive measures: this approach involves the use of conventional and generally conservative design rules. Particular attention is given to specification and control of materials, workmanship, protection, and maintenance procedures to avoid exceeding limit states in design situations where calculation models are not available or not necessary. For example, the application of charts, tables and procedures following local convention derived based on comparable experience can be used. Such design rules are provided in the UK NA to EC7-1 and in the NCCI referred to in the NA. Such prescriptive measures may be applied to ensure durability, for example, by specifying additional thickness of steel for steel sheet pile walls to prevent the adverse effects of corrosion loss.

zz

By experimental models and load tests: this approach involves the use of load tests or tests on largeor small-scale models to justify a design. The differences between the actual construction and test in the ground conditions, and the time and scale effects should all be carefully considered by the designer. Tests may be carried out on a part of the actual construction or on either a full-scale or smaller scale model.

zz

By the Observational Method (OM): this approach is permitted, but EC7-1 provides little guidance on how to implement it. Using this method can achieve significant economy in particular circumstances and can be effective where prediction of geotechnical behaviour is difficult or uncertain. Before construction is started, acceptable limits of behaviour must be established, the possible behaviours must be assessed, and a plan of monitoring must be prepared to check that actual behaviour is within acceptable limits. During construction, monitoring should be carried out as planned and assessed at appropriate stages, and planned contingency actions put into place if the

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limits of behaviour are likely to be exceeded. Detailed guidance on implementing the OM within the context of the Eurocodes is provided in Section 7.4.

Judgement and experience Every stage of the design process demands the application of empiricism based on sound judgement and experience. The rationale of this process, including details of the experience upon which it is based, should be communicated and explicitly recorded as part of the design. This is essential and should not be overlooked. It is not sufficient to proceed simply based on ‘in my experience...’ with no further explanation, but to record the information that complies with the definition of comparable experience in Section 2.3.3.

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EC7-1 requires the designer to carry out the following: zz

establish the relevant design situations

zz

identify the relevant limit states

zz

verify that the limit states are not exceeded by adopting one or more of the design approaches.

The appropriate design approach is selected based on experience. Sometimes only one approach is used in design, however depending on particular circumstances, the design approaches can be used in combination. More detailed guidance in this regard is provided in Chapter 7.

2.4

DESIGN REQUIREMENTS AND PERFORMANCE CRITERIA

The design of an embedded retaining wall requires a holistic approach. It is a ground-structure interaction system where the retaining wall derives both loading and support, at least in part, from the ground. The wall transfers load from the retained ground so that it is resisted elsewhere in the ground mass and by the wall and its support system. How this transfer occurs depends upon the type of wall, the in situ stress state, strength and stiffness of the ground, the wall and its support system, and also on the method and sequence by which the wall and support system are constructed. This requires a full understanding of the role of the wall in the overall structure and how it interacts with its support system. Figure 1.4 shows some of these considerations for a typical (Geotechnical Category 2) basement retaining wall. It is important to establish the design requirements, loading conditions (actions) and performance criteria that the wall should satisfy at the start of the project. The wall should be stable and satisfy important performance criteria during construction and throughout its design life. Table 2.1 lists some of the key issues that should be considered at this stage. In addition to those listed in Table 2.1, in many cases, project-specific design specifications or employer’s requirements will highlight specific needs, such as crack width criteria for reinforced concrete walls, which the retaining wall should also satisfy.

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Table 2.1

Key considerations

Site-specific constraints

Project specific requirements Wall design life

Site location zz

adjacent to buildings, services, roads, railways etc

zz

zz

permissible limits for ground movements and wall deflection

Role of wall in overall structure

zz

delivery of materials to site.

durability requirements.

Wall watertightness requirements

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Site geometry

Construction programme

zz

shape and dimensions

zz

site topography

Lateral support to wall

zz

working space for construction plant

zz

headroom restrictions

temporary requirements

zz

zz

is an embedded wall necessary?

permanent requirements.

zz

Ground and groundwater conditions

Vertical support from wall

zz

geology and hydrogeology

zz

temporary requirements

zz

ground stratigraphy, fabric and permeability

zz

permanent requirements.

zz

soil/rock strength and stiffness

zz

wall cut-off requirements

zz

temporary groundwater control measures

zz

drainage (eg by leakage through the wall or installation of drains and weepholes)

zz

permanent groundwater control measures

zz

rising groundwater levels.

2.5

LIMIT STATES

The Eurocodes adopt a common limit state design philosophy to ensure rational integration of geotechnical design with structural design. Simpson and Driscoll (1998) define limit state design as an approach in which attention is concentrated on avoidance of limit states, ie states beyond which either the retaining wall no longer satisfies the design performance requirements or there is a possibility of damage, economic loss or unsafe situations arising. In limit state design, attention is directed to undesirable states in which the construction is failing to perform satisfactorily. This is done by distinguishing between ultimate and serviceability limit states. Ultimate limit states are concerned with the safety of people and the structure (Section 2.5.1). Serviceability limit states are concerned with the functioning of the structure under normal use, the comfort of people, and the appearance of the construction works (Section 2.5.2). In compiling the list of required performance criteria and relevant limit states, the designer should consider the various design situations that can be foreseen during the construction and design life of the wall. Limit state design involves verifying that the relevant limit states are not exceeded in each design situation.

2.5.1

Ultimate limit states

Ultimate limit states are those associated with collapse or with other similar forms of structural failure (EC0 Clause 1.5.2.13). As previously stated, they are concerned with the safety of people and of the structure. For embedded retaining walls, EC7-1 (Clause 9.2(1)-(3)P) lists the following to be considered, where relevant: zz

loss of overall stability

zz

failure of a structural element, such as a wall, anchor, waler or strut, or failure of the connection between such elements

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zz

combined failure in the ground and in the structural element

zz

failure by hydraulic heave and piping

zz

movement of the retaining structure, which may cause collapse or affect the appearance or efficient use of the structure or nearby structures or services, that rely upon it

zz

unacceptable leakage through or beneath the wall

zz

unacceptable transport of soil particles through or beneath the wall

zz

unacceptable change in the groundwater regime

zz

failure by rotation or translation of the wall or its parts

zz

failure by lack of vertical equilibrium.

It is important to note that these criteria do not mention what type of analysis should be used in studying the limit state, or whether the materials will be responding elastically or in a plastic mechanism. Rather, they are based entirely on the practical issues of degrees of danger, damage and, by implication, cost of repair. For example, if a structure supported by a retaining wall collapses because of wall displacement, an ultimate limit state has occurred despite the fact that the wall has merely deflected without forming a mechanism in the ground. For embedded retaining walls, there are several possible ultimate limit states that may be reached. Some of these are shown in EC7-1 Clauses 9.7.4(1)-(3)P, 9.7.5(1)-(5)P, 9.7.6(1)-(4)P, 9.7.7(1)-(4)P, and are reproduced in Figure 2.5. Where possible, the wall should be designed so that adequate warning of danger (ie approaching an ultimate limit state) is given by visible signs. The design should guard against the occurrence of brittle failure, eg sudden collapse without conspicuous preliminary deformations. Particular caution should be applied where this is not possible. Case studies of embedded walls exhibiting outright and obvious collapse are few. Except in the case of a loss of structural integrity of the wall or its supports, failure is likely to manifest as a large movement, which in most cases gives a stabilising change in earth pressures or wall geometry. (For example, large rotation of an embedded wall will reduce the lateral stresses behind the wall and the effective retained height, and increase the lateral stresses in front of the wall and the effective embedment depth, all of which will act to stabilise the wall). Most failures occur due to inadequate design or control of the support to a wall in the temporary works condition, for which sheet pile walls are often used. Malone (1982) reports six sheet pile wall collapses and two cases of gross movement of sheet pile walls. Causes of failure were identified as: zz

inadequate support to the wall from the ground due to insufficient embedment

zz

buckling of the struts providing lateral support to the wall

zz

structural inadequacy of the connection between the strut and the wall

zz

inadequate foundations of raking struts

zz

over-excavation of the soil berm or its premature removal before installation of the struts.

Rowe (1986) describes the significant progressive outward movement of the toe of an anchored sheet pile wall due to the softening of the clay below the excavation in front of the wall from water seeping through the interlocks along confined permeable horizons in the ground. Inadequate understanding of the geological and hydrogeological conditions at the site was a significant contributory factor. Other case studies of sheet pile wall failures are provided by Sowers and Sowers (1967), Broms and Stille (1976), Daniel and Olsen (1982) and Powrie (1996). Problems experienced in reinforced concrete bored pile and diaphragm walls typically relate to difficulties in concreting leading to concrete contamination, insufficient cover to reinforcement and lack of watertightness at joints.

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Wall failures are seldom due to inadequacies of modern earth pressure theories or structural failure of the wall itself, although the misuse of sophisticated design tools can be a factor. Wall failures are most likely to be caused by: zz

Inadequate understanding of the geological and hydrogeological conditions, including soil properties and groundwater effects

zz

Poor design and construction details and poor standard of workmanship, particularly of support systems.

zz

Construction operations and sequences that result in earth pressures differing from those assumed in design.

zz

Inadequate control of construction operations, eg over-excavation of berms and formation, excessive surcharge loads from soil heaps and construction equipment.

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2.5.2 Serviceability limit states EC0 Clause 1.5.2.14 defines serviceability limit states as those that correspond to conditions beyond which specified requirements for a structure or structural member are no longer met, for example predefined limits on the amount of water seepage, wall deflections etc. The code also requires the designer to distinguish between reversible and irreversible serviceability limit states (EN 1990 Clause 3.4(2)P). Serviceability limit states concern the functioning of the structure or structural members under normal use, the comfort of people, and the appearance of the construction works (EN 1990:2002+A1:2005 Clause 3.4(1)P). For example, the following should be considered where relevant: zz

unacceptable wall deflections and associated ground movements

zz

unacceptable leakage through or beneath the wall

zz

unacceptable transport of soil grains through or beneath the wall

zz

unacceptable change to the flow of groundwater.

The permissible movements specified in design should take into account the tolerance of nearby structures and services to displacement. Limiting values should be assigned to allowable wall deflections and the movement of the ground adjacent to the wall. A cautious estimate of the distortion and displacement of the retaining wall and the effects on nearby structures and services should be made on the basis of comparable experience (as defined in Section 2.3.3) and the design verification of the relevant serviceability limit states using one or more of the approaches listed in Section 2.3.4, and discussed further in Section 7.3.2. This should include the effects of wall construction. The estimated displacements should not exceed the limiting values.

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Figure 2.5

Ultimate limit state examples

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2.6

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1

KEY POINTS AND RECOMMENDATIONS

Design is a continuous process, requiring regular review to ensure that the client’s needs are being met. There should be clear allocation of design responsibility between the various parties involved in a retaining wall project. The following are essential: a

good communication between all parties

b

a team approach to problem-solving

c

an integrated total project process

d

a risk-based approach to design and construction management.

2

Clear allocation of design responsibility between the various parties is important, but not sufficient to maintain continuity of design communication over the full project life cycle. To do this effectively, one party should maintain an overview, and it is recommended that a lead designer be identified for all projects where more than one designer is engaged to oversee this process.

3

Construction methods should be explicitly considered at the start of the design stage. This is because different construction methods may require different detailed design considerations. In this regard, early contractor involvement is recommended.

4

Risk assessment and management are statutory safety obligations under CDM 2015. For a typical retaining wall project, it is good practice to complete one of the forms shown in Figures A2.1 and A2.2 (or similar).

5

The designer should assign a geotechnical category to each retaining wall structure. The degree of effort required in site investigation and design should be appropriate to the category. Also, where calculations and analyses are undertaken, these should be appropriate for the category. As a minimum, these calculations and analyses should demonstrate that equilibrium is achieved without overstressing materials. Procedures of higher categories can be adopted to justify more economic design, where appropriate.

6

It is inappropriate to adopt advanced technological solutions that minimise the dimensions and material costs of the retaining structure while prolonging the design and construction periods and long-term maintenance requirements resulting in increased overall costs. Similarly, it is inappropriate to take undue risks during construction to minimise construction duration and reduce costs. Balance is required. Whole-life costs should be considered. A robust design that minimises long-term maintenance requirements may be appropriate in some circumstances.

7

Knowledge of the principles of soil and rock mechanics (as appropriate), ground fabric, permeability, stress history, and in situ strength and stiffness is important for understanding ground behaviour. However, there is no substitute for the application of empiricism based on sound judgement and experience at every stage of the design process. The rationale of this process, including details of the experience upon which it is based, should be communicated and explicitly recorded as part of the design. This is essential.

8

Limit states are states beyond which the retaining wall no longer satisfies the design performance requirements. The following steps should be taken to verify the safety and serviceability requirements of a retaining wall:

9

24

a

list the performance criteria that the wall should satisfy

b

list the limit states at which the various performance criteria will be infringed

c

demonstrate that the limit states are sufficiently unlikely to occur.

The designer should carry out the following: a

establish the relevant design situations (ie persistent, transient, accidental, seismic)

b

identify the relevant limit states

c

verify that the limit states are not exceeded by adopting one or more of the design approaches permitted by EC7-1 (by calculation, by prescriptive measures, by experimental models and load tests, by the OM). The appropriate design approach is selected based on experience. Sometimes only one approach is used in design, however, depending on particular circumstances, these design approaches can be used in combination. More detailed guidance is provided in Chapter 7.

CIRIA, C760

10 The assumptions, data, methods of calculation and results of verification of safety and serviceability that comprise the design of the embedded retaining wall should be recorded in a geotechnical design report, which complies with the requirements of EC7-1 Section 2.8(1). The lead designer should be responsible for compiling the inputs if more than one designer is involved. 11 Where possible, the wall should be designed so that it will give adequate visible warning of distress (ie approaching an ultimate limit state). The design should also guard against the occurrence of brittle failure, eg sudden collapse without conspicuous preliminary deformations. Particular caution should be applied where this is not possible.

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12 Wall failures are seldom due to inadequacies of modern earth pressure theories or structural failure of the wall. They are most likely to be caused by: a

inadequate understanding of the geological and hydrogeological conditions, including soil properties and groundwater issues

b

poor design and construction details and poor standard of workmanship, particularly relating to support systems

c

construction operations and sequences that result in earth pressures differing from those assumed in design

d

poor control of construction operations, eg over-excavation of berms and formation, excessive surcharge loads from soil heaps and construction equipment.

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25

3 Construction considerations and wall selection

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Major economies are possible at the scheme design stage by reviewing the construction method and type of wall to be used. This is also the stage when maximum carbon emissions reduction can be achieved through design. This chapter is intended primarily for the designer, but is also aimed at the constructor who is considering alternatives early in the design process to help select an economic solution for the retaining wall and the construction sequence. The designer must consider the method with which the wall will be constructed, and in-line with CDM 2015, must ensure that the wall can be safely constructed by eliminating health and safety risks associated with constructability. Figure 3.1 illustrates the decision paths for the selection of the appropriate wall type, emphasising the importance of considering constructability before any design work is carried out. This chapter presents the different types of embedded retaining wall and the key considerations in the wall selection process. It discusses construction methods for the excavation sequences from wall installation to the completion of the permanent works facilitated by the retaining wall. The chapter emphasises the importance of reviewing the temporary and permanent conditions so that the solution accounts for the differing requirements at each stage of the wall’s life. Many factors may affect the choice of construction sequence and type of retaining wall, not all of which relate to the basic design parameters. Personal preference and a history of successful projects using a specific approach often play a large part in the choice of methods. This chapter reviews the advantages and disadvantages of each method to help guide the designer and the constructor towards suitable choices. The main issues covered in this chapter are: zz

types of embedded retaining wall, including a review of available wall types and associated construction methods and tolerances

zz

relative construction cost data for various types of embedded wall

zz

wall selection criteria to be considered before detailed design

zz

construction sequences appropriate for temporary and permanent works

zz

temporary and permanent support systems, props, berms, ground anchorages

zz

selection of appropriate construction sequence.

3.1

TYPES OF EMBEDDED RETAINING WALLS

3.1.1 Review of wall types Wall types can be categorised by their material (usually steel or reinforced concrete) and by their installation method. In general, steel walls are driven, vibrated or pushed into the ground without any material being removed. Reinforced concrete walls are created by first removing the ground and, where necessary, providing some form of temporary ground support before placing the concrete and the reinforcement. This is a generalisation and some construction methods fall outside these definitions, but these techniques are relatively unusual and often costly.

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CIRIA, C760

The available wall types are described in detail in ICE (2007) and summarised in Appendix A3, demonstrating in particular the differences between contiguous and secant piled walls. For all wall types, ground conditions or environmental constraints may prevent their installation. Table 3.1 lists the advantages and limitations of various wall types.

Establish ground model based on site investigation and ground level surveys

Establish groundwater regime

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Establish maximum retained height

Establish design life of wall (eg temporary or permanent)

Review environmental considerations including site restraints that will influence construction method and wall size (eg access, headroom restrictions, noise and vibration)

Is the wall required to retain water in the temporary or permanent case? No

Yes

Establish depth at which interlock for water cut-off is required

Identify most economic piling technique to suit ground conditions and achieve required interlock (secant piles or sheet piles). If the required interlock for water cut-off or retained height cannot be achieved with secant or sheet piles then a diaphragm wall is required. A diaphragm wall may be required due to permanent work requirements, eg bending moment capacity (see Tables 3.1 and 3.2)

Identify most economic piling technique to suit ground conditions and satisfy environmental considerations. Contiguous piles, king post piles and sheet piles are typically more economic compared to secant or diaphragm wall techniques, but are limited in the maximum retained height achievable (see Table 3.2).

Is a diaphragm wall required? Yes

Identify optimum pile spacing and pile diameter or sheet pile size based on retained height/ capacity required

Identify panel width based on retained height/capacity required

No

Identify optimum pile spacing and pile diameter or sheet pile size based on retained height/ capacity required

Design wall based on proposed temporary and permanent supports Figure 3.1

Decision paths for selection of wall type

Guidance on embedded retaining wall design

27

Wall types

Hard/hard secant

Hard/firm secant

Hard/soft secant(3)

Contiguous pile

King post

Sheet piles

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

both primary and secondary piles are constructed using a ‘standard concrete mix’, which is typically C28/35

a stiff wall with high bending capacity, primary piles may be reinforced, if required

a permanent water-retaining wall(2)

the firm material for the primary piles is a reduced strength concrete mix, typically C8/10 with 10N/mm2 cube strength at 56 days

a permanent water-retaining wall(2)

the use of soft piles enables the hard piles to be formed using lower-torque rigs than for hard/hard secant piles

slower to construct compared to other types of secant wall support fluids are required if drilling in silts/sands below the water table walings for shear/bending transfer required

zz zz

the depth is limited by the verticality tolerance, which may determine the extent of the secant interlock (see Appendix A3)

zz

zz

the cutting of the hard primary piles requires high-torque rigs or oscillators

walings for shear/bending transfer required

zz zz

support fluids are required if drilling in silts/sands below water table

zz

the depth is limited by the verticality tolerance, which may determine the extent of the secant interlock (Appendix A3)

walings for shear/bending transfer required

zz

zz

zz

support fluids are required if drilling in silts/sands below the water table

zz

zz

zz

the soft pile mix is not significantly cheaper than concrete. The local concrete plant is often unable to batch the soft material, so site batching is required

not a permanent solution to retain water

zz

zz

zz

zz

support fluids will add to the construction carbon, if used

more installation emissions compared to a contiguous wall

extra emissions compared to a contiguous wall from extra concrete and steel (if present) in the primary piles

lower carbon emissions than an equivalent secant wall

normally used for temporary support only – associated carbon emissions can be reduced by considering extracting for reuse if appropriate

significant carbon advantage if extracted for reuse (see Section 3.2.6)

generally lower embodied carbon than equivalent reinforced concrete type

Carbon considerations

the depth is limited by the verticality tolerance, which may determine the extent of the secant interlock (Appendix A3)

zz

zz

zz

walings for shear/bending transfer required

zz

acts as a water-retaining temporary wall

support fluids are required if drilling in silts/sands below the water table

zz

a stiff wall with typical clear spacing of 150 mm between piles

not a permanent solution in any soil due to the gaps between piles, unless a structural facing is applied, safety issues with ground instability

not a water-retaining solution

zz

potential safety issues with ground instability (clay clods falling through gaps)

zz zz

potential ground movements in drained conditions

zz

not suitable to retain water so cannot be used for excavation below the groundwater table in coarse-grained soils

walings for shear/bending transfer required

may not be suitable where tight deflection criteria are specified

limited end bearing in cohesive soil

potential declutching in hard driving conditions if poorly supervised

maximum pile length about 30 m

the cheapest and quickest to construct type of concrete piled wall

economies due to less drilling time and less concrete due to spacing

zz

zz

wall can be fully watertight by sealing and welding interlocks

zz

can be installed around obstructions at isolated points and under low headroom

zz

can be used as both temporary and permanent wall

zz

zz

zz

suitable as a stand-alone Grade 1 water-retaining wall(2)

zz

no arisings to be removed

zz

zz

provide an economic wall with a predictable surface finish

zz

Limitations

zz

Wall Type(1) Advantages

Table 3.1

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CIRIA, C760

zz

zz

zz

zz

D-walls are generally far more robust if high groundwater flows are anticipated

walings for shear/bending transfer often not required

efficient reinforcement design with the use of asymmetric reinforcement to suit bending moment distribution

fewer joints compared with piled walls

in some circumstances (where Grade 1 water retention is acceptable) the face of the diaphragm wall can form the final finish subject to some surface cleaning and removal of protuberances

can be installed to great depths, provided the verticality tolerances can be accepted

a permanent water-retaining wall

(2)

zz

zz

zz

zz

disposal of the support fluid may be costly

the installation equipment is extensive, requiring a large site area for accommodation of the support fluid plant, reinforcement cages and the excavation plant

cannot follow intricate plan outlines

horizontal continuity is difficult to achieve between panels

Limitations zz

potentially more emissions than a secant wall

Carbon considerations

3 Hard/soft secant walls are not so common due to the appreciable cut required between primary and secondary piles and difficulties in obtaining the soft mix for the primary piles.

2 Sheet pile walls, hard/firm secant pile walls, hard/hard secant pile walls or diaphragm walls may provide an acceptable level of water retention if a low grade of substructure/basement water retention is required (not greater than Grade 1 in BS 8102:2009). For higher grades of water retention, structural facing walls and/or drained cavities should also be provided as detailed in Maloney et al (2009). Drained cavities should be designed to be kept free of water and adequately ventilated, to prevent penetration of water vapour into the substructure. For higher grades of water retention, sheet piles can be sealed and welded (see ArcelorMittal, 2016).

1 Wall types are discussed in Appendix A3. Enhanced capacities can be achieved with a combination of different wall types, eg combi or high modulus walls, as defined in EC3-5. For other wall types not covered in this guide, such as double embedded wall gravity structures, see the guidance given in BS 6349-2:2010.

Notes

Diaphragm wall

zz

zz

zz

Wall Type(1) Advantages

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Guidance on embedded retaining wall design

29

3.1.2 Wall construction methods and tolerances FPS and ICE (1999) contains some useful guidance for the designer on wall construction methods and tolerances. Table 3.2 shows typical applications and tolerances of embedded retaining walls. Table 3.2

Typical applications of embedded retaining walls Range of retained height

Wall type

Technique(1)

Sheet piles King post

Groundwater control

Best Temporary Permanent achievable

Cantilever(9)

Propped

Typical

Driven

≤5m

5–10 m

1:75

1:100

Yes

Yes

Conventional rotary bored or driven

≤4m

5–10 m

1:75

1:75

No

No

CFA

≤8m

5–16 m(4)

1:75

1:100

Conventional rotary bored

≤8m

5–25 m

1:75

1:125

No

No

CFA

≤8m

5–16 m(3)

1:75

1:100

Hard/soft secant

Conventional rotary bored

≤8m

5–25 m

1:75

1:125

Yes(4)

No(5)

Hard/firm secant

CFA

≤8m

5–16 m(3)

1:75

1:100

Yes(6,7)

Yes(8)

≤8m

5–25 m(3)

1:150

1:200

Yes(7)

Yes(8)

5–30 m

1:100

1:150

5–40 m

1:150

1:200

Yes

Yes

20–50 m+

1:200

1:400

Contiguous pile

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Verticality(2)

Cased rotary or Hard/firm and cased-CFA using hard/hard secant thick-wall casing Rope grab Diaphragm wall

Hydraulic grab Mill

≤8m

Key CFA – continuous flight auger. Rotary bored – open bore with temporary casing. Notes 1

Techniques: the indicated techniques are those that are most appropriate to the particular form of construction.

2 The verticality of an embedded wall is primarily dependent on the piling technique used. ‘Typical’ verticality is achievable without special measures under normal conditions. ‘Best achievable’ represents a reliable improvement in verticality, which requires additional control measures. In specific cases a higher degree of verticality may be possible, but this should be discussed with the piling contractor, as the final achieved verticality will be a function of a variety of factors including driver experience, depth of excavation, speed of construction, drilling tool stiffness, ground conditions, use of casing, provision of guide wall and drilling/excavation equipment. 3 CFA piling techniques require the cage to be pushed into the wet concrete, which limits the reinforced pile depth (commonly 15 m to 20 m, sometimes less). This constraint in turn limits the retained height that can be achieved with CFA piling. 4 The depth to which hard/soft secant pile walls can provide water resistance is restricted by the construction tolerances of the boring rig and the groundwater pressure to be resisted. This type of wall has been historically used to resist groundwater flow to maximum depths of about six metres, although up to eight metres head of groundwater has been retained. 5 The long-term resistance of the soft elements of hard/soft secant pile walls to groundwater flow relies on the wall remaining in a damp environment. Long-term water resistance is usually provided by additional works such as reinforced concrete lining walls, which transfer the groundwater load into the hard piles. 6 The depth to which hard/firm secant pile walls can reliably provide water resistance is restricted by the power and construction tolerances of the boring rig. Where a cut-off is required beyond 7 m to 8 m depth consideration should be given to using cased-CFA or rotary bored techniques. Further guidance on the interlock that can be achieved with different piling techniques is given in Appendix A3. 7 The depth to which groundwater control can be achieved with hard/firm or hard/hard secant piling is a function of the degree of overlap (cut) between the piles and the achievable verticality. The achievable cut is usually in the range 150 mm to 200 mm, but can be increased with the use of more powerful rigs. 8 In aggressive ground conditions the use of hard/firm secant pile walls for permanent groundwater control may be limited by the durability of the firm concrete. 9 The typical range of cantilever height is dependent on geology, for example in very stiff glacial till, cantilever walls up to 12 m height have been successfully used with minimal ground displacements.

Table 3.3 lists the range of pile diameters that are available with CFA, cased CFA and segmental casing rotary rigs forming a secant piled wall. Where a segmental cased rotary system is used to construct a secant wall, the cased section would typically provide a minimum one metre seal into a low permeability layer or one metre below formation level, whichever is the lower. It is important to note that the diameter of the secondary pile below the cased section would correspond to the drilling tool size and so would be of smaller diameter.

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CIRIA, C760

Table 3.3

Range of typical wall sizes for piled embedded retaining walls

Wall type

Pile diameter/casing size Casing size (mm)

Drilling tool size (mm) 600 750

Secant CFA (hard/firm)

N/A

900 1050 1200

700

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Secant cased CFA (hard/firm)

Secant segmental cased rotary(1) (hard/firm, hard/hard)

600

850

750

880

780

1000

900

508

450

620

520

750

650

880

750

1000

900

1180

1050

1300

1200

1500

1350

1650

1500

2000

1830

Notes 1 Some variation may exist in diameters between different piling contractors. For diameters of 300 mm or less, refer to a specialist piling contractor. Where headroom restrictions apply, the diameter of the piled wall will be limited by the rig capability. Specialist piling contractors are able to install large diameter piles under restricted headroom, but the exact pile diameters and headroom conditions should be discussed with the specialist piling contractor.

For example, Figure 3.2 shows that for a 1000 mm/900 mm secant wall, over the cased section there is increased cover to the main reinforcement. In this case a cage with an outside diameter of 750 mm would be installed into a 1000 mm pile. For economic design, the increased cover to the main reinforcement over the cased section can be used to reduce the amount of reinforcement required. The example shown in Figure 3.2 refers to the case where the clay is below formation level. However, if the clay was above formation level the designer would typically terminate the casing of the secondary pile at one metre below formation level, and could possibly leave the casing of the primary piles above the formation level if this is compatible with the facing requirements. In selecting the appropriate secant pile diameter, the designer must consider the optimum pile diameter, rig verticality and the minimum cut between primary and secondary piles to achieve the required interlock for water cut-off. Typically this cut would range between 125 mm and 225 mm depending on the type of secant wall (hard/firm or hard/hard) and the pile diameter. For practical reasons, to maintain a minimum primary pile neck thickness that will not compromise the integrity of the primary piles, cuts in excess of 225 mm should be avoided. Appendix A3 provides a graphical method of deriving the required interlock as a function of pile diameter and rig verticality.

Guidance on embedded retaining wall design

31

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In specifying construction tolerances, the designer should consider the overall substructure retaining wall thickness. This may include: zz

a guide wall, where required

zz

an allowance for clearance of driving equipment to boundary walls (for sheet pile walls)

zz

the embedded wall (diaphragm wall, contiguous/secant pile wall etc)

zz

an allowance for construction tolerances, eg position plus verticality and an allowance for protrusions (for cast in situ concrete walls)

zz

a facing wall, where required (eg for hard/soft secant pile walls)

zz

a drained cavity (eg 150 mm, but 50 mm is possible in extreme circumstances)

zz

a blockwork wall (typically 150 mm)

zz

an allowance for wall deflections, Figure 3.2 Cross section through a secant hard/firm wall constructed if applicable. with segmental cased rotary pile 1000 mm/900 mm (casing/tool size)

In addition there should be sufficient clearance from any adjacent buildings to install the wall. Typically, a bored piling rig operating at right angles to an existing boundary wall, requires a minimum clearance of one metre from the pile centreline to the face of the boundary wall (also, if a guide wall is required, a minimum clearance of 0.3 m is required from the edge of the piled wall, which means that the minimum clearance from the pile centreline to the face of the boundary wall must be greater than one metre for pile diameters greater than or equal to 1500 mm). This minimum clearance will also be greater at corners and the proposed layout should be produced in consultation with a specialist piling contractor. In addition, the nodding motion of the rig mast should be considered especially where building overhangs exist. Restricted access rigs can operate about 0.5 m from a vertical face, but consultation with a specialist contractor is recommended. For sheet pile walls, an allowance of 0.75 m to the centreline of the piles for hydraulic jacking equipment or conventional driving equipment is usually appropriate. Closer proximity is possible, but it is recommended to consult a specialist installation contractor. Figure 3.3 shows typical guide wall details for a piled secant wall. The guide wall should be cast against the ground to provide additional lateral support and the designer must ensure that the ground at the base of the guide wall will prevent any washout of fines that could undermine the stability of the wall. A guide wall should be at least 0.75 m deep with a minimum width of 0.3 m at its narrowest part. In the case of diaphragm walls, where the reinforcement is likely to consist of multiple cage sections, consideration must be given to the minimum safe working space required by the site team working on splicing the cages.

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Notes 1

Finished face of the guide wall to be vertical to within 1:200.

2 The minimum diameter of the profile should be pile casing diameter +20 mm, with a tolerance of +10 mm, -0 mm in order to achieve the ±25 mm positional tolerance. 3

Recommend that blinding concrete is tied into guide walls with mesh.

4 Reinforced concrete section to be designed to accommodate the forces applied to the guide wall, including forces due to trapping of reinforcement cage sections or additional surcharges, if present.

Figure 3.3

Typical guide wall details for a piled secant wall

It is recommended that a reinforced concrete capping beam is always used to connect individual piles at ground level to form a wall. A capping beam provides a mechanism for the distribution of bending moment, shear force and vertical load onto a number of wall piles. In addition, failure to achieve the required penetration of an individual wall pile, or installation of a defective pile, can often be rectified in piled walling if enhanced capacity of adjacent piles is employed via a robust capping beam. Once the wall piles are constructed, the contractor should allow sufficient time for the capping beam to be formed and reach sufficient strength before any excavation takes place in front of the wall.

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33

3.1.3 Relative construction cost data for various embedded wall types

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It is possible to install an embedded retaining wall to retain the ground in the temporary case only and then to construct a permanent structure inside the resulting excavation, disregarding the presence of the temporary wall completely. This approach is common in some parts of the world (for example the Middle East) as it provides absolute clarity on design responsibilities. However, the use of a single wall to perform the retaining function in the temporary and permanent conditions will almost certainly result in cost and programme savings. The following factors influence the overall cost and carbon emissions: zz

the space occupied by the retaining wall and any sequencing required

zz

interlock requirements for retaining wall

zz

watertightness criteria for the inner face of the permanent wall

zz

the use of an inner lining wall to form a drained cavity or to conceal the formed face of the embedded wall

zz

the connection details between the wall and the permanent slabs (for embedded walls, these details can be difficult and costly to form, see Section 3.3.3)

zz

logistical issues including propping, headroom and other space restrictions

zz

site location, which may affect material supply

zz

ground conditions that require special measures such as long casings or support fluid.

Figure 3.4 gives an indication of the relative cost comparison of various types of wall to assist in the choice of wall. Although typical wall depths (L) are shown, the actual cost is not a direct function of the wall depth, but it depends on all the factors listed here. It is stressed that this figure can only be indicative and the designer is encouraged to discuss the choice of wall with a specialist contractor.

Figure 3.4

34

Relative cost of different wall types with reference to typical wall depths (based on 2015 figures)

CIRIA, C760

3.2

WALL SELECTION

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The type of wall affects the design parameters. The designer should determine the type of wall before undertaking the detailed design because the choice of wall can have a fundamental effect on the design. It is difficult for the designer to determine the type of wall without considering the practical aspects governing its installation. The final choice is often a compromise between several of the following criteria (these factors are discussed below in no particular order): zz

cost (see Figure 3.4 for relative cost comparison)

zz

ground conditions – the ground will influence the method of construction, eg when excavating into inter-bedded soils comprising gravel, sand, silt and clay, support fluid will be required to support the bore during construction

zz

groundwater conditions, including pressures, especially if there is a risk of sub-artesian conditions, and variations of ground permeability with depth

zz

the risk of erosion failure when constructing below the water table in fine sands/silts

zz

interlock for water cut-off, if required

zz

speed of construction/programme

zz

excavation depth (see Table 3.2 for tolerances for different wall types), constructability requirements (see Section 3.2.3)

zz

health and safety related hazards relating to the nature of the site and its surroundings

zz

the presence of obstructions, including any remains of archaeological interest

zz

the need to restrict ground movements to within acceptable limits

zz

extent of the site to accommodate construction plant (particularly important for the use of diaphragm walling and rotary bored piling techniques, with the need to accommodate support fluid mixing and storage facilities and reinforcement cages)

zz

headroom restrictions

zz

site access

zz

construction tolerances

zz

compatibility with the permanent works durability

zz

degree and nature of ground contamination

zz

environmental issues including noise and vibration

zz

carbon emissions.

3.2.1 Ground conditions and obstructions The ground conditions may dictate the type of equipment needed to install the wall, which will then affect the cost. Obstructions and boulders can prevent the installation of sheet piles and some types of reinforced concrete walls (eg CFA) without pre-treatment of the ground to remove or break up the boulders. This is often possible near the surface, but becomes more difficult at depths beyond 3 m to 4 m. The presence of hard strata above the required toe level will necessitate special measures to ensure that the wall is installed (see Table 3.4).

Guidance on embedded retaining wall design

35

Table 3.4

Measures for dealing with obstructions

Wall type

Sheet piles

Cast in situ piles (contiguous or secant piled walls)

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Diaphragm walls

Installation measures

Potential issues

zz

special techniques etc

zz

probing or pre-augering

zz

jetting (limited ability to deal with hard strata)

zz

zz

zz

(1)

cased rotary or cased CFA rather than CFA, allowing chiselling etc(2)

zz

noise and vibration. Appropriate sheet pile section required additional settlement, change in soil parameters may affect wall movements additional settlement, heave, change in soil parameters may affect wall movements

zz

longer installation programme with some vibration and noise

vibration, noise and over-break

zz

chiselling for grabbed walls(3)

zz

zz

reverse circulation mill(4)

zz

expensive mobilisation. Some systems are inefficient at dealing with fine-grained materials

Notes 1 An appropriate sheet pile section may enable obstructions to be pushed aside or broken up. Where obstructions exist, the panel driving method of installation may be more suitable compared to the pitch and drive method, as described in ArcelorMittal (2016). 2 A cased pile system, such as rotary bored or CFA, allows a range of drilling tools to be used within the casing to remove obstructions. The teeth on the casing are also used to drill through obstructions. This is a way of drilling through existing foundations, which may contain steelwork. 3 Diaphragm wall grabs are able to remove smaller boulders from the trench, although this may cause some over-break where the boulder extends beyond the sides of the trench. 4 The reverse circulation mill requires a more extensive cleaning plant for the drilling fluid than for a grabbed diaphragm wall, to remove the spoil from suspension. On small sites, the cleaning plant may be located off site with the drilling fluid piped on and off the site.

3.2.2 Groundwater If groundwater is present above excavation level, the wall will usually be required to act as a groundwater cut-off to prevent flow directly from the retained ground into the excavation. This is an important design decision that has cost implications for the wall. The designer must confirm that the verticality tolerances needed to produce the interlock for groundwater cut-off can be achieved with the proposed rig. The alternatives to using a temporary wall as a groundwater cut-off are: zz

Dewatering to lower the groundwater level during construction, provided any ground movements that may occur can be tolerated.

zz

In impermeable ground, excavation with sump pumping only to deal with water inflow due to rainfall etc.

zz

Excavating and placing part of the permanent works under water either using tremie techniques or precasting.

zz

Provision of a separate groundwater cut-off around the outside of the wall.

In at least the first three of these, the permanent wall would still need to act as a groundwater cut-off in the long term.

3.2.3 Constructability requirements The designer must consider the constructability requirements as detailed in the relevant standards (BS EN 1536:2010+A1:2015, BS EN 1537:2013, BS EN 1538:2010+A1:2015, BS EN 12063:1999, BS EN 14199:2015). For sheet pile walls, guidance can also be obtained from ArcelorMittal (2016), BS 6349-1:2000 for maritime structures and Williams and Waite (1993) for cofferdams. Constructability requirements will often govern the final design solution and will control the detailing of reinforced concrete piled walls. Particular attention is drawn on the reinforcement requirements given in BS EN 1536:2010+A1:2015, eg the minimum clear distance between longitudinal bars shall not be less than 100 mm to allow proper flow of concrete. The clear distance can be reduced to 80 mm if concrete with less than 20 mm aggregate is used. Where grout or mortar is used, the minimum distance between bars may be less.

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CIRIA, C760

3.2.4 Durability Durability is not usually a concern for a temporary wall, unless the soil contains particularly aggressive contaminants. When it forms part of the permanent works, the wall should satisfy the durability requirements specified for the permanent works in order to provide the required design life. Permanent sheet piles are required to be structurally verified to take into account potential loss of steel thickness during the life of the wall (Section 7.5.1). The durability of concrete walls is satisfied by reference to the applicable CoP for structural concrete (Section 4 of EC2-1-1). The durability requirement of the concrete is satisfied by means of a minimum cement content, maximum water cement ratios, minimum strength and minimum cover to the reinforcement, subject to an acceptable standard of workmanship on site (Section 7.5.2).

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3.2.5 Contaminated ground Contaminated ground creates additional safety risks for any wall system that involves removal of the ground. The handling and disposal of the arisings should be planned and carried out to minimise the risks to the site operatives and to the environment. The use of wall types that avoid soil removal (eg sheet piles) should be considered in these circumstances, although there is still a risk from jetting out of soils or smearing of contaminants between layers. Extensive guidance in this area is beyond the scope of this publication. If ground contamination is deemed a risk on a construction site, advice should be sought from a geo-environmental specialist and reference made to the latest guidance documents. Certain aggressive chemicals prevent the use of cast in situ concrete without special protection measures (BS 8500-1:2002, BS 8500-2:2015+A1:2016, BS EN 206:2013). This limits the choice of wall to sheet piles, although some of the cast in situ systems could be adapted to satisfy these requirements (eg precast diaphragm wall panels).

3.2.6

Environmental issues

Due to the growing concern about the impact of construction on the environment, the designer and constructor should consider all the relevant environmental issues. The choice of wall can affect the environment during installation, throughout its life and on demolition. As a result, the designer should consider the environmental aspects of the whole-life cycle of the wall. The issues to be considered are shown in Table 3.5 (see also Pantelidou et al, 2012). Table 3.5

Environmental issues throughout the life cycle of the wall

Life cycle of wall

Installation

Working life End of life – removal

Environmental issues zz

carbon embedded and emitted

zz

noise and vibration

zz

number of vehicle movements associated with the wall construction

zz

use of sustainable materials (for guide wall construction for example)

zz

dust, gases and leachate from contaminated spoil, before disposal

zz

disposal of any contaminated spoil.

zz

effect on the local groundwater.

zz

ease of removal

zz

reuse of materials.

Carbon footprint Although historically carbon emissions were not explicitly considered in the design of a geotechnical structure, recent advances in the understanding of, and the urgency in, reducing the carbon footprint of the built environment are rapidly changing the design brief and requirements. The importance of reducing carbon emissions is well researched and documented (IPCC, 2014) and many clients now request a design that strikes an appropriate balance between reduced carbon emissions and cost.

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In the UK, the Climate Change Act 2008 establishes a legal obligation for UK carbon emissions to be reduced by 80 per cent to the 1990 levels by 2050. The construction industry has made significant advances in understanding how substantial its impact is (HM Treasury, 2013) and to route-map the steps required to achieve this ambitious reduction commitment.

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Significant background research has been undertaken into understanding the embodied energy associated with different types of embedded retaining systems (Soga et al, 2011, Inui et al, 2011). Carbon emission calculations are now commonplace, with various tools readily available to facilitate them, such as the EFFC-DFI (2013) carbon calculator specific for piles and embedded walls.

Note Embodied energy (EE) is an alternative indicator to carbon emissions (CO2e) used for quantifying the environmental impact of a structure and is defined as the total primary energy consumed during the lifetime of a product (Jones and Hammond, 2008). The approximate correlation between the two is 1GJ EE = 0.098t CO2e.

For most construction materials, the carbon emissions can vary significantly depending on raw material or recycled content, country of origin, energy mix, manufacturing and transportation methods. There are various databases of carbon emission factors, relating emissions to unit weight or unit volume of typical construction materials, but no standardised database yet exists. References most widely used in the UK include Hammond and Jones (2008, 2011), Defra (2014) and Gillenwater (2005), but note that these will be substantially different in other parts of the world. Figure 3.5 shows an example of the estimated capital carbon footprint for a typical piled wall scheme with reference to the materials and energy used. The carbon footprint is expressed in terms of tCO2e, ie tonnes of carbon dioxide equivalent, which is a measure that is commonly used to compare the emissions of other greenhouse gases relative to one unit of CO2. In this example, the majority of the carbon is related to the concrete used.

Figure 3.5

Example of estimated carbon embedded and emitted for a typical piled wall project

The assessment of the capital carbon footprint is similar for diaphragm walls, although the plant and support fluid requirements are likely to be relatively more carbon intensive (see also De Wit et al, 2002). For steel pile walls, the majority of the carbon is embedded in the steel and can vary significantly with the recycled and/or recoverable steel content, its initial manufacturing process and country of origin. Most hot rolled sheet piles manufactured in Europe are processed from recycled scrap, although there is currently no declaration of recycled content of the steel delivered. With market globalisation and political and economic variants favouring geographically different sources, the carbon implications of steel used in construction can be a complex assessment, which should be taken into account. Conversely, steel piles are probably the only type of embedded retaining system that can be extracted and reused several times, which can substantially reduce the associated carbon emissions. Reuse of structural elements is the most sustainable and carbon-efficient construction option, as the material-

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embodied carbon is zero (it is only accounted for the first time it was used) and the installation, transportation and extraction emissions are only a fraction of the material one.

Websites Greenhouse gas protocol: Calculation tools: www.ghgprotocol.org/calculation-tools/all-tools

PAS 2080:2016 provides a standardised methodology on the management of whole-life carbon across all infrastructure sectors. It sets common rules across the process not only on how the carbon emissions must be calculated, but also how to consider carbon reduction throughout the life of the project.

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Geothermal piles Consideration of whole-life carbon emissions also favours the use of retaining wall elements for energy generation during the operation of a structure. Using geothermal energy saves on carbon dioxide emissions and gives long-term energy and cost savings. Geothermal piles are innovative piles that could also be applied to basement wall piles to transport and store geothermal energy from the surrounding ground. When required, the energy from these geothermal wall piles is used to heat or cool the superstructure. Creating a geothermal wall involves the introduction of geothermal pipework, which is achieved by fitting flexible plastic loops to the pile reinforcement cages (see Figure 3.6). Once the geothermal wall is constructed the loops are linked to further plastic pipes embedded in the concrete floors, walls or ducting within the building, leading to a heat pump in the plant room. Fluid circulating within the geothermal wall runs through the heat pump. This uses the temperature differential between the ground and inlet temperature, via compression (heating) or expansion (cooling), to generate building heating or cooling. Geothermal systems can result in appreciable future fuel cost savings and can significantly reduce the amount of annual carbon emissions depending on the number and depth of geothermal wall/ pile elements. For example, the system installed on the site shown on Figure 3.6a comprising both geothermal piles and geothermal walls can deliver 150kW of heating and cooling leading to savings of 96T of carbon emissions annually. This is provided through a combination of 49 geothermal piles (up to 24 m deep) and loops within a diaphragm wall (up to 36 m deep). A larger geothermal scheme was implemented on the site shown on Figure 3.6b, where the system can deliver an order of magnitude greater heating and cooling (1600kW) by installing geothermal loops in 219 energy piles (up to 2.5 m diameter, 38 m deep) and two open loop deep wells. Further guidance on geothermal piles can be found in GSHPA (2012).

a

b

Figure 3.6 Geothermal loops in a diaphragm wall at the Bulgari Hotel in Knightsbridge (a), and geothermal loops in a large rotary bored pile at One New Change (b) (courtesy Cementation Skanska)

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3.3

CONSTRUCTION METHODS FOR SOIL SUPPORT

3.3.1

Construction sequence

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The construction sequence from existing ground level should usually start with the installation of the embedded retaining wall, once site preparation works have been completed. Any excavation before wall installation, while reducing the depth of the wall, may involve additional temporary works, potential difficulties with plant access and extra ground movement. These temporary works may affect the ground conditions and may induce further loading conditions that need to be considered in the design of the permanent wall. This is particularly critical in cases where the design of temporary works is carried out by others, so it is important that the details of temporary works are communicated to the designer of the permanent wall in a timely manner. Designers should consider the whole of the construction sequence, up to the completion of the permanent structure. An efficient retaining wall design should avoid the need to design for particularly high section forces for isolated construction stages. This may not always be possible due to other constraints, for example, a large span between temporary supports during the construction stages may dominate the section design. A review of the support levels to reduce this span may enable a smaller section to be used. A balanced design should make full use of the section capacity at each construction stage.

3.3.2 Temporary and permanent works The use of one wall that can be used for both temporary and permanent works has an economic advantage as it requires the installation of only one wall (see Figures 3.7 and 3.10). When making this decision, the designer should consider the form of the permanent internal face and particularly the watertightness requirements. Where the wall is used to form a basement, guidance on the groundwater protection for various grades of basement use is given in BS 8102:2009, EC2-3, Johnson (1995) and Maloney et al (2009). Table 3.1 identifies wall types that can act as permanent water retaining elements. Alternatively, the structural capacity of the wall can be used to support the soil loads and any vertical loads from the permanent works, while a secondary wall (eg a reinforced concrete lining wall connected to the inside face of a contiguous piled wall) provides watertightness.

Figure 3.7 Temporary and permanent works at an underground car park. An example of the use of a sheet pile wall as the permanent wall, exposed and painted (courtesy ArcelorMittal)

Some specified permanent works details may prevent the use of the temporary retaining wall as part of permanent works. For example, a specified tanking membrane around the outside of the permanent structure will necessitate that the retaining wall only serves a temporary function in most circumstances.

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Any connections between the wall and internal slabs require careful consideration to ensure the buildability of the solution (Section 3.3.3).

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For an efficient design the wall capacity required for the permanent condition should also satisfy the requirements of the temporary condition, ideally avoiding the need to provide a stronger or stiffer wall for the temporary conditions. However, it is important to take a holistic view of the whole project rather than concentrating only on the wall (Section 1.5). It may be preferable to provide an increased wall strength and stiffness specifically for the temporary conditions, to reduce the amount of temporary support. Where the temporary conditions are more onerous than the permanent, it may be possible to design more economically for a reduced durability in the short-term. This may take the form of an increased allowable crack width for the temporary case of reinforced concrete piled walls or in the case of sheet pile walls, it may be appropriate to use a higher steel grade or plastic section properties of the steel in an elastic analysis. These decisions should be taken for each project based on a full understanding of the design, cost and programme requirements of the options.

Cantilever wall Figure 3.8 illustrates the sequence for a cantilever wall.

Figure 3.8

Cantilever wall construction sequence

Table 3.6 lists the advantages and limitations of adopting a cantilever wall. While this may be the simplest option, it may not be suitable because of unacceptable deflections during the temporary excavation stages. For deeper excavations, the large depth and strength of the wall required to support the cantilevered excavation may make this an uneconomic option. Table 3.2 indicates the typical range of retained heights for various types of wall. Table 3.6

Advantages and limitations of adopting a cantilever wall

Advantages zz

zz

zz

A simple construction sequence with no temporary propping to the wall. The permanent works are constructed in an open excavation free from the restrictions of working around or under temporary props. No requirement for ground anchors or structure support outside the wall. Better use of the confines of the site.

Limitations zz zz

zz

May be uneconomic for deeper excavations. The deflections generated by the unpropped excavation may be unacceptable depending on the ground conditions and nearby structures The depth and strength of the wall to ensure stability against overturning may be considerable.

A series of carefully-defined construction stages, allowing propping of the wall to be inserted during a staged excavation sequence, is an alternative to a cantilevered solution to reduce the embedment depth and stiffness of the retaining wall, and control the wall deflections. These may be categorised as either ‘top-down’ or ‘bottom-up’ construction sequences.

Propped wall – top-down sequence Top-down is defined by the use of the permanent internal structure as the temporary propping to the retaining wall, cast in a top-down sequence. The higher-level slabs are cast before the lower-level slabs to act as horizontal frames for wall support as the excavation progresses. This process is shown in Figure 3.9.

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Figure 3.9

Top-down construction sequence

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Table 3.7 lists the advantages and limitations of adopting a top-down construction sequence. Excavation work takes place through openings in the permanent works beneath the previously cast slabs (Figure 3.10).

Figure 3.10

Top-down excavation at the Victoria and Albert Museum, London (courtesy Arup)

A top-down solution requires: zz

Support for the vertical load of the permanent slabs in the temporary condition. This may take the form of temporary piles, hangers or the permanent columns formed ahead of the excavation by means of piles, barrettes etc. For short spans between opposing walls, it may be sufficient to connect the slabs to the walls and to rely on the shear capacity of this connection to support the vertical load of the slab.

zz

Access for the removal of soil and the supply of materials, which may be through the ground floor and substructure slabs.

zz

Ventilation for the work below ground beneath permanent slabs. Consideration should also be given to safe methods of working.

zz

A method for the excavation and construction of the substructure that is compatible with the available headroom and limited access.

The main advantage of the top-down approach is that it allows the superstructure to be constructed at the same time as the substructure. Certain site planning and design issues are associated with this method, and these should be addressed in addition to those previously noted: zz

Sufficient vertical load capacity of the wall and the internal column supports (if applicable) to support the increasing superstructure load throughout the construction sequence.

zz

Access through the superstructure works for the substructure works. This can become a critical issue for small confined sites.

Common requirements that necessitate the use of a top-down sequence are: zz

the need to make an early start on superstructure construction

zz

the need to minimise ground movements.

In general, it is uneconomic to use a top-down sequence to reduce the programme time to complete the superstructure unless there are more than two levels of substructure.

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Table 3.7

Top-down construction

Advantages zz

zz

zz

zz

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zz

Limitations

The superstructure construction can proceed at the same time as the substructure, provided the necessary vertical supports, generally piles, are in place. Temporary propping is replaced by the use of the permanent slabs.

zz

zz

Provides a stiff support system for the wall, minimising movement.

zz

Piling can be take place from existing ground level.

zz

Plunge column bearing piles will require less heave reinforcement due to the beneficial effect of the short-term permanent actions (Gk).

The excavation works and substructure construction are slower and more expensive due to the restrictions on the size of plant and the limited access. Holes may have to be left in the slabs to provide access for the subsequent excavation. Vertical support for the permanent slabs is required in the temporary condition. The stiffer construction during the intermediate construction stages attracts higher loads into the permanent structure.

The vertical support to the permanent slabs during the temporary stages can be provided by temporary piles that are either later removed or used as part of the permanent works after suitable surface preparation, to remove any over-break for cast in situ concrete piles for example. The most economic foundation option is a single pile supporting each column. The plunge column technique is frequently used. The placing tolerances of top-down piles are shown in Table 3.8, but depend on the degree of construction control. The table does not include any allowance for the rolling tolerances of the plunged steel beams, which can become critical when working to reduced tolerances. Table 3.8

Tolerances for top-down construction (plunge columns in rotary bored piles)

Quality control

Pile installation tolerances

Column installation tolerances

Position

Verticality

Position

Verticality

Conventional(1)

±75 mm

1:75

±75 mm

1:75

Controlled

±25 mm

1:150

±25 mm

1:200

±25 mm

1:150

±10 mm

1:400

(2)

Optimum(3) Notes 1

Conventional: rotary piling using thin-wall casings, column position controlled with rudimentary guide at pile head.

2 Controlled: pile position controlled with bespoke guiding frames, bore formed with segmental thick-wall casings, column placed with adjustable placing tube or similar, and verticality measured at top of bore. 3 Optimum control: guide walls: thick-wall casing, high degree of column control using hydraulically adjustable frame and optical/laser control of verticality. zz

T he pile diameter must be sufficient to accommodate these tolerances, the reinforcement cage and the column, without risk of a clash at the full column depth. For deep columns this may dictate a larger diameter than otherwise required for simple bearing capacity requirements.

zz

These tolerances do not include the steel column fabrication tolerances which should also be considered in design.

zz

The pile and column tolerances are independent of each other, so the pile design should address the potential cumulative effects of these tolerances.

Propped wall – bottom-up sequence A bottom-up construction sequence is defined by the construction of the permanent works from the lowest level upwards, casting the foundation slab before the internal walls and slabs above. Figure 3.11 illustrates the sequence for a typical excavation with two levels of temporary props. A semi top-down construction sequence can also be used in bottom-up construction sequences to omit the requirements of temporary props. This is achieved by casting a basement slab as a doughnut slab, to prop the wall in the temporary condition, and omit the requirements of temporary props.

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Figure 3.11

Bottom-up construction sequence

Temporary props are likely to be heavy steel sections and the safety risks associated with their use should be considered (see Figures 3.12 and 3.13). The safety issues are the risks to the workforce during the installation and removal of these elements, together with the risk of accidental damage or unforeseen loading on the props during the excavation and construction operations (see Figure 3.13). The sequential construction of the superstructure and the substructure minimises conflicts between different operations on site and normally results in a less congested critical path compared with a top-down sequence.

Figure 3.12 Bottom-up construction sequence (courtesy Figure 3.13 Bottom-up construction sequence (courtesy Cementation Skanska) Bachy Soletanche) Table 3.9

Bottom-up construction

Advantages zz zz

zz

Deflections are controlled by the use of propping to the wall.

Limitations zz

Compared to a cantilever solution the wall strength, stiffness and depth may be reduced. Sequential construction of the substructure and the superstructure.

zz

Compared to a cantilever wall, there are cost and programme penalties with the use of temporary props. The propping impedes the final excavation and the construction of the permanent works.

An alternative means of achieving a bottom-up construction sequence is by means of a circular cofferdam that relies on hoop compression to resist the external soil and water loads without the need for internal propping. The compression member can either be the retaining wall (eg a diaphragm or secant piled wall) or ring beams at suitable levels as the excavation progresses. Sheet pile circular cofferdams may be considered for diameters 4 m to 50 m. Standard sheet piles are normally used based on a rotation of the interlock of up to five degrees although alternate piles may be bent to suit a tighter radius.

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Examples of circular shafts constructed with diaphragm wall techniques and secant piling methods are shown in Figures 3.14 and 3.15 respectively. A circular shaft minimises the wall length for a fixed floor area and can provide an efficient solution where there is sufficient space to accommodate the circular plan area. Circular cofferdams are particularly efficient in providing storage for tanks.

Figure 3.14 Circular shaft constructed with diaphragm wall Figure 3.15 Circular shaft constructed with secant piles panels, Hackney, East London (courtesy Cementation Skanska) at Eastney, Portsmouth (courtesy Bachy Soletanche)

3.3.3

Construction requirements of temporary and permanent support system to retaining wall

The designer should consider the whole construction sequence when designing an embedded retaining wall to ensure that the design satisfies the requirements of each stage and to minimise the overall construction costs. For example, it is not sufficient to consider the excavation down to formation level without reviewing how the permanent works will be constructed for a bottom-up sequence. A cantilever or anchored wall results in no Figure 3.16 Various support systems to sheet pile walls at Thelwall Viaduct, propping interfering with the internal works, and is likely to be Merseyside (courtesy Bachy Soletanche) the constructors’ preferred solution. The wall may be either a temporary structure or a permanent wall. There should be sufficient system stiffness if ground movements are to be limited (Chapter 6). Figure 3.16 shows various support systems to sheet pile walls at Thelwall Viaduct, Merseyside.

Props Design guidance for temporary propping is given in Twine and Roscoe (1999) and Williams and Waite (1993). This is discussed further in Chapter 8. Where propping to the retaining wall is required, it may need temporary works and should be removed in a defined manner as the permanent works are constructed and are able to replace the role of the temporary props. Temporary props are usually made of steel (tubular props are often used for their efficiency in compression) although concrete props are sometimes used, particularly as corner braces across the ends of excavations. For narrow excavations, props can span the full width of the excavation (see Figure 3.17).

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Figure 3.17

Temporary props spanning full width of excavation (courtesy Groundforce and Bachy Soletanche)

For wide excavations, vertical support may be required to the propping system. The prop removal sequence should be carefully considered to avoid the following problems, where possible: zz

removal of the temporary props from beneath already constructed slabs

zz

the location of temporary props through internal walls or slabs, which may need to be cast before removal of the props.

Consideration should also be given in bottom-up construction sequences due to the fact that critical prop forces are often developed during removal of props. Twine and Roscoe (1999) provides some guidance on prop removal.

Berms The use of a berm adjacent to the wall allows the excavation to proceed to a deeper level in the centre of the site, with the advantage that the major part of the excavation can be carried out unimpeded by props. The option of using raking props down to the formation level is shown in Figure 3.18. The berm is removed once the raking props are in place allowing the permanent structure to be completed, then removing the props at a suitable stage. The design of the end prop support at formation level for a raking prop should be carefully considered to avoid unacceptable movement as the prop is loaded. As noted in Section 2.5.1, the premature removal or over-excavation of a berm is one of the causes of failure identified by Malone (1982).

Figure 3.18

The use of a berm and raking props

Figure 3.19 shows an alternative use of a berm in conjunction with construction of part of the permanent works.

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Figure 3.19

The use of a berm and a prop to the permanent structure

The berm is removed once the permanent works are sufficiently advanced to provide support to the wall. A berm is only a realistic option where a lower level of permanent propping is required and usually for a wide excavation.

Ground anchorages If a cantilever wall is not a suitable solution, the use of ground anchorages to provide the horizontal support to the retaining wall can be considered. For marine structures other types of anchors can be considered, eg use of dead-man anchors. The main advantage of the use of ground anchorages is that the excavation remains unobstructed by propping (Figure 3.20). Table 3.10 lists the advantages and limitations. Table 3.10

zz

Anchored secant bored pile wall (courtesy Cementation

Advantages and limitations of ground anchorages

Advantages zz

Figure 3.20 Skanska)

Once installed, the excavation is free of any obstructions allowing for efficient construction of the permanent works. The ground anchorage prestress may reduce wall deflection and settlement behind the wall, depending on the magnitude of the prestress.

Limitations zz zz

zz

zz

zz

The time to install and stress the ground anchorages increases the excavation time. The ground anchorages often extend outside the site boundaries and the necessary permissions are required. The ground anchorages require de-stressing and occasionally removing at the end of construction. Increased risk with ground anchors compared to struts, eg risk of delay and cost if ground conditions vary. Installation of ground anchors (drilling and grouting) can induce wall movements.

Ground anchorages are greatly underused in the UK compared with experience elsewhere in Europe. Their increased use may result in significant savings over propping schemes where programme time is available for the construction of ground anchorages and the space is available to locate them. However, ground anchor installation and stressing times will usually be greater than the time required to install temporary props. Any potential savings should also consider the impact on the construction programme.

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The following design issues should be considered: zz

The ground anchorages will be pre-stressed to a percentage of their working load upon installation. The wall designer should specify anchor pre-stress/lock off load and maximum working load required and allow for the effects of this preload.

zz

The ground anchorages will usually be installed at an angle to the horizontal, imposing a vertical component of load to be resisted by the retaining wall. Depending on the fixing detail, a moment may also be induced in the wall.

zz

A condition of the permission to install the ground anchorages beneath a nearby owner’s property may be that the ground anchorages are removed at the end of construction and the anchor hole grouted up (there are proprietary systems, which allow the removal of the steel tension member).

zz

Space is necessary outside the wall, free from services and obstructions such as existing piles/ foundations and basements, to install the ground anchorages. Appropriate site investigation of this space is also required.

Permanent slabs as props Use of the permanent slabs of a basement is a common way of providing the permanent support to a wall where the wall forms part of the permanent works. In addition to the prop loads (see Chapter 8), the connection may also support vertical loads from the slabs. These may include the slab permanent (Gk) and variable (Qk) actions and, in the case of the base slab, any heave and groundwater uplift loads. The connections between the wall and the slabs may be costly and time-consuming to form, negating the advantages of using the temporary wall as part of the permanent works. Some examples of slab to wall connections are shown in Figures 3.21 to 3.27. For flexibility, post-drilled slab connections may also be used. The limitations of various connection details are given in Table 3.11. Table 3.11

Wall/slab connection types

Connection type

Applicable wall types

Welded bars

zz

Welded universal beams (UB)

zz

zz

sheet piles (Figures 3.21 and 3.22). sheet piles and cast in situ concrete walls where UB is used as reinforcement. all concrete wall types.

Drilled in bars

Couplers cast in the wall(1)

zz

diaphragm walls (Figure 3.23).

zz

secant walls (Figure 3.25).

Limitations zz

site welding required.

zz

site welding required.

zz

zz

need to locate and avoid the wall reinforcement

zz

drilled in bars are associated with health and safety issues.

zz

zz zz

diaphragm walls (see Figure 3.24)

Bent-out bars(1)

zz

zz zz

Hinged joint(3)

zz

all concrete wall types (Figures 3.26 and 3.27).

often of limited capacity due to the inability to drill sufficient length to anchor large-diameter bars

zz

tolerance in vertical position of the diaphragm wall cage (± 100 mm recommended)(4) increased congestion(2). tolerance in vertical position of the diaphragm wall cage (± 100 mm recommended) increased congestion(2) limitation on the size of the bars, which can physically be bent out (16 mm mild steel bars or occasionally 20 mm). none.

Notes 1 Couplers and bend-out bars are not usually recommended for cast in situ piles due to the difficulties of ensuring their angular and vertical position. 2 The increased congestion may prevent the concrete flowing around the bars, which could lead to honeycombing in an area of high concrete stress. The bars attached to the couplers and bend-out bars should not impede any concrete or tremie pipe (typically 300 mm in diameter) used to place the concrete. The designer must provide as a minimum 100 mm clear bar spacing to allow for the proper flow of concrete between bars. 3

Hinged joints have been used on several road schemes in the UK to accommodate the heave of the underlying clay.

4 An additional ±50 mm should be included for each section of cage that is spliced, so two sections would give ±100 mm. In addition, the designer should allow for a minimum of 10 degrees rotational tolerance.

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Figure 3.21

Typical connection detail at sheet pile wall/concrete slab

Sheet pile wall/concrete slab connection at Bristol underground car park Figure 3.22 (courtesy ArcelorMittal)

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Figure 3.23

Typical detail for couplers cast within a diaphragm wall panel

Figure 3.24

Typical details of bent-out bars in diaphragm wall panel

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a Connection details using couplers

b Shear connection details Figure 3.25 Possible connection details between secant piled wall and slab level (a) connection using couplers and shear connection (b)

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Figure 3.26

Hinged slab, A406 North Circular Road, London

Figure 3.27

Hinged joint, A406 North Circular Road, London

One solution to the limitations in Table 3.11 is to avoid the need for a shear or moment connection between the slabs and the wall, designing the joint to resist only compression forces from the wall. There are various options for supporting the vertical loads on the slabs as an alternative to the shear connection to the wall: zz

Hangers may be used to carry the vertical load to support points above the slabs, often to the wall capping beam. The hangers may operate in the temporary and/or in the permanent condition (eg at the Copenhagen Metro deep stations as described by Beadman et al, 2001).

zz

Internal columns, designing the slab to cantilever out from the column position to the wall.

zz

A permanent wall, cast against the temporary wall to provide the long-term support for the vertical loads. This may prove to be the cheaper option where it allows an inexpensive temporary wall to be specified and avoids costly wall/slab connections. However, in this circumstance, the designer should give careful consideration to strain compatibility between the temporary and permanent wall (Wharmby et al, 2001).

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A further variation, where high uplift loads are applied to the structure from the base slab, is to form a corbel above the base slab to support uplift loads. The corbel prevents any high fixed end moments from the base slab being transferred into the wall, which may be advantageous where the base slab edge support moments risk over-stressing the wall.

3.3.4

Selection of appropriate construction sequence

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The selection of the appropriate construction sequence involves many aspects of design and construction. The designer is aware of the design constraints and the contractor may have preferred ways to construct the works, based on equipment and propping availability or personal experience. It is difficult for a designer in isolation to appreciate the full picture and define the construction sequence. In selecting an appropriate construction sequence, the following issues should be considered: zz

excavation depth and propping requirements

zz

deflection limits for the retaining wall

zz

use of the retaining wall as part of the permanent works

zz

sufficient space within the site boundaries to install the temporary and permanent wall

zz

permanent works details

zz

depth at which interlock is required between secondary and primary piles to achieve seal into low permeability layer.

3.4

KEY POINTS AND RECOMMENDATIONS

1

Major economies are possible at the scheme design stage by reviewing the construction method and type of wall to be used. The designer should consider the whole of the construction sequence, up to the completion of the permanent structure.

2

The designer must develop a suitable ground model onto which the proposed walls and substructure can be drawn so that the ground engineering risks can be identified and appropriately dealt with.

3

Before detailed design, the designer must consider the constructability requirements and rig limitations such that the wall can be safely constructed by eliminating the associated health and safety risks.

4

Subject to the permanent works details, there are economies to be gained where one wall can be constructed to meet the requirements of the temporary and permanent conditions.

5

The constructor will always prefer a clear excavation with no propping to constrain the permanent works. This can be realised by the use of a cantilever wall or an anchored wall. The limitations of the use of a cantilever wall may include a substantial wall with unacceptable deflections during the excavation.

6

A propped wall with a top-down construction sequence provides a stiff support system with no temporary propping and is typically adopted where there is a need to: a

make an early start on superstructure construction

b

minimise ground movements.

7

In general, it is uneconomic to use a top-down sequence to reduce the programme time to complete the superstructure unless there are more than two levels of substructure.

8

A bottom-up construction sequence is commonly adopted when the wall requires propping. Ground anchorages may be used as an alternative to temporary props to provide an obstruction-free construction zone, but the programme time should be available for the construction of the ground anchorages and the space available to locate them. Where this is possible, use of ground anchorages may result in significant savings over schemes requiring propping. However any potential cost savings should be considered against potentially increased construction programme to install and stress the ground anchors.

9

Different wall types are discussed to identify their suitability in particular circumstances and relative costs are provided to allow a crude comparison to be made between the options.

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4 Analysis

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This chapter provides background information on lateral earth pressures and guidance on the methods of analysis that may be used in the design of an embedded retaining wall. It is assumed that the reader is familiar with the basic principles of soil and weak rock mechanics needed for embedded retaining wall design as summarised in Appendix A4. These are: zz

the concepts of total, effective and shear stress

zz

the representation of the stress state within the cross-sectional plane of a long retaining wall using the Mohr circle construction

zz

the distinction between undrained (short-term) total stress analysis and drained (long-term) effective stress analysis for soils

zz

key aspects of soil behaviour relevant to embedded retaining walls, including the effect of stress history, soil strength and soil stiffness

zz

the formulation of limiting (active and passive) lateral stresses (earth pressures) based on appropriate failure criteria for the materials involved

zz

basic limit equilibrium calculations for embedded retaining walls.

This chapter: zz

discusses the evolution of earth pressures in soils

zz

provides guidance on the evaluation of limiting lateral earth pressures

zz

outlines the two main classes of analysis that may be used as the basis for design (limit equilibrium, based on conditions at collapse, and SSI analyses, which may be used either at collapse or to give an estimate of working conditions)

zz

discusses the application of these methods to ULS and SLS design calculations, providing guidance where appropriate for different structural forms.

It does not address the determination of water pressures, which is covered in Section 5.6. Also, the focus is on the calculation of lateral earth pressures that are then used to assess the equilibrium of the wall, as this is the approach most commonly adopted with embedded retaining walls. In cases where the stability of a block (comprising the wall together with an adjacent zone of ground acting as one) may be more critical than the stability of the wall alone, the equilibrium of this larger block should be investigated further.

4.1

EARTH PRESSURES

Horizontal (lateral) stresses in soils are usually described and quantified by means of a lateral earth pressure coefficient, K:

(4.1)

where p′v is the effective overburden pressure σ′h is the horizontal (lateral) effective stress at the same point within the soil mass. The effective overburden pressure is p′v is given by

(4.2)

where

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γ is the bulk density z is the depth below ground surface u is the pore water pressure q is any uniform surcharge at the ground surface. Earth pressure coefficients are expressed as the ratio of horizontal effective stress to effective overburden pressure, p′v , (rather than horizontal to vertical effective stress, σ′v) because soil/wall friction makes the local vertical effective stress in the soil or weak rock adjacent to a retaining wall difficult to calculate.

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4.1.1 In situ lateral stress Unlike some other retaining walls, embedded walls usually retain predominantly natural ground. So the pre-existing or in situ horizontal (lateral) earth pressure, as modified by wall installation, is potentially important. The symbol K0 is used to denote the earth pressure coefficient describing the initial in situ stress state in the ground before the wall is installed. The in situ earth pressure coefficient may be of particular concern in a clay deposit, in which K0, like the specific volume, depends on the geological stress history. Deposition (or burial under a glacier) corresponds approximately to one-dimensional (1D) compression, during which the horizontal effective stress σ′h increases in proportion to the effective overburden pressure p′v (Figure 4.1). Clays may also become consolidated by desiccation (drying) on exposure to air, by vegetation or by freezing, where the effective stress is increased through a reduction in pore water pressure while the total stress remains constant.

Figure 4.1

Schematic stress history of an overconsolidated clay deposit

On unloading (eg due to the erosion of overlying soil, re-saturation after desiccation, the melting of an overlying glacier or a rise in groundwater level), the horizontal effective stress, σ′h , tends to remain ‘locked-in’, decreasing proportionately less quickly than the effective overburden pressure, p′v . So the in situ earth pressure coefficient K0 in an overconsolidated clay stratum is usually greater than unity. In heavily overconsolidated clays, a zone of soil extending to a depth of several metres from the surface may be close to its limiting passive pressure because of geological unloading.

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The earth pressure coefficient in granular material or weak rock is not uniquely related to the stress history of the deposit as it is in clay. In situ lateral earth pressures may be estimated using an equation of the form:

(4.3)

where ϕ′ is the drained angle of shearing resistance OCR is the over consolidation ratio

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β is the slope angle (to the horizontal) of the soil surface. For normally consolidated deposits (OCR = 1) and level ground (β = 0), this reduces to Jaky’s empirical formula K0 = (1 – sin ϕ′). EC7 indicates λ = 0.5 while Mayne and Kulhawy (1982) suggest λ = sin ϕ′. Further details are given in Section 5.5.3.

4.1.2 Effect of wall installation The process of installing a diaphragm or bored pile retaining wall (see Box 6.1) in an overconsolidated deposit is potentially important in three respects: 1

Wall installation by boring or excavating panels may reduce the horizontal effective stresses close to the wall to below their in situ values. Wall installation by ground displacement methods (eg driving) may increase the horizontal effective stresses close to the wall.

2

During wall installation, the surrounding soil may be subjected to various stress paths involving lateral unloading and reloading (Box 6.1). These define the recent stress history of the soil, which may influence the soil stiffness during bulk excavation in front of the wall (Powrie et al, 1998).

3

Ground movements during wall installation may require consideration in their own right (Section 6.1.1). Experience (Thompson, 1991) indicates that there are unlikely to be significant ground movements arising from the installation of a cast in situ wall in stiff ground where the water table is low and the workmanship is good. However, where the ground is soft and/or the water table is high, workmanship is poor, or local construction difficulties (eg obstructions in the ground) are encountered, ground movements arising from wall installation can be large (see Section 6.2).

Numerical analyses of the post-excavation behaviour of the wall that do not take into account the stress relief due to wall installation may overestimate wall bending moments and prop loads (eg Potts and Fourie, 1984, Powrie and Batten, 2000a, Batten and Powrie, 2000). The magnitude and extent of lateral stress reduction during wall installation will depend on: zz

the initial in situ earth pressure coefficient

zz

soil properties and groundwater conditions

zz

individual pile or panel geometry

zz

the detailed method (eg whether pile bores are cased, supported using support fluid or open) and sequence of construction.

Installation of an in situ wall can be modelled explicitly in finite element analyses, or its effects may be taken into account empirically (Section 5.5.4). Either approach will require a degree of approximation and leave an element of uncertainty. The field data and finite element analyses reported in the literature and summarised in Section A4.6 indicate that the installation of a diaphragm wall in panels might be expected to reduce the in situ lateral earth pressure coefficient in an overconsolidated clay deposit by about 20 per cent, and the installation of a bored pile wall by about 10 per cent. Analyses of diaphragm wall panel installation in normally or lightly overconsolidated deposits indicated either no significant net change in lateral effective stress during wall installation, or an increase to an average of approximately the lateral pressure of the support fluid.

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The selection of appropriate values of K in design calculations is discussed in Section 5.5.3. An assumed pre-excavation lateral stress distribution should not normally be less than that exerted by the wet concrete during construction. For a wall cast under a support fluid, this is reasonably well represented by the hydrostatic pressure of wet concrete from the top of the wall to a critical depth hcrit ~ H/3, where H is the overall depth of the wall. For depths greater than hcrit, the rate of increase of lateral stress with depth is equal to the unit weight of the support fluid, ie:

for

(4.4)



for

(4.5)

where

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σ is the horizontal total stress h γ is the unit weight of the concrete c γ is the unit weight of the support fluid h z is the depth below the top of the wall (Lings et al, 1994). This stress distribution may be considered to represent the effect of the concrete starting to consolidate or set before the pour is complete. Vibration-induced settlements associated with driven walls in coarse-grained deposits can also be significant.

4.1.3 Limiting values When soil is removed from in front of an embedded retaining wall, the wall will tend to move into the excavation. This will result in a reduction in the lateral stress in the ground behind the wall, eventually bringing it to the active condition in which the ground is at failure with the horizontal effective stress as small as it can be for the effective overburden pressure. In the ground that remains in front of the wall below formation level, the horizontal effective stress at failure is as large as it can be for the effective overburden pressure. Approximations to these limiting pressures may be calculated by considering either the stresses in a zone of ground at failure, or the equilibrium of an assumed sliding wedge. The first approach, following Rankine (1857), is based on a stress state that can be in equilibrium without exceeding the limiting strength of the ground. In a uniform deposit, the limiting ratio of horizontal to vertical effective stresses is constant, ie if the vertical effective stress increases linearly with depth, so will the limiting horizontal effective stress. The limits calculated in this way ensure stability, but may be unnecessarily severe, and are inherently safe (assuming that the correct boundary conditions, ground strength parameters and pore pressures have been identified). In the second approach, following Coulomb (1776), the force that must be exerted by a retaining wall to prevent a wedge of soil from sliding down an assumed slip surface is determined, and is then usually assumed to arise from a stress that increases linearly with depth. The limits obtained will prevent sliding along the slip surface assumed in the calculation, but may not be sufficient to prevent failure from occurring in some other, unidentified mechanism, so they could be unsafe. Neither approach considers wall deformation explicitly. Traditionally, the limiting calculation would be distanced from collapse by the application of a suitable factor to the soil strength. Idealised geostructural deformation mechanisms (eg Bolton and Powrie, 1988) offer a way of linking wall movement to the factored (mobilised) soil strength, as discussed in Section 6.2.4. For a frictionless wall, the Rankine and Coulomb analyses give the same results (Figures 4.2 and 4.3).

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Figure 4.2

Rankine plastic equilibrium for a frictionless wall or soil interface translating horizontally

Coulomb’s method to calculate the limiting active force for a frictionless wall/soil interface Figure 4.3 translating horizontally

Mechanism-based solutions are arguably more versatile than stress field calculations in dealing with features such as line loads and variability in strength parameters, ground levels and pore water pressures in the ground around the wall. Caquot and Kerisel (1948), Sokolovski (1965) and Kerisel and Absi (1990) developed methods to account for realities such as wall friction and sloping ground surfaces. This was achieved by assuming a failure mechanism and then determining the limiting force acting on the wall between the ground surface and a point at any given depth. Like Coulomb’s original analysis, such methods do not give the equivalent pressure distribution explicitly (it is usually assumed to increase linearly with depth), and produce limits that are not, strictly, inherently ‘safe’. However, experience overwhelmingly demonstrates that these more recent theories give accurate values of the limiting lateral stresses for use as a basis for design, assuming that variations in pore water pressures and strength properties etc in the ground around the wall have been identified and taken into account. In summary, the conventional approach to the design of an embedded retaining wall is something of a hybrid, with earth pressure coefficients derived using a mechanism-based approach to take account of

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factors such as wall interface friction, the profile of the retained ground surface and the presence of line loads etc used in an equilibrium analysis of the wall.

4.1.4 Wall friction and adhesion In reality, the interface between the ground and the wall is not frictionless and so the resultant force between the wall and the ground is inclined rather than normal to the wall.

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Figure 4.4 shows schematically the expected relative movement between a propped embedded retaining wall and the surrounding ground. On the retained (active) side, the soil slumps downward relative to the wall. On the excavated restraining (passive) side, the soil heaves upward relative to the wall. These directions of relative soil/wall movement will tend to reduce Ka on the retained side and increase Kp in the restraining soil in front of the wall. Both of these changes are beneficial to wall stability.

a Wall friction: downward movement of the soil relative to the wall

b Wall friction: upward movement of the soil relative to the wall

c

Common situation – wall friction beneficial on both sides of the wall

Figure 4.4

Effect of wall friction

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In some cases, the directions of relative soil/wall movement behind or in front of the wall may not be as conventionally assumed. These include: zz

Load-bearing walls, which may move downward relative to the soil on both sides (the downward component of the load in an inclined anchor might have a similar effect).

zz

Where an activity such as dewatering a compressible horizon or excavating an underlying tunnel causes the soil in front of the wall to settle relative to the wall.

zz

Unusual loading conditions, for example a wall being used as a tension member subjected to an uplift force.

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Care should also be exercised in selecting passive earth pressure coefficients in zones where the wall is moving into the retained soil, eg above a prop just below the top of the wall (Section 4.1.7), or below the pivot point in the case of an unpropped cantilever wall, where the direction of soil/wall friction is uncertain (Bica and Clayton, 1998).

Wall friction The ground adjacent to a wall surface will generally have been disturbed by installation of the wall, and will probably have no tendency to dilate. So, the maximum friction angle, δmax , which can be mobilised against the surface of a wall, would be expected to be equal to the constant volume (critical state) angle of shearing resistance of the soil, ϕ′cv (Rowe, 1963, Bolton and Powrie, 1987, Powrie, 1996). If the wall roughness is less than the typical particle size of the soil (D50), lower values of δ will apply (Jardine et al, 1993). If the wall is very rough relative to the average particle size of the soil, forcing a rupture surface to develop within the undisturbed body of the soil rather than along the interface, some or all of the dilatant strength of the soil might be mobilised, giving an upper limit ϕ′peak (Subba Rao et al, 1998). Rowe and Peaker (1965) show that the wall friction actually developed depends on the direction and magnitude of the movement at the soil/wall interface and that quite large movements might be required to develop full friction on the passive side. However, Subba Rao et al (1998) report results using shear box tests showing that small (less than 5 mm) relative movements can be sufficient to develop full friction at the soil/wall interface. Walls formed from driven piles require careful consideration, particularly in overconsolidated clays where the large displacements at the soil/wall interface may have reduced the soil/wall friction angle to the residual strength of the soil. CoP have traditionally advocated the use of values of soil/wall friction angles δ that are somewhat less than the soil angle of shearing resistance, ϕ′ . This is partly because if the peak angle of shearing resistance ϕ′peak were used as a design parameter, a soil/wall friction angle of δ = ϕ′peak would be unrealistically high in many circumstances. So, δ < ϕ′ takes account of the fact that the wall may not be perfectly rough, and that it might need more wall displacement to mobilise a given interface friction than the same degree of soil strength. Also, consideration of the vertical equilibrium of the wall may indicate that, in some circumstances, the wall friction angle δ may not attain the same value uniformly on both sides of the wall. Recommendations regarding values of δ/ϕ′ applicable to different wall materials and types are given in Chapter 7.

Wall adhesion For total stress analysis in stiff clays, the undrained soil/wall adhesion, cw , is often assumed to be smaller by a factor of about 2 than the undrained shear strength of the soil cu (ie cw = α × cu, where α ≈ 0.5). This is at least in part to account for softening of the soil at the soil/wall interface during wall installation. Smaller values of wall adhesion may apply in particular circumstances (see Chapter 7).

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Values of wall friction and adhesion for use in design calculations Recommended limiting values of wall friction and adhesion for use in design calculations are given in Chapter 7. The calculation of lateral earth pressure coefficients taking into account the effects of shear stresses at the soil/wall interface is addressed in Section 4.1.5.

4.1.5 Determination of limiting lateral earth pressures

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Earth pressures can be determined assuming either drained (effective stress) or undrained (total stress) conditions. The factors that must be noted when determining whether drained or undrained conditions apply are discussed in Appendix A4 and Chapter 5.

Effective stress analysis The effective horizontal active and passive earth pressure equations in generalised form are given by:

(4.6)



(4.7)

where σ′ is the effective active pressure acting at a depth in the soil a

σ′ is the effective passive pressure acting at a depth in the soil p

∫

p′ is the effective overburden pressure (p′ = γdz + q – u) v v c′ is the soil cohesion (if any) c′w is the soil/wall adhesion (if any). Ka and Kp are earth pressure coefficients, whose values depend on ϕ′ , δ , and β, where ϕ′ is the effective angle of shearing resistance of the soil, δ is the soil/wall friction and β is the slope of the soil surface. For an unbonded (uncemented) granular material, c′ = 0. For a fractured rock mass whose strength is characterised according to Byerlee’s Law (Sanderson, 2012, see also Appendix A4), c′ = 0. For intact weak rocks, a degree of real cohesion may be present as discussed in Appendix A4 and Chapter 5. In any effective stress analysis, it is usual to adopt c′w = 0. A Mohr-Coulomb failure envelope of the form τ = σ′ tanϕ′ + c′ was in the early days of soil mechanics taken as a convenient representation of a peak strength failure envelope. However, it over-predicts the peak shear stress attainable at low normal effective stresses and under-predicts it at higher normal effective stresses. An effective stress peak strength failure envelope for soils is better described by: zz

a variable (secant) value of ϕ′peak , (τ/σ′)peak = tanϕ′peak , where ϕ′peak is a function of the normal effective stress σ′

zz

a non-linear envelope of the form τpeak = A.σ′ b , where A and b are soil-specific parameters

zz

a tri-linear failure envelope, representing tensile failure at very low stresses, rupture at intermediate stresses (the Hvorslev surface), and the critical state line at higher stresses (see Schofield, 1980, Muir Wood, 1991, Atkinson, 2007, or Powrie, 2014).

These representations of the peak strength are analytically complicated as the resulting earth pressure coefficient depends on the normal effective stress and hence varies with depth. However, in a design according to EC7, the strength used in a limit equilibrium analysis should be that governing the occurrence of the limit state in question. For soils, the collapse (ULS) of an embedded retaining wall that is either unpropped or singly-propped at the crest would be governed by the constant volume (critical state) strength ϕ′cv , at which c′ = 0 and τ/σ′ = tanϕ′cv . The appropriateness of this interpretation of this requirement of EC7 is discussed further in Chapter 7.

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For weak rocks, failure conditions at effective stresses relevant to embedded retaining walls are better described by a tensile cut-off and a line representing shear failure as outlined in Appendix A4 and by Sanderson (2012). Although the Mohr-Coulomb failure envelope with a non-zero value of c′ is more acceptable in this case because there is likely to be a component of real cohesion, care must still be taken to ensure that the strength envelope used in analysis does not overestimate the true strength at low mean effective stresses. An empirical failure relationship for weak rocks that rounds off the sharp transition from tensile to shear failure in a bi-linear model was proposed by Hoek and Brown (1980):

(4.8)

where

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q is the deviator stress (= σ′1 – σ′3) σci is the unconfined compressive strength s, a and mb are parameters determined as outlined in Chapter 5. The total horizontal active and passive earth pressures acting on the wall (which govern its structural behaviour) are given by:

(4.9)



(4.10)

where u is the pore water pressure. Published values of active and passive earth pressure coefficients Ka and Kp usually relate the horizontal component of earth pressure to the notional overburden. In some cases, values relating to the resultant stress (which acts at an angle δ normal to the wall) are also given and care should be taken to ensure that the correct (horizontal component) values are used. Wall shear stresses may be obtained by multiplying the horizontal effective stress component σ′p or σ′a by the tangent of the angle of wall friction, tanδ. Charts indicating the horizontal components of earth pressure coefficients are given in Appendix A4 for the following cases: zz

Kah vs. ϕ′ for a vertical wall and backfill slopes β/ϕ′ = -1, -0.75, -0.5, -0.25, 0, +0.25, +0.5, +0.75 and +1 for δ/ϕ′ = -1, -0.75, -0.66, -0.5, 0, +0.5, +0.66 and +1.

zz

Kph vs. ϕ′ for a vertical wall and backfill slopes β/ϕ′ = -1, -0.75, -0.5, -0.25, 0, +0.25, +0.5, +0.75 and +1 for δ/ϕ′ = -1, -0.75, -0.66, -0.5, 0, +0.5, +0.66 and +1.

These have been calculated based on the equations given in EC7-1 (2013). Such equations facilitate programming and calculations using spreadsheets. Simpson and Driscoll (1998) show that the equations in EC7-1 (2013) give earth pressure coefficients that are generally close to those of Kerisel and Absi (1990). Exceptions occur for high values of ϕ′ and (δ/ϕ′), for which EC7-1 is more conservative.

Total stress analysis Total stress analysis is applicable only to clay soils in the short term while there is no drainage of water or air into or out of the soil and the specific volume remains constant. Total stress analysis is not applicable to coarse-grained soils or weak rocks.

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In a total stress analysis, the generalised horizontal active and passive earth pressures are given by:

(4.11)

and

(4.12)

where σa is the total horizontal active earth pressure σp is the total horizontal passive earth pressure cu is the undrained shear strength

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cw is the wall adhesion. It is normal practice to apply limits to the value of cw adopted in design (see Chapter 7). The term 2 √[1 + (cw /cu)] is an approximation and the value used should not exceed 2.57 (= 1 + π/2).

4.1.6 Tension cracks For a wall with no soil/wall adhesion and no available ground or surface water, the minimum active lateral total stress needed to support a clay soil in undrained conditions is, in theory, negative to depths of (2cu – q)/γ and 2 (2cu – q)/γ by the Rankine and Coulomb analyses respectively (see Box 4.1), where cu is the undrained shear strength, q is a uniform surface surcharge and γ is the unit weight of the soil. Rather than rely on tensile stress acting across the soil/wall interface to help support the wall, it is usual to assume that a tension crack develops over the depth below the retained surface where the calculated active lateral total stress is negative. Box 4.1

Theoretical depths of tension cracks by the Rankine and Coulomb analyses

For a retaining wall where there is no soil/wall adhesion and no available ground or surface water, the theoretical depth of tension cracks by the Rankine and Coulomb analyses is given by:

Rankine The depth of tension cracks, z, is given by:



∴ z = (2cu – q)/γ

Coulomb

Resolving horizontally:

Pa = N cos 45° – cu √2 z sin 45° = 0



∴ N = cu √2 z

Resolving vertically:

W + qz tan 45° = N sin 45° + cu √2 z cos 45°



where



W = ½ γz² tan 45°



N = cu √2 z



∴ z = 2 (2cu – q)/γ

This is twice the depth of the tension crack derived from the lower bound stress field (Rankine) analysis.

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Where water is not expected, the depth of (dry) tension cracks ztc may be taken as the value given by Equation 4.11 with σa set to zero. (For cw = 0, ztc = (2cu – q)/γ, if cw is non-zero, ztc will be slightly greater). The total pressure acting on the retained side of the wall at any depth z (in metres) below ground level should be taken as the greater of Equation 4.11 and a minimum equivalent fluid pressure (MEFP) of 5z kPa (Figure 4.5). The adoption of this MEFP ensures a degree of robustness short of assuming that the crack will flood completely.

Figure 4.5

Tension cracks: minimum total horizontal stress

In the case of an embedded cantilever wall or where access to water is possible, consideration should be given to the possibility that a tension crack may flood. If this happens, the clay will be supported by the hydrostatic pressure of water in the tension crack, which may then open to a depth of at least (2cu – q)/(γ – γw), where γw is the unit weight of water. In such circumstances, in design, a hydrostatic pressure of γwz kPa (where z is the depth in metres below the retained surface) should be adopted on the retained side of the wall to the depth where the total stress calculated using Equation 4.11 becomes equal to this value (Figure 4.5). For an embedded wall propped or anchored near or at the top, the increase in lateral stress associated with the movement of the wall into the retained soil and/or stress redistribution onto a relatively stiff prop or anchor should prevent the ingress of surface water into a tension crack. In this case, provided that a lateral stress greater than the hydrostatic pressure of water at the same level can be demonstrated over a minimum depth of one metre near the top of the wall, the possibility of a flooded tension crack developing behind a propped or anchored wall in a uniform homogeneous isotropic stratum of clay may be discounted. Consideration would still need to be given to the possibility of water entering a tension crack, for example through: zz

a sand parting or other more permeable horizon in the ground at a lower level

zz

preferential drainage paths that may have developed during wall installation (eg sheet piles driven through coarse-grained soils into fine-grained soils dragging down permeable soil behind the wall, or the effects of pre-augering the soil to ease pile installation).

The development of a dry or flooded tension crack must also be considered in weak rocks with a high component of cohesive strength (c′). The development of a flooded tension crack could result in higher lateral stresses against the wall than would be indicated by the application of Equation 4.6.

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4.1.7

Factors affecting limiting lateral earth pressures

Ground stratigraphy Within each stratum, the theoretical limiting active and passive pressures calculated using Equations 4.6 to 4.12 generally increase with depth. At the interface between two strata, the overburden pressure takes a single value, but the lateral stress changes because of the different soil properties in each layer. In design analyses, the theoretical limiting lateral stresses should be used in each stratum as appropriate.

Adjacent highway traffic and railway vehicle loading The effect of adjacent highway and railway loading should be determined as described in Section 5.7.1.

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Adjacent permanent vertical loading A uniform surcharge acting over either the retained or the excavated soil surface is easily taken into account, as it simply increases the vertical total stress at every depth. A strip load running parallel to the wall may be modelled using the procedure suggested by Pappin et al (1986), illustrated in Figure 4.6a or by the 45° distribution approach proposed by Georgiadis and Anagnostopoulos (1998), illustrated in Figure 4.6b.

a

b

where δ is the angle of friction between the soil and the wall

Figure 4.6 Additional lateral effective stress acting on the back of a wall due to a strip load running parallel to it (from Pappin et al, 1986 and Georgiadis and Anagnostopoulos, 1998)

Finite element analyses (Georgiadis and Anagnostopoulos, 1998) show that a small lateral movement of the wall resulting from a surcharge significantly reduces lateral pressures and wall bending moments to below those determined from elastic theory. So the use of elastic (Boussinesq) lateral stress distributions to model a strip surcharge is not recommended unless the wall is rigid with no deformation. A line load of magnitude QL (kN per metre run of the wall) may be considered to exert an additional lateral force on the wall of Pn per metre run, given by:

Guidance on embedded retaining wall design

(4.13)

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The pressure distribution is as shown in Figure 4.7.

Figure 4.7

Pressure diagram for a line load (after Williams and Waite, 1993)

A point load or a line load of limited extent may be converted to an equivalent infinite line load, and a surcharge of limited area to an equivalent infinite strip load, using a method presented by Williams and Waite (1993). This is shown in Figure 4.8.

(4.14)

where QC is the concentrated load (kN) QL the equivalent line load (kN/m) A is the minimum distance of the loaded area from the wall L is the lateral extent of the loaded area, which in the case of a point load will be zero.

Figure 4.8

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Sloping ground Earth pressure coefficients for use when the ground in front of, or behind, the wall is sloping are given in Appendix A4 (β/ϕ′ ≠ 0).

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When the slope behind or in front of the wall is uneven or non-uniform, one of the following approximate methods may be used: 1

Adopt a uniform design slope that approximates or envelopes the actual ground profile. The error associated with an approximation to the actual ground profile may be difficult to quantify, while the adoption of a uniform envelope to the actual slope may be unduly conservative.

2

Carry out a succession of Coulomb wedge analyses to determine the active lateral thrust at different depths down the wall. This is discussed in more detail in the context of earth berms by Daly and Powrie (2001) for total stress and by Smethurst and Powrie (2008) for effective stress analysis, but can be a long and complex process.

3

Model the effect of the slope as a surcharge averaged over the extent of the active zone behind the wall, or as a series of surcharges as outlined in Figure 4.6. This method does not explicitly account for the horizontal shear stress needed to keep the slope standing, which must be calculated and applied as an additional force at the top of the wall. Failure to apply this horizontal force will lead to a potentially unsafe design.

Difficulty in analysis may be encountered if the slope behind the retaining wall is so steep that, according to a stability analysis, it would fail. In these circumstances, once due consideration has been given to whether the steepness of the slope really does pose an unacceptable hazard or risk, it is recommended that the slope is modelled as a single or series of equivalent surcharges over the active zone behind the retaining wall, with the horizontal stress needed to keep the slope standing with the required partial factor on soil strength applied as an additional force. Note that this is an extension of the third approach (given above), in that: zz

an additional force is required, so as to ensure the theoretical stability of the actual slope at the required partial factor on soil strength

zz

this additional force may need to be determined from a separate slope stability analysis

zz

the additional force may act at a point other than the top of the wall.

Interacting walls in close proximity In some circumstances, for example on either side of a highway or railway, walls may be located relatively close to each other. Finite element analyses have shown that, for rough walls separated by a distance less than the wall embedment in homogeneous isotropic soil, can result in significantly enhanced values of passive pressure coefficient. The analyses assumed homogeneous isotropic soil with an internal angle of shearing resistance of ϕ′ , with no cohesion and zero angle of dilation. The walls were not vertically restrained. Interaction effects were found to increase with the soil angle of shearing resistance, ϕ′ , but became insignificant when the separation exceeded the wall embedment (Figure 4.9). Frictionless walls were unaffected at any spacing.

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Notes 1

Homogeneous isotropic soil with no cohesion and no dilation.

2

No groundwater.

3

Wall free to move vertically.

Figure 4.9

Enhancement factor on passive earth pressure coefficient for rough walls in close proximity

Based on these results the passive earth pressure coefficient, Kp* , due to interaction between walls is given by: Kp * = ξ Kp

(4.15)

where ξ is the enhancement factor from Figure 4.9 Kp is the passive earth pressure coefficient for an isolated wall (ie with no interaction effects). The enhancement in Kp values illustrated in Figure 4.9 applies only for the conditions modelled in the finite element analysis. Further work would need to be carried out to derive similar enhancement factors for different ground and groundwater conditions or more general application.

Props at the top of the wall Many singly-propped walls are propped just below, rather than exactly at the top of the wall. Also, real props are often one metre or more in depth. In these conditions, the top of the wall may tend to rotate backwards into the retained soil, leading ultimately to the development of passive rather than active conditions. However, the soil behind the wall is still moving downward relative to the wall, rather than upward, as is the case in a conventional passive zone. The resulting downward shear stress on the back of the wall will reduce (rather than enhance) the passive earth pressure coefficient, generally resulting

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in a value of about 1 (see Figures A4.15 to A4.23 in Appendix A4). The overall effect of the enhanced lateral stresses behind the wall above the level of the prop on the calculated depth of wall embedment is generally small, although neglecting it may significantly reduce the calculated prop load. So, the assumption of normal active conditions behind the wall, both above and below the prop, is usually acceptable in limit equilibrium analysis for calculating the depth of embedment of walls having one level of props at a depth of up to one-third of the retained height below the top of the wall. However, it will tend to underestimate the prop load.

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4.1.8 Factors potentially increasing earth pressures in serviceability limit state conditions The earth pressures that develop around an embedded retaining wall depend on how the initial stress state of the ground is changed by wall installation and the subsequent sequence of excavation and support. In most cases, design for ultimate and serviceability limit states using the procedures set out in Chapter 7 are adequate, but in some circumstances further consideration of the possibility of enhanced lateral stresses under serviceability limit state conditions is appropriate. These are discussed as follows.

Compaction pressures As embedded walls usually retain predominantly natural ground, the development of high lateral stresses due to compaction of the backfill in layers is unlikely to be relevant for this type of wall. However, compaction pressures may be relevant where made ground is compacted against embedded or king post walls or where the ground level adjacent to the wall is raised after construction of the wall. Theoretical treatments of compaction stresses are given by Broms (1971), Carder et al (1977) and Ingold (1979) for coarse-grained soils, and by Carder et al (1980), Symons et al (1989) and Clayton et al (1991) for fine-grained soils. Summaries are provided by Powrie (2014) for both coarse-grained and fine-grained soils, and in Hong Kong Government (1994) for coarse-grained soils (Chapman et al, 2000).

Long-term pressures on walls in overconsolidated deposits Designers are sometimes concerned about the possibility of the in situ lateral stresses in overconsolidated deposits becoming re-established against the wall, for example due to creep. Long-term field measurements behind embedded walls retaining London Clay at Walthamstow, Hackney, Reading and Malden, made over periods of up to eight years following construction, generally indicate a slight reduction in the measured lateral stresses near the wall (Carder and Darley, 1998). Measurements around a retaining wall on the High Speed 1 (HS1) railway at Ashford indicate a longterm decline in total lateral stresses over a 15 year period, which was associated with a reduction in pore water pressure due to through-the-wall drainage (Richards et al, 2007). The evidence is that for walls embedded in stiff overconsolidated clay, the long-term total lateral effective stresses remain largely unchanged or even reduce from those at the end of the construction period.

4.2

METHODS OF ANALYSIS

4.2.1 Introduction Modern CoP generally require the designer to check the adequacy of the retaining wall and its supports against an ULS, eg global collapse, and a SLS, eg cracking of the concrete. For a retaining wall, this distinction can be particularly significant – in many real situations the loads imposed by the ground

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on the wall at the ULS (collapse) may be smaller than those under working conditions, as under working conditions the strength of the soil is unlikely to be fully mobilised. However, the partial factors introduced into ULS calculations lead, in many cases, to more severe structural action effects than those calculated for SLS.

Limit equilibrium analysis

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In this publication, the term limit equilibrium analysis is used to describe an analysis in which the equilibrium of the wall is assessed under the action of assumed lateral pressure distributions, usually based on limiting (active and/or passive) lateral earth pressure coefficients. The active and passive earth pressure coefficients are calculated from the soil strength, to which values of partial factors have been applied as appropriate to obtain the design geometry and to distance the wall from collapse. Limit equilibrium calculations are usually based on simple linear lateral stress distributions consistent with the uniform mobilisation of soil strength around the wall. However, there are ways of modifying the linear stress distributions to account for the effects of differential strength mobilisation due to, for example, wall flexibility. Assumed mechanisms of deformation can be used as a basis for estimating wall movements. Limit equilibrium methods are better developed and more directly applicable for some structural forms (eg unpropped cantilever walls) than others (eg multi-propped walls and walls propped near formation level). However, linear distributions of lateral stress are unlikely to represent the real in-service conditions around complex walls in reality, and their use to calculate SLS bending moments may lead to an over-conservative design. For these reasons, together with the general availability of powerful computers, most retaining wall design is carried out with the aid of computer software that enables the interaction between the wall and the ground to be considered.

Soil-structure interaction (SSI) analyses Subgrade reaction and pseudo-finite element methods In the simplest SSI analyses, the wall is modelled as a beam and the ground as a series of horizontal springs (subgrade reaction method) or as an elastic continuum (pseudo-finite element method). The soil stiffness is characterised by means of spring stiffness, a modulus of subgrade reaction or the stiffness of the elastic continuum. Spring stiffness increasing with depth may be specified, and maximum and minimum spring forces (corresponding to the passive and active limiting stresses) imposed. Beam-onsprings (subgrade reaction) and pseudo-finite element (elastic continuum) analyses will calculate wall movements, bending moments and prop loads, but not ground movements around the wall. Props are generally modelled as springs or point loads and there may be some difficulty in representing real support conditions, especially where moment restraint is provided. Although actual construction sequences can be modelled, these methods and solutions are not exact. Their relevance to reality depends on the appropriate selection of design input parameters. These should ideally be calibrated against reliable field measurements of well-monitored comparable excavations and wall systems (see Chapter 6). Even then, the inherent approximations and the relative simplicity of the methods mean that the results obtained are only approximate.

Finite element and finite difference methods More complex SSI analyses model the ground as well as the wall and its construction sequence explicitly, using finite element or finite difference techniques. In a finite element or finite difference analysis, it is possible to model: zz

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zz

actual construction sequences

zz

structural and support details

zz

consolidation and groundwater effects.

Ground movements as well as wall movements, bending moments and prop loads are calculated, but may be of limited value unless a well-developed soil or rock constitutive model has been used and the results calibrated against reliable measurements of well-monitored comparable excavations and wall systems in similar ground (see Chapter 6).

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Finite element and finite difference methods give theoretically complete solutions, yet are still idealised in their modelling of ground behaviour. They require the user to have significant and specific experience of the software package being used, and experience of modelling the ground conditions and construction sequence envisaged. It is unlikely that two users of the same software, modelling the same problem independently, will obtain identical results.

Selection of method of analysis The appropriate method of analysis to use in any given circumstances will depend on factors such as the complexity of the structure and the construction process, the information needed from the calculation, the input data available and the potential economic benefit from refining the analysis. For example, if the wall depth is governed by cut-off requirements or if a sheet pile wall section is governed by considerations of driveability, there may be little benefit in carrying out complex SSI analysis. Similarly, there is little benefit in using complex numerical analysis to reduce material costs of walls, where there is little or no interaction between the ground and the structure (eg cantilever walls). Table 4.1 summarises the most widely used methods of analysis. Although some give a large amount of design information, reliability depends on the quality and suitability of the input data. It is sensible to carry out some simple calculations as a check on more advanced methods. For example, where possible, it is prudent to carry out limit equilibrium calculations with appropriate simplifying assumptions to obtain a conservative bound before carrying out complex finite element or finite difference analyses. It is generally better to use a simple analysis with appropriate soil parameters than a complex analysis with inappropriate soil parameters. Table 4.1

Advantages and limitations of common methods of retaining wall analysis

Type of analysis/ software Limit equilibrium (eg ReWaRD, bespoke spreadsheet)

Advantages zz

needs only the ground strength

zz

simple and straightforward

zz

follows design standards/codes

zz

easy to check input and output.

Limitations zz

zz

zz

zz

does not calculate deformations. Hand calculations of deformations possible by relating mobilised strength, shear strain and wall rotation, or through empirical databases statically indeterminate systems (eg multipropped walls), non-uniform surcharges and berms require considerable idealisation can model only drained (effective stress) or undrained (total stress) conditions

zz

two-dimensional (2D) only

zz

takes no account of pre-excavation stress state

zz

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does not model interaction between the ground and the structure, wall flexibility and construction sequence

pore pressures need to be specified in an effective stress analysis.

71

Type of analysis/ software Subgrade reaction/ beam on springs (eg WALLAP, CADS)

Advantages zz

zz zz

SSI analysis is possible, modelling construction sequence etc soil modelled as a bed of elastic springs interaction between the ground and the structure taken into account

zz

wall movements are calculated

zz

relatively straightforward

zz

results take account of pre-excavation stress state.

Limitations zz

zz

subgrade moduli can be difficult to assess

zz

2D only

zz

zz zz

zz

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Pseudo-finite element (eg FREW, WALLAP)

zz

zz

zz

Finite element and finite difference (eg SAFE (2D FE) PLAXIS (2D and 3D FE) CRISP (2D and 3D FE) FLAC (2D and 3D FD) ABAQUS (3D FE) DYNA (3D FE))

SSI analysis is possible, modelling construction sequence etc ground modelled as an elastic solid with soil stiffness matrices calculated using a finite element program interaction between the ground and the structure taken into account

zz

wall movements are calculated

zz

relatively straightforward

zz

takes account of pre-excavation stress state.

zz

zz

zz zz

zz zz

zz

zz

full SSI analysis is possible, modelling construction sequence etc complex ground models can represent variation of stiffness with strain and anisotropy

zz zz

zz

zz zz

zz

zz

zz

zz

takes account of pre-excavation stress state can model complex wall and excavation geometry including structural and support details

zz

wall and ground movements are computed potentially good representation of pore water response can model consolidation as soil moves from undrained to drained conditions can carry out two (2D) or three dimensional (3D) analyses.

idealisation of ground behaviour is likely to be crude

zz

zz

zz

berms and certain structural connections are difficult to model global effects not modelled explicitly ground movements around wall are not calculated pore pressures need to be specified in an effective stress analysis. 2D only linear elastic ground model, with active and passive limits berms and certain structural connections are difficult to model global effects not modelled explicitly ground movements around wall are not calculated pore pressures need to be specified in an effective stress analysis. can be time-consuming to set up and difficult to model certain aspects, eg wall installation quality of results dependent on availability of appropriate stress strain models for the ground extensive high-quality data (eg pre-excavation lateral stresses as well as ground stiffness and strength) needed to obtain most representative results simple (linear elastic) ground models may give unrealistic ground movements structural characterisation of many geotechnical finite element and finite difference packages may be crude significant software-specific experience required by user pore pressures can be calculated as part of the analysis.

4.2.2 Limit equilibrium analysis The main attributes of a limit equilibrium analysis were summarised in Section 4.2.1. Determination of the lateral stress distribution acting on an embedded wall for use in an effective or total stress limit equilibrium analysis typically involves the steps given in Box 4.2.

Cantilever walls Unpropped embedded walls rely entirely on an adequate depth of embedment for their stability – they are not supported in any other way. They will tend to fail by rotation about a pivot point near the toe, above which active conditions are developed in the retained ground and passive conditions in the restraining ground in front of the wall. The idealised stress distribution at failure, together with the corresponding bending moments and implied wall deflections, is shown in Figure 4.10.

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a

b

c

d

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Note The maximum bending moment occurs at the point of zero shear at level X-X

Figure 4.10 Idealised stress distribution for an unpropped embedded cantilever wall at failure, effective stresses (a), pore water pressures (see Section 5.6.2) (b), wall bending moment distribution (c), wall deflection (d)

These conditions are known as fixed earth support. Given the retained height h and the soil angle of shearing resistance, ϕ′ , the depth of embedment required to prevent collapse, d, of a cantilever wall can be determined, together with the depth of the pivot point (about which the wall can be imagined to rotate) below formation level, zp . The equations of horizontal and moment equilibrium can be used to find these two unknowns, so the system is statically determinate. Box 4.2 Steps involved in a typical limit equilibrium analysis 1 Identify key depths on each side of the wall, ie the retained and excavated soil surfaces, groundwater levels and interfaces between different strata, between which the vertical and lateral effective stresses are expected to vary linearly. 2 Identify which zones of ground adjacent to the wall would be in the active and passive condition at failure and the depths at which the stress state at failure changes between active and passive. 3 At each key depth, calculate the overburden pressure, pv, and for an effective stress analysis the pore water pressure, u. The overburden pressure is given by the depth z below the relevant free surface multiplied by the average unit z weight of the soil, γ, plus any uniform surface surcharge q (pv = ∫ γdz + q). The pore water pressure should be 0 calculated according to the site hydraulic boundary conditions, based on a suitable analysis, eg a flownet or the linear seepage approximation (Section 5.6.2). In the case of a contiguous pile wall or a wall with weep holes, through-thewall seepage may reduce the pore water pressures in the retained ground significantly, if the integrity of such drainage can be relied upon in the long term. 4 a For an effective stress analysis, select values of wall friction based on the considerations listed in Section 4.1.4 and Chapter 7 and evaluate the appropriate active and passive earth pressure coefficients from Section 4.1.5 and Appendix A4.

b For a total stress analysis, select the values of wall adhesion based on the considerations listed in Section 4.1.4 and Chapter 7. z

5 a For an effective stress analysis, calculate the effective overburden pressure (p’v = ∫ γdz + q – u). and the 0 horizontal effective stress using Equation 4.6 (active) or 4.7 (passive), with appropriate values of Ka and Kp from Step 4a.

b For a total stress analysis, calculate the horizontal total stress using Equation 4.11 (active) or 4.12 (passive). In this analysis, increase the total horizontal stress to a minimum of 5z kPa (where z is the depth in metres below the soil surface) for a dry tension crack, or to γw.z kPa for a water-filled tension crack. Further details of the treatment of tension cracks are given in Sections 4.1.6 and Chapters 5 and 7.

6 The wall is required to be in horizontal and moment equilibrium under the combined actions of the effective stresses and pore water pressures (effective stress analysis) or the total stresses (total stress analysis), anchor forces and/ or prop forces. From the two equations of horizontal and moment equilibrium, two unknowns can be determined, from which all remaining earth pressures and structural action effects may be derived. For an unpropped wall, the unknowns are the depth of embedment d and the depth zp to the point near the bottom of the wall at which active and passive pressures interchange. For a propped or anchored wall, assuming a ‘free earth’ stress distribution (Section 4.2.2), the two unknowns are the wall depth and the prop or anchor force, F. For a propped or anchored wall assuming a ‘fixed earth’ stress distribution (Section 4.2.2), the designer is required to make a further assumption. This could relate to the level at which the wall bending moment is zero or maximum, the depth of the wall, the prop or anchor force or the maximum bending moment. This is discussed further in Section 4.2.2. 7 Check vertical equilibrium.

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If the linear approximation to the steady-state pore water pressure distribution is used (Section 5.6.2 and Figure 5.15a), the two equilibrium equations are simultaneous and quartic in the two unknowns, and can be solved using a spreadsheet or iteratively as outlined by Bolton and Powrie (1987).

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The inconvenience of the iterative solution in the days before computers led to the development of an approximation to the exact calculation, in which the resultant of the stresses below the pivot point is replaced by a single point force Q acting at the pivot (Figure 4.11).

a

b

c Figure 4.11 Approximate stress analysis for unpropped walls, effective stresses (a), pore water pressures (b), check that the added depth can mobilise at least the required force, Q (c).

The portion of the wall below the pivot does not feature in the analysis. The two unknowns are now the depth to the pivot zp and the equivalent point force Q. Solution is simpler in this case as moments can be taken about the pivot, eliminating Q from the moment equilibrium equation. The value obtained for zp is multiplied by an empirical factor, historically 1.2, to arrive at the overall depth of embedment, d. This factor of 1.2 does not relate to distancing the wall from collapse (ie it is not a partial factor), but is necessary because the calculation is approximate. If the simplified procedure is used, a check should be carried out to ensure that the added depth is sufficient to mobilise at least the calculated value of Q (Figure 4.11c). To determine the design depth of embedment, the calculation indicated in Figure 4.11 should be carried out with the appropriate partial factors, known surface surcharges and allowance for known and unforeseen (unplanned) excavation (Chapter 7). Bending moments and shear forces, either at true limiting equilibrium or for the design embedment depth with the specified partial factors and modifications to geometry and loading applied, may be calculated from the appropriate equilibrium pressure distribution. Powrie (1996) and Bica and Clayton (1998) show that the stress distribution illustrated in Figure 4.11 gives a realistic estimate of the geometry of an unpropped cantilever wall at collapse, allowing for likely uncertainties in the soil angle of shearing resistance ϕ′ and the direction and magnitude of the soil/wall friction angle δ (Figure 4.12 for walls in dry sand).

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Note Full method 1 assumes δ = 2 φ′/3 with Kah and Kph determined using Caquot and Kerisel (1948). Full method 2 assumes wall friction downwards below the centre of rotation of the wall with Kph at this location determined using Coulomb’s method.

Figure 4.12

Normalised depth of embedment at failure (after Bica and Clayton, 1998)

Embedded walls propped at the top If the possibility of a structural failure of the wall or excessive movement of the props is discounted, an embedded wall propped at the top can only fail by rotation about the position of the prop. A simple equilibrium effective stress distribution at failure is shown in Figure 4.13a, and pore water pressures according to the linear seepage approach (Section 5.6.2) in Figure 4.13b. The resulting bending moment diagram and implied wall movements are shown in Figures 4.13c and 4.13d. The conditions giving rise to the effective stress distribution shown in Figure 4.13a are known as free earth support, because no fixity is developed at the toe. In this case, the two unknowns are the prop force P and the depth of embedment, d, required to prevent failure. The depth of embedment, d, can be calculated by taking moments about the prop, and P then follows from the condition of horizontal force equilibrium. To determine the design depth of embedment, the calculation indicated in Figure 4.13 must be carried out with the appropriate partial factors etc as discussed in Chapter 7. For several reasons, the earth pressure distribution illustrated in Figure 4.13 may be less representative of what actually happens at collapse than Figure 4.11 is for unpropped walls. In particular, real props are of finite depth/thickness and provide a kinematic restraint that may inhibit the development of fully active conditions in the immediate area, This is not taken into account in the derivation of the lateral earth pressure coefficients likely to be used in analysis (Bolton and Powrie, 1987). If the prop is located below the top of the wall, the wall above prop level may rotate back into the retained soil. There may be a local increase in the lateral stress in the vicinity of the prop (compared with Figure 4.13), and a decrease in the lateral stress below it. This redistribution of lateral stresses would result in an increase in prop load and a reduction in wall bending moments in comparison with those obtained using the simple linear lateral stress distribution shown in Figure 4.13. This is discussed further in Section 4.3 and Chapter 7.

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a

b

c

d

Figure 4.13 Idealised stress distribution at failure for a stiff wall propped rigidly at the top: effective stresses (a), steady-state pore water pressures for a wide excavation where the differential water head dissipates uniformly (b), wall bending moment distribution (c), wall deflection (d)

Some authors (eg Williams and Waite, 1993, and ArcelorMittal, 2016) describe the use of a ‘fixed earth support’ calculation for a propped wall. The idealised and simplified effective stress distributions are shown, together with indicative wall bending moments and deflections, in Figure 4.14.

a

b

c

d

Note The point of zero bending moment (at level Y–Y) is assumed to occur where the active and passive pressures balance, ie the net pressure is zero.

Figure 4.14 Fixed earth support effective stress distributions and deformations for an embedded wall propped at the top: idealised stresses (a), simplified stresses (b), wall bending moment distribution (c), wall deflection (d) (from Williams and Waite, 1993)

This stress distribution might correspond to a mechanism of failure involving the formation of a plastic hinge at the point of maximum bending moment. The fixed earth support analysis is unlikely to be appropriate for stiff, propped walls in clay soils whose embedment depths are governed by considerations of lateral stability. For such walls, the embedment depth calculated assuming fixed earth support conditions will be greater than in a free earth support analysis, and the fixed earth support analysis gives a very conservative bound for wall toe depth. There may be other reasons why the embedment depth of the wall is greater than that required to satisfy lateral stability, eg to provide an effective groundwater cut-off or for adequate vertical load bearing capacity. In such cases, fixed earth conditions may provide a more realistic basis than Figure 4.13 for the estimation of lateral stresses. In the absence of a plastic hinge (which would define the wall bending moment at this point), both the idealised and the simplified stress distributions shown in Figure 4.14 are statically indeterminate. To calculate the prop force and the depth of embedment, the designer must introduce a further requirement

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or simplification. Williams and Waite (1993) suggest assuming that the point of contraflexure (ie where the bending moment is zero) occurs at the level where the net pressure acting on the wall is zero (Figure 4.14). The stress distribution shown in Figure 4.14 would correspond to the correct failure mechanism for a propped or anchored wall where the prop or anchor yields at a constant load. Such a system is statically determinate, providing the prop or anchor yield load is known.

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Walls propped at formation level

a

b

Figure 4.15 Stress analysis for an embedded wall propped at formation level, division of soil zones (a), and idealised effective stress distributions (b) (from Powrie and Li, 1991)

A wall propped rigidly at formation level might be expected to rotate into the excavation, leading to active conditions in the retained ground above the wall and in the ground remaining in front of the wall (zones 1 and 3 in Figure 4.15), and passive conditions behind the wall below formation level (zone 2). As the depth of wall embedment is increased, however, the sense of wall rotation given by a simple limit equilibrium analysis reverses and the implied partial factor on soil strength begins to decrease with increasing embedment depth (Powrie and Li, 1991). This is unrealistic and for walls of deeper embedment, such an analysis is over-conservative. However, the simple analysis does suggest that increasing the ratio of wall embedment to retained height beyond a certain limit will bring no additional benefits. In reality, a wall propped at formation level is often supported in a different way (eg by a higher level temporary prop and/or earth berms) while excavation to formation level is carried out. So careful consideration of the stability of the wall at each stage during construction will be required. It is likely that the wall embedment will be governed by an interim excavation stage.

Retaining walls with a stabilising base In some circumstances, a wall with a stabilising base (ie a platform extending a short distance in front of the wall with a rigid connection at formation level) can represent a more economic solution than either a rigidly propped wall or an unpropped wall of deeper embedment (see St John et al, 1993). The stabilising base works because the contact pressure between it and the excavated soil surface: zz

leads to a restoring moment on the retaining wall

zz

increases the passive pressures in the ground in front of the wall by acting as a surcharge.

The degree of wall movement needed to mobilise both of these effects may be minimised by using (possibly pre-loaded) temporary props to support the wall until the stabilising base has been cast and gained strength (Richards et al, 2004). Based on a comparison with finite element analyses and centrifuge model tests, Daly and Powrie (1999) recommend the use of the limit equilibrium stress analysis shown in Figure 4.16 for this type of wall.

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a

b

Figure 4.16 Forces acting on a stabilising base retaining wall, horizontal effective stress (a) and pore water pressure acting on the wall (b)

The bearing pressure on the underside of the stabilising base is calculated using conventional bearing capacity theory with zero friction between the stabilising base and the underlying ground, and the same (mobilised) angle of friction as used to calculate the lateral stresses on the wall. However, Daly and Powrie (1999) note that the calculation lacks rigour and should be confirmed by a more detailed SSI analysis. Finite element analyses by Powrie and Chandler (1998) suggest an optimum stabilising base width of about half the retained height for a stabilising base and wall having the same EI (where E is the Young’s modulus and I is the second moment of cross-sectional area of the wall).

Retaining walls with a stress-relieving platform If some excavation and/or fill is needed on the retained side of the wall, there may be an advantage in constructing a stress-relieving platform, attached rigidly to the wall stem some distance below the top and protruding horizontally into the retained soil (Tsagareli, 1967, St John et al, 1993). The relieving platform will reduce bending moments in the wall by: zz

applying a reverse moment at platform level, due to the weight of the soil on top of it

zz

reducing vertical and also horizontal stresses in the retained soil below platform level.

It is straightforward to take both of these effects into account in a limit equilibrium analysis. For maximum efficiency, however, the platform should extend far enough into the retained soil to reduce vertical stresses adjacent to the wall, and there may be a void below it. In large maritime structures, the relieving platform may be supported on vertical load carrying piles as described in BS 6349-2:2010 and EAU (2015).

Multi-propped walls In the permanent condition, embedded retaining walls are often propped at more than one level. Examples include underground car parks, basements and cut-and-cover tunnels, which may be propped by reinforced concrete floor and roof slabs. A multi-propped wall is likely to act in different ways at different stages during its construction (eg as an unpropped cantilever, an embedded wall propped at or near its top, and as an embedded wall with more than one prop). When investigating the design bending moments and prop loads, it is necessary to consider each stage of construction individually, including stages during which the wall is supported by temporary props, to determine the largest load in each part of the structure. The cumulative effect of the incremental changes in lateral stress and wall movement that occur during each stage of construction should also be considered in detail in a rigorous analysis of the final condition (see Chapter 7).

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A multi-propped wall is statically indeterminate, and SSI analysis is recommended for their design. A limit equilibrium calculation that may be used as an approximate check is discussed in Section 7.3.1.

King post walls A king post wall comprises a series of vertical soldier piles (king posts) installed into the ground at intervals, which support a retaining wall made up of horizontal laggings (Chapter 3 and Appendix A3). This is a potentially very economical form of construction, but the movements associated with it can be relatively large.

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The horizontal laggings may be made of steel, concrete or timber. The earth pressure that must be resisted by the king posts and laggings will depend on the stiffness of the support system. The design calculations should consider all stages of the excavation and support conditions for the wall. The king post wall should ensure satisfactory overall stability of the wall and the individual king posts should be designed to resist the calculated lateral loads. (If necessary, lateral load tests can be conducted quite economically by loading piles in pairs reacting against each other).

Overall stability of king post walls For cantilever walls, ULS limit equilibrium stability calculations should be based on values of lateral stress and load per metre length that are determined from the assumption of fully active lateral stresses in the retained soil with the partial factors enumerated in Table 7.1, together with the assumptions on unplanned excavation etc stated in Section 7.3. Where ground anchorages or props are installed to support the king posts, the king post piles and the walings should be designed to accommodate the failure of an individual anchorage to carry its full design load.

Lateral loading of king posts The king posts should be designed as piles in lateral loading, giving an ultimate net effective resisting force P′u per metre embedded depth, per metre length along the wall of: P′u = Kp.b.p′v /s

at embedment depths z ≤ 1.5 b (4.16)

and P′u= Kp2.b.p′v /s

at embedment depths z ≥ 1.5 b (4.17)

where b is the king post width s is the spacing of the king posts (s > 3b) Kp is the passive earth pressure coefficient defined as (1 + sin ϕ′)/(1-sin ϕ′) p′v is the effective overburden pressure at depth z (Fleming et al, 2008). These expressions for P′u are based on the work of Barton (1982) as reported by Fleming et al (2008). Originally considered applicable for Kp values of between 3.0 and 5.3 (ie 30° ≤ ϕ′ ≤ 43°), they have been shown by Pan (2013) to be equally suitable for 2≤ Kp ≤3 (ie 20° ≤ ϕ′ ≤ 30°) and so to cover the likely range of ϕ′ values for most soils. Broms (1964) suggested calculating the limiting lateral force at depth using the expression P′u = 3.Kp.b.p′v /s. This gives values greater than Equation 4.17 for soils having ϕ′ < 30°, and smaller values for soils having ϕ′ >30°. Pan (2013) shows that, in comparison with finite element analyses, Equation 4.17 gives conservative results for ϕ′ ≤ ∼40°. The Broms expression, P′u = 3.Kp.b.p′v /s, is unconservative for soils having ϕ′ 30°. Equation 4.17 is preferable in most soils, but it must be remembered that both are empirical approximations.

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An expression that gives results closer to the numerical values calculated by Pan (2013) for the limiting line load at depth is: (4.18) where P′u is the ultimate net effective resisting force, as calculated here σ′v is the vertical effective stress Kp is the passive earth pressure coefficient D is the pile diameter (Figure 4.17).

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This gives an equivalent net resisting force per metre embedded depth, per metre length along the wall, of (8.5Kp – 10).b.p′v /s, for king posts of diameter b at a spacing s, where p′v is the effective overburden pressure at depth z. The depth over which the limiting lateral stresses were reduced by surface effects increased with soil strength, up to about seven times the pile diameter for ϕ′ < 50°. Further details are given in Pan (2013) and Powrie and Smethurst (2015). In a total stress analysis, the ultimate net lateral resisting force per metre embedded depth, per length Pu is given by Fleming et al (2008) as: Pu = [2 + (7z/3b)].c .b/s at embedment depths z ≤ 3b u

(4.19)

and Pu = 9. c .b/s u

at embedment depths z ≥ 3b (4.20)

where b is the king post width s is the spacing of the king posts (s > 3b) c is the undrained shear strength u at embedment depth z.

King posts socketed into rock For posts socketed into a weak rock whose strength is characterised by both a frictional strength δ′ and a real (effective stress) cohesion c′, limiting lateral resistances may be estimated by applying Equations 4.16 and 4.17 or 4.18 to the frictional component and Equations 4.19 and 4.20 to the cohesion component of strength. The resultant resistance is then the sum of that arising Figure 4.17 Limiting at-depth lateral stress on a pile moving into a soil from the two strength components. characterised by the effective stress failure criterion, τ = σ′tanϕ′, empirical The depth z for determining which of expressions compared with finite element calculations run for both full strength (δ = φ′) and zero strength (δ = 0) pile-soil interfaces, overall stability Equations 4.16 and 4.17 or 4.19 and (a) and lateral loading of individual king posts (b) (from Pan, 2013) 4.20 to use should be taken from the top of the rock stratum, owing to its likely much greater strength than the overlying soil.

Vertical loading of king posts It is prudent to design for vertical loads applied to king posts to be carried in end (base) bearing. This leaves the interface friction available to assist in the generation of lateral resistance. However, Equation 4.20 corresponds to the theoretical solution for a frictionless pile, while the value of Kp used in Equations 4.16 and 4.17 is for an interface friction angle δ = 0, so this approach will be more conservative.

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Mobilisation of soil strength with wall displacement

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In a normally consolidated soil, the in situ lateral earth pressure coefficient K0 is closer to the active limit than the passive. In these conditions, the stresses in the soil behind the wall fall to their active values after only a small movement of the wall. In front of the wall, larger movements than are acceptable under working conditions may be required for the stresses to rise to the passive limit. So, the wall may, under working conditions, be in equilibrium under the action of active pressures in the retained soil, and lower-than-passive pressures in the soil in front of the wall (Terzaghi, 1943 and Rowe et al, 1952). This is the reasoning behind early design approaches for retaining walls, which involved the reduction of the passive earth pressures by a factor Fp. In an overconsolidated clay, the in situ lateral effective stress is likely to be closer to the passive limit (Figure 4.1) than the active owing to the geological stress history (Skempton, 1961, Burland et al, 1979). Although the in situ lateral stresses are likely to reduce during wall installation, the high in situ lateral stresses in overconsolidated clays led to a concern that the assumption of fully-active conditions in the retained soil may be inappropriate for an overconsolidated clay at deformations small enough to be acceptable in service. However, bending moments and/or lateral earth pressures measured in centrifuge model tests (Bolton and Powrie, 1988) and in the field (Tedd et al, 1984, Carder and Darley, 1998, Richards et al, 2007, Montalti, 2015) do not evidence this concern in practice. This is probably a result of stress relief due to wall installation and the relatively high stiffness of the soil behind the wall in lateral unloading (Powrie et al 1998).

Pressure redistribution due to arching and wall flexibility Local variations in wall movement and rotation can, for propped or anchored walls, lead to nonlinearities in lateral stress distributions. This redistribution of stress away from the linear-with-depth variations assumed in simple limit equilibrium analyses can be exploited to reduce design bending moments and wall depth if a SSI analysis is carried out. Stress redistribution may occur due to: zz

the kinematic restraint imposed by a rigid prop, both under working conditions and at collapse

zz

wall flexibility.

Rowe (1952) showed that for rigid props, the horizontal stress distribution on the retained side of the wall is non-linear, with load ‘arching’ on to the relatively stiff prop (Figure 4.18). This is accompanied by a reduction in the lateral stress over the mid-section of the wall, leading to a reduction in wall bending moment. If the wall is propped just below the top, the increase in lateral stress in the vicinity of the prop will be more pronounced because of the tendency of the upper part of the wall to rotate back into the retained ground. Rowe (1952) also found that an outward movement at the anchor point of less than H/1000 (where H is the overall wall height) was sufficient to generate fully active conditions, and a linear variation in lateral stress with depth behind the wall. The flexibility of an embedded retaining wall may also affect deformations and bending moments. In general terms, wall deformation occurs partly due to

Guidance on embedded retaining wall design

Figure 4.18 into rigid prop

Reduction of lateral stress in the retained soil due to arching

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rigid body rotation (in the case of a propped or anchored wall, about the position of the prop or anchor), and partly due to bending (Figure 4.19). For a propped or anchored wall of given overall height H and flexural stiffness EI, bending effects are most significant when the wall is supported at the top. Rowe (1952) found that if the wall is stiff (ie the deflection at the level of the excavated soil surface is of the same order as the deflection at the toe), the stress distribution in front of the wall under working conditions is approximately triangular. If the wall is flexible (ie the deflection Figure 4.19 Components of wall displacement and definition of a ‘stiff’ wall at excavation level is significantly greater than at the toe), the centre of pressure of the stress distribution in front of the wall under working conditions is raised (Figure 4.20b). This leads to smaller anchor loads and bending moments than those given by a limit equilibrium calculation based on triangular lateral stress distributions.

a

b

Figure 4.20

Stress distributions behind and in front of stiff (a) and flexible (b) embedded walls (after Rowe, 1952)

Diakoumi and Powrie (2013) demonstrate that the relative importance of wall movements resulting from bending and rigid body rotation may be characterised by means of a dimensionless flexibility number R, given by:

(4.21)

where G* is the rate of increase in soil shear modulus with depth in kN/m3 H is the total height of the wall (= d + h), in m EI is the flexural rigidity of the wall, in kNm2/m ρ = H4/EI is the wall flexibility as defined by Rowe (1952).

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Bending moment and prop load reduction curves developed by Diakoumi and Powrie (2013) on the basis of an adaptation of the mobilised strength design (MSD) method (Bolton, in prep) for flexible embedded retaining walls, are presented in Figures 4.21a and 4.12b for soils having ultimate or critical state angles of effective shearing resistance ϕ′ult = 20° and 30°, with pore pressures of zero and corresponding to steady state seepage from a full-height groundwater level behind the wall. These are normalised with reference to a calculation carried out according to EC7, based on the same mobilised soil strength (ie the same partial factor applied) in the soil behind and in front of the wall.

a Comparison between MSD and conventional limit equilibrium (EC7) maximum bending moments for different values of log(G*ρ) and different conditions of pore water pressure when ϕ′ult = 20° and 30°

b Comparison between MSD and conventional limit equilibrium (EC7) maximum prop forces for different values of log(G*ρ) and different conditions of pore water pressure when ϕ′ult = 20° and 30° Figure 4.21 Comparison of bending moment and prop loads from MSD and limit equilibrium analyses (after Diakoumi and Powrie, 2013)

Stress redistribution due to the kinematic restraint of the prop might be modelled by means of rectangular, rather than triangular, stress distributions. This is discussed by Simpson and Powrie (2001), but is not a practice that has traditionally been followed in the UK. Simple analyses and look-up charts offer a useful check, but in general a designer wishing to take account of SSI effects to achieve economies in design by taking account of stress redistribution should carry out a SSI analysis as described in Section 4.2.3.

4.2.3 SSI analysis The main advantage of using an analysis capable of modelling SSI is the ability to estimate wall deflections and surrounding ground movements by considering some or all of the following: zz

soil conditions and behaviour (eg variation of stiffness with strain, effective stress and stress path, anisotropy, in situ earth pressures)

zz

pore water pressures at various stages, including during consolidation/swelling from undrained to drained conditions

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zz

the wall and its support system

zz

the sequence of construction, possibly including wall installation.

The following discussion relates mainly to the use of a SSI analysis to investigate SLS. If a SSI analysis program is used to investigate ULS, a different approach to the selection of certain parameters would be required. This is discussed further at the end of Section 4.2.3. Numerical analysis can provide output to a high level of detail provided the required input data are available. The user of any analysis package must understand the principles on which it is based and the data input requirements sufficiently to interpret and appreciate the limitations of the output. The designer should validate the output of the analysis with reference to simpler calculations, previous comparable experience and/or field data. The main points that should be considered are as follows.

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Ground conditions and behaviour zz

Is the in situ stress state known or can it be reliably estimated? How much will it affect the results of the analysis?

zz

Will wall installation be modelled explicitly, or will an empirical adjustment to the in situ stress state be made to account for its effects (Sections 4.1.2 and 5.5.4)?

zz

Will the ground model include: {{

a transition from elastic to plastic behaviour (essential)

{{

consolidation effects

{{

strain-dependent stiffness moduli

{{

stress-path-dependent moduli

{{

anisotropy?

zz

Are the required parameters available?

zz

How will the groundwater conditions (including any temporary dewatering) be modelled?

zz

How will the transition from short-term (undrained) to long-term (drained) conditions be handled?

zz

How will 3D geometrical effects (eg corners) be modelled?

zz

What other actions (eg loads from nearby structures, or for a load-bearing wall) need to be included and when?

Soil strength and stiffness must be appropriate for the ranges of stress and strain expected, especially with simple (linear elastic-perfectly plastic) models.

Wall support and sequence of construction The wall interacts with its support system. In a typical cut-and-cover-structure, the wall interacts with the base slab, the roof slab and intermediate props and slabs for both vertical and lateral stability. In finite element and finite difference analytical models, these effects are considered as an integral part of the analysis, although care must be taken in modelling joints as pinned, butted (ie simply cast up against) or having continuity of bending stiffness. With subgrade reaction and pseudo-finite element techniques, appropriate assumptions regarding the input data and boundary conditions should be made to allow for these effects, eg the application of fixed-end bending moments and rotational stiffness at slab/ wall connections. Similarly, the effects of an earth berm will need to be modelled by the application of appropriate lateral stresses in a subgrade reaction or pseudo-finite element analysis (Section 8.2). In finite element, finite difference, subgrade reaction and pseudo-finite element analyses, the effects of construction sequence, which may result in the wall being supported in different ways at different excavation depths, can be modelled explicitly.

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Wall flexural stiffness Table 4.2 summarises how basic wall stiffness parameters of Young’s Modulus, E, and second moment of cross sectional area, I, (usually expressed per metre run in plane strain) are typically used in SSI analysis. The calculated load effects and wall deflections will depend on the wall flexural stiffness, EI, adopted. In an SLS analysis, the value of EI used should be appropriate to the construction stage under consideration, whether in the short or the long-term. For reinforced concrete walls, this will require consideration of the effects of cracking (due to wall flexure), creep, possible shrinkage and early age temperature effects. For steel sheet pile walls, loss of section due to corrosion should be considered. However, ULS bending moments and structural loads should be determined based on the full flexural rigidity of the wall.

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Table 4.2 wall types

Values of Young’s modulus E and second moment of cross-sectional area I for various embedded retaining

Wall type

Second moment of cross sectional area, I

Young’s modulus, E

Reinforced concrete diaphragm walls

I = d3/12 m4/m run, where d is the wall thickness in metres

E = Young’s modulus of concrete, making due allowance for creep and relaxation (see main text)

Reinforced concrete bored pile walls

I = πD4/64s m4/m run, where D is the pile diameter in metres, and s is the spacing between piles for contiguous bored pile walls and hard/hard secant bored pile walls, or between hard piles for hard/soft and hard/firm secant bored pile walls

E = Young’s modulus of concrete, making due allowance for creep and relaxation

Steel sheet pile walls

I = second moment of area of sheet pile section per m run

E = Young’s modulus of steel comprising sheet pile section

Reinforced concrete walls For concrete walls, the value of EI should strictly be determined for the reinforced section. The approximation indicated in Table 4.2 is commonly adopted and has been found to be appropriate in conjunction with the design procedures presented in Chapter 7. For a reinforced concrete section, the value of EI changes over time, with creep and relaxation causing a ~50 per cent reduction from the short-term uncracked value over the long term. The flexural stiffness EI of a concrete wall should be calculated at each construction stage and in the long term. It is often considered appropriate to adopt 0.7E0I and 0.5E0I during the construction and long-term stages respectively, where E0 is the uncracked short-term Young’s modulus of concrete (typically, E0 = 28 to 30 GPa as a characteristic value) and I is the second moment of area of the reinforced concrete section as defined in Table 4.2. The way in which the reduction in EI is applied in the analysis should be considered carefully. Box 4.3 shows the procedure for a SSI analysis. This approach is required in most computer programs in which the stiffness represents the response to load increments only. The same approach may be used to model corrosion of steel sheet piles, with I reducing over time.

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Box 4.3

Changing wall EI to allow for cracking, creep and relaxation of concrete

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The following shows the type of bending moment-curvature curve requires for a change from short-term to long-term stiffness.

The high short-term stiffness on OA is required to drop to the lower long-term stiffness on line OBC. Consider an element of structure that in the short-term has been stressed to point A. Over time, its state will move to somewhere on line BC. If there is no change of strain during this change, stresses will simply relax and the stress/strain state will move to point B. If, on the other hand, the load on the element cannot change, it will creep and move to point C. If an element is at point A and the only change is in the Young’s modulus, further behaviour will proceed along line AD. This does not capture creep or relaxation. The soil-structure interaction analysis should ensure that even if nothing moves, stresses will change from point A to point B. If these new stresses are no longer in equilibrium, the analysis should then indicate further strains such that the stress state moves up line BC.

Steel sheet pile walls Values of I for hot rolled sheet piles to BS EN 10248-1:1996 are given in ArcelorMittal (2016). The stiffness and bending strength of walls comprising Z-profile steel sheet piles, which have their interlocks in the flanges (Appendix A3), may be maintained by crimping or welding (see ArcelorMittal, 2016). With Z-profile piles, the effective section modulus will be reduced if the piles are allowed to rotate about the vertical axis during driving – roughly, 5° of rotation will result in a 15 per cent reduction in the combined section modulus (Williams and Waite, 1993). To avoid this, the wall should be installed within construction tolerances and excessive rotation at the interlock avoided by using guide walings to control the verticality and depth of the wall. In walls made up of U-profile steel sheet piles, the connecting section incorporates an interlock located on the centreline or neutral axis of the wall. If two piles are able to displace relative to one another along the interlock, the full modulus of the combined sections will not be realised. These piles rely on the transfer of longitudinal shear stress between adjacent piles (by friction at the interlocks) to develop the full modulus of the combined section. It is likely that shear will be generated by surface irregularities, rusting, lack of initial straightness and soil particle migration into the interlocks during driving (Williams and Waite, 1993). However, shear transfer may not be effective in the case of piles: zz

forming cantilever walls

zz

cantilevering a significant distance above or below walings

zz

driven into and supporting silts and/or soft clay

zz

retaining free water over a part of their length

zz

that are prevented (eg by rock or obstructions) from penetrating to their required toe level.

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These conditions are termed ‘unfavourable’ or ‘highly unfavourable’ and for U-profile steel piles, reduction factors to the nominal stiffness and bending strength apply to the corroded section properties used in section verification to EC3-5. For the UK, Table 2 of the UK NA to EC3-5 (BS EN 1995-11:2004+A1:2008) gives the reduction factors that apply for the limit states. The values given for the loss of thickness of steel are cautious estimates. In reality, the loss of section will vary over the length of the pile. The full stiffness should be used in an analysis to obtain the maximum ULS moments and the section verified taking into account corrosion effects in accordance with EC3-5. The reduced stiffness can then be calculated or averaged to indicate deflections and movements in the SLS checks. Reduction factors β should be treated in the same way, and also for the buckling and combined effects of actions in the verification of sections in accordance with EC3-5.

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In soft soils, U-profile piles may be installed as triples or Z-profile piles used to avoid oblique bending effects. Otherwise, the NA to EC3-5 gives the required reduction factors for U-piles used singly or in pairs in soft soils.

Effective normal stiffness of supports In subgrade reaction and pseudo-finite element analyses, it is necessary to input explicitly the stiffness of any temporary or permanent props as an equivalent stiffness in the direction normal to the wall. The effective normal stiffness k (in kN/m per m run) of a prop should be calculated as follows:

(4.22)

where A is the prop cross sectional area E is the Young’s modulus of the prop material s is the spacing between props L is the ‘free length’ of the prop, ie the distance between the wall and the point at which is the prop is rigidly supported. (For symmetrical walls, the free length is half the length of the prop). The effective stiffness of the propping system has to be resolved perpendicular to the face of the wall taking account of any inclination of the prop in the horizontal or vertical planes. The stiffness of ground anchors may be more difficult to model in an idealised way, owing to uncertainties concerning the degree of fixity and the effects of shear stress transfer between the anchor and ground along the length of the anchor. If concrete slabs are used to support the wall (eg in a top-down construction sequence), the calculated axial stiffness of the slab should be reduced to allow for any openings. For concrete slabs and props, the Young’s modulus should be reduced to allow for the effects of creep and relaxation as previously described for concrete walls. Thermal expansion of a thick concrete slab during curing can also have a noticeable effect (Batten and Powrie, 2000).

Selection of pre-excavation earth pressure coefficient K for use in design calculations The value of K adopted in design calculations should allow for the effects of wall installation. This can be modelled explicitly in SSI analysis using numerical methods (finite element or finite difference methods) or its effects may be taken into account empirically as described in Section 5.5.3.

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ULS using finite element or finite difference analysis When carrying out an ultimate limit state analysis to EC7 Design Approach 1 Combination 2 (DA1C2), in which partial factors greater than unity are applied to the soil strength parameters, the analyst faces a choice between: 1

Carrying out the whole analysis with the soil strength already reduced by the requisite partial factors (Strategy 1 as defined by Simpson, 2012).

2

Starting and continuing the analysis to each key stage with the full soil strength, and then reducing the soil strength by the required partial factor to investigate the effect on each stage separately (Strategy 2 as defined by Simpson, 2012).

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Several authors have investigated the effect of the choice of strategy (Bauduin et al, 2000, Simpson and Yazdchi, 2003, Simpson and Driscoll, 1998, Schweiger, 2005, Simpson and Hocombe, 2010), without drawing consistent conclusions. At present, either approach is acceptable, although there is an emerging preference towards the second as it can lead to the calculation of larger bending moments in multi-propped walls. Strategy 2 has been adopted in the design example presented in Appendix A7.

4.3

EFFECT OF METHOD OF ANALYSIS

Appendix A4 compares the results obtained from the analysis of four generic retaining wall problems: 1

Cantilever wall – effective stress analysis.

2

Cantilever wall – total stress analysis.

3

Singly-propped wall – effective stress analysis.

4

Singly-propped wall – total stress analysis. Using the following methods and commercially available software: a

limit equilibrium (spreadsheet)

b

subgrade reaction/pseudo-finite element (FREW, WALLAP)

c

finite element (PLAXIS).

The design approaches and combinations adopted were: 1

A ULS calculation using EC7 DA1C2, with partial factors of 1.25 applied to tanϕ′, 1.4 to the undrained shear strength, 1.0 to permanent loads and 1.3 to variable unfavourable loads.

2

A ULS calculation using EC7 DA1C1, with partial factors of 1.0 applied to tanϕ′, 1.0 to the undrained shear strength, 1.0 to permanent loads, 1.1 to variable unfavourable loads and 1.35 to calculated effects of actions (wall bending moments, shear and anchor/prop forces).



SSI analysis to DA1C1 were carried out using the depth of wall embedment calculated using DA1C2. This approach cannot be used in limit equilibrium analyses (because by definition such a wall would not be in equilibrium under the assumed distribution of lateral stresses), for which a new depth of embedment was calculated (see Section 7.3).

3

An SLS calculation with the depth of embedment determined for the ULS calculation and partial factors of 1.0 applied to soil strengths, all actions and calculated effects of actions.

The wall was assumed to be rough and the soil to have ϕ′peak = ϕ′cv , so that (δ/ϕ′)des = 1. The scenarios analysed are defined in Appendix A4 and illustrated in Figures A4.24 to A4.27. The assumptions made in the calculations and the results obtained are summarised in Table A4.1 and Figures A4.28 to A4.31. The overall conclusion is that limit equilibrium analyses can be used with confidence to calculate depths of wall embedment and the ULS effects of actions where stress redistribution due to SSI is not

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significant, and to provide an approximate check on the results of more sophisticated SSI analysis. For walls where the effects of stress redistribution are significant, or where interactions with nearby structures or more accurate estimates of ground movements are required, and for wall types where simple limit equilibrium earth pressure distributions are less certain (eg walls that are singly-propped at low level or multi-propped walls), an appropriate SSI analysis should be carried out.

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4.4

KEY POINTS AND RECOMMENDATIONS

1

Analysis of retaining walls may be carried out in terms of effective stresses using a frictional failure criterion based on the drained angle of shearing resistance ϕ′, or (for walls in clay in the short term) in terms of total stresses using an undrained shear strength failure criterion. Assessment of the medium to long-term stability of a retaining wall should be carried out in terms of effective stresses. It is good practice to carry out a total stress analysis using the undrained shear strength cu to check the short term stability (especially when cu is low), but this is only valid in the short term while there is no significant dissipation of the excess pore water pressures induced on excavation.

2

In an effective stress limit equilibrium analysis, lateral earth pressures are usually calculated from earth pressure coefficients giving the ratio of horizontal effective stress to the effective overburden pressure, p′v. It is necessary to consider carefully the relative soil/wall movement (both horizontally and vertically) and the vertical equilibrium of the wall in assessing the shear stresses on the wall and selecting appropriate values of earth pressure coefficients. Appendix A4 provides equations and charts for the determination of earth pressure coefficients.

3

For walls subject to external vertical loads, the magnitude and direction of the friction or adhesion assumed at the soil/wall interface should be appropriate for each construction stage and in the long term. For walls that support very large vertical loads and may settle relative to the ground, it is generally prudent to assume the limiting values for wall friction and adhesion given in Section 7.3.1 over the embedded portion of the wall and zero friction or adhesion on the retained side above excavation level.

4

In total stress analysis, the following minimum total horizontal stress should be assumed on the retained side of the wall: a

where water is not expected: the greater of a minimum equivalent fluid pressure (MEFP) = 5z kPa and the total stress calculated from Equation 4.11 (Figure 4.5)

b

flooded tension cracks: the greater of a hydrostatic pressure = γwz kPa and the total stress calculated from Equation 4.11 (Figure 4.5).



For an embedded wall propped or anchored near its top, provided that a lateral stress greater than the hydrostatic pressure of water can be demonstrated over a minimum depth of one metre near the top of the wall on the retained side, the possibility of a tension crack in a uniform homogeneous isotropic stratum of clay flooding from the surface may be discounted. However, the possibility of water entering such a tension crack by other means should be considered, for example through a sand parting or other more permeable horizon in the ground, or through preferential drainage paths that may have developed during wall installation.

5

Appendix A4 summarises the field data and finite element analysis reported in the literature regarding wall installation effects. While it is difficult to give general guidance, wall installation might cause a 10 per cent reduction in the in situ lateral earth pressure coefficient for bored pile walls and 20 per cent for diaphragm walls installed in overconsolidated clays. In a simple elastic SSI analysis (in which the pre-failure deformation behaviour of the soil is assumed to be linear and no allowance is made for the effect of recent stress history on the soil stiffness), a pre-excavation lateral earth pressure coefficient of unity is likely to give reasonably realistic bending moments and prop loads for walls embedded in stiff overconsolidated clays. If K0 < 1, it is suggested that the pre-excavation value of lateral earth pressure coefficient is taken to be the same as K0 (ie wall installation does not change the in situ value). Corresponding guidance for analyses in which the soil is represented using a constitutive model that allows for the effect of recent stress history on soil stiffness (ie assigns a higher stiffness to the soil behind the wall than that in front) is given in Appendix A4.

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6

For embedded walls in stiff clay, field evidence indicates that the long-term lateral effective stresses remain largely unchanged from those at the end of the construction period, with changes in lateral stress being largely associated with changes in pore water pressure. There is no evidence that at-rest pressures are reinstated behind such a retaining wall over time.

7

Limit equilibrium analyses can be used with confidence to calculate depths of wall embedment and the ULS effects of actions where stress redistribution due to SSI is not significant, and to provide an approximate check on the results of more sophisticated SSI analysis. For walls where the effects of stress redistribution are significant, or where interactions with nearby structures or more accurate estimates of ground movements are required, and for wall types where simple limit equilibrium earth pressure distributions are less certain (eg walls that are singly-propped at low level or multipropped walls), an appropriate SSI analysis should be carried out.

8

In an effective stress analysis, SSI offers opportunities for stress redistribution away from the simple linear increase with depth assumed in a limit equilibrium calculation. In such circumstances, a shorter wall, smaller calculated wall bending moments and possibly greater calculated prop or anchor loads will likely be obtained from SSI analysis than from limit equilibrium calculations. Limit equilibrium calculations in which active conditions are assumed behind the wall above the prop will tend to underestimate prop or anchor load and should be treated with caution in design (Section 8.1.3).

9

The wall flexural stiffness EI adopted in analysis should be appropriate for each construction stage and in the long term. For reinforced concrete walls it is often appropriate to adopt 0.7 E0I and 0.5 E0I during the construction and long-term stages respectively, where E0 is the uncracked short-term Young’s modulus of concrete (typically E0 = 28 GPa) and I is the second moment of area of reinforced concrete section as defined in Table 4.2.

10 Reduction factors for section properties for sheet piles should be applied in accordance with EC3-5 and the NA where piles:

90

a

form cantilever walls

b

cantilever a significant distance above or below walings

c

are driven into and support silts and/or soft clay

d

retain free water over a part of their length

e

are prevented (eg by rock or obstructions) from penetrating to their required toe level.

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5 Determination and selection of parameters for use in design calculations

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This chapter is intended for the geotechnical designer, consulting engineers, contractors and those involved in the design of temporary and permanent works. It assumes that the reader has some experience and understanding of the principles of engineering design and the requirements of input parameters for the associated analyses. As discussed in Section 2.3.4, EC7-1 permits the design of the wall to be undertaken by calculation, prescriptive measures, experimental models and load tests, and by use of the OM. The selection of parameters set out in this chapter is aimed at design by calculation or by the OM, as described in Chapters 2, 4, and 7. Design by prescriptive measures has not been considered as it does not require the determination of parameters and design by experimental models has limited applicability to embedded retaining walls. This chapter provides guidance on the determination and selection of parameters for use in design calculations and analyses. Figure 5.1 outlines the required process. The chapter provides guidance on: zz

description and classification of soil and rock

zz

site investigation requirements

zz

the determination of ground stratigraphy, fabric and permeability and the assessment of drained or undrained ground behaviour

zz

the determination of the soil and rock parameters relevant to retaining wall design

zz

the determination of groundwater pressure

zz

typical actions (load cases)

zz

circumstances where provision should be made for unplanned excavation of formation

zz

the selection of appropriate parameters for use in design and analysis with regard to temporary works and permanent works design.

The appropriate determination and selection of parameters for use in design calculations can lead to economies in wall materials and construction.

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Assign geotechnical category to wall

Understand requirements of design calculations and analysis, and focus site investigation to satisfy them Determine site investigation requirements

zz

Geotechnical Category 1

Geotechnical Category 2

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No site-specific ground investigation

Geotechnical Category 3 As Geotechnical Category 2, with additional specialist requirements

Carry out desk study zzCarry out site-specific ground investigation zz

Note: the design of Category 3 structures is beyond the scope of this publication

Understand geological and hydrogeological setting Determine soil stratigraphy and fabric

zz

Assess whether ground conditions are drained or undrained in the short term

Determine soil and rock parameters

Determine groundwater pressures

Determine load cases

Determine design geometry

Consider factors for safety, uncertainty and acceptable deformations

Select parameters for use in design calculations for: transient design situation (temporary works design) persistant design situation (permanent works design) zzaccidental design situation zzseismic design situation (when applicable) zz zz

Figure 5.1

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5.1

DESCRIPTION AND CLASSIFICATION OF SOIL AND ROCK

5.1.1 Soil The identification and description of soil should be in accordance with the requirements of BS EN ISO 14688-1:2002+A1:2013 (Table 5.1 and Figure 5.2). The classification of soil should conform to BS EN ISO 14688-2:2004+A1:2013. Table 5.1

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Soil fractions

Very coarse soil

Coarse soil

Particle size fractions (after BS EN ISO 14688-1 and BS EN ISO 14688-2) Sub-fractions

Particle sizes mm

Comments

Large boulder

>630

Wet soil does not stick together

Boulder

>200 to 630

Cobble

>63 to 200

Gravel

>2.0 to 63

Coarse gravel

>20 to 63

Medium gravel

>6.3 to 20

Fine gravel

>2.0 to 6.3

Sand Coarse sand

>0.63 to 2.0

Medium sand

>0.2 to 0.63

Fine sand

>0.063 to 0.2

Silt

Fine soil

>0.063 to 2.0

>0.002 to 0.063

Coarse silt

>0.02 to 0.063

Medium silt

>0.0063 to 0.02

Fine silt

>0.002 to 0.0063

Clay

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2 mm?

Are most particles > 200 mm?

zzDescribe

Boulders

Yes

Coarse soil

No

Yes

Yes

No

Very coarse soil

Yes

Do they weigh more than the rest of the soil?

Remove cobbles and boulders (> 63 mm)

No

Is the soil low density?

No

Does the soil comprise organic materials and have organic odour?

Natural soils

Yes

Was the soil laid down by natural processes?

Fine soil

No Clay

secondary fractions according to 4.3.3 of BS EN 14688-1 zzDescribe plasticity according to 4.4 of BS EN 14688-1 zzDescribe organic content according to 4.5 of BS EN 14688-1 zzDescribe colour according to 5.5 of BS EN 14688-1 zzDescribe consistency according to 5.14 of BS EN 14688-1 zzReplace cobbles and boulders zzAdd other information and minor constituents zzAdd geological origin according to 4.10 of BS EN 14688-1

zzDescribe

Silt

Yes

Does soil display low plasticity, dillatancy, silky touch, disintegrate in water and dry quickly

Yes

Describe according to 5.11 BS EN 14688-1

Organic soil

secondary mineral soil fractions according to 4.3.3 of BS EN 14688-1 zzDescribe plasticity according to 4.4 of BS EN 14688-1 zzDescribe structure according to 4.8 of BS EN 14688-1 zzDescribe colour according to 5.5 of BS EN 14688-1 zzDescribe consistency according to 5.14 of BS EN 14688-1 zzAdd other information and minor constituents zzAdd geological origin according to 4.10 of BS EN 14688-1

zzDescribe

Describe according to BS EN 14688-1

Volcanic soil

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No Describe proportion, condition and type of soil components between fill (controlled placement) and reconstituted ground (uncontrolled placement)

zzDistinguish

Describe as for natural soils

Yes

Does soil display low plasticity, dillatancy, silky touch, disintegrate in water and dry quickly

Made ground

Fine-grained soils BS EN ISO 14688-1 distinguishes between consistency and strength. Consistency has no quantitative connotation and is determined from hand tests (Table 5.2). The terminology used is the same as that given in BS 5930:1999, except that in BS 5930:1999 quantitative boundaries were defined, which are now no longer recognised.

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Table 5.2

Consistency index Ic of silts and clays (after BS EN ISO 14688-1 and BS EN ISO 14688-2)

Consistency of silts and clays

Consistency index, IC

Manual test

Very soft

< 0.25

Exudes between the fingers when squeezed in hand

Soft

0.25 to 0.50

Can be moulded by light finger pressure

Firm

0.50 to 0.75

Cannot be moulded by the fingers, but can be rolled in the hand to 3 mm thick thread without breaking or crumbling

Stiff

0.75 to 1.00

Crumbles and breaks when rolled to 3 mm thick threads but is still sufficiently moist to be moulded to a lump again

Very stiff

> 1.00

Can no longer be moulded but crumbles under pressure. It can be indented by the thumbnail

Note Ic = (wL – w)/ IP where wL is the liquid limit, w is the water content and IP is the plasticity index.

Where laboratory test results or field measurements of strength are available, Table 5.3 should be used in addition to the qualitative field descriptive terms given in Table 5.2. Table 5.3

Undrained shear strength of fine soils (after BS EN ISO 14688-2)

Undrained shear strength of clays

Undrained shear strength, cu kPa

Extremely low

< 10

Very low

10 to 20

Low

20 to 40

Medium

40 to 75

High

75 to 150

Very high

150 to 300

Extremely high1

> 300

Note 1 Materials with undrained shear strength greater than 300 kPa may behave as very weak rocks and should be described as rocks according to BS EN ISO 14689-1.

Very coarse and coarse grained soils Classification is in terms of density index ID where ID is defined as:

(5.1)

ID is dependent upon the void ratio (e) and the void ratios corresponding to the minimum density (emax) and the maximum density (emin), as measured in the laboratory.

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Table 5.4

Correlations to classify density of coarse-grained soils (after BS EN ISO 14688-2 and EC7-2) BS EN ISO 14688-2

Term

density index

BS EN 1997-2 SPT N1(60) blows/300 mm

ID % Very loose

0 to 15

0 to 3

Loose

15 to 35

3 to 8

Medium dense

35 to 65

8 to 25

Dense

65 to 85

25 to 42

Very dense

85 to 100

42 to 58

Note N1(60) = N*[(ERr)/60]*CN

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where zz

N1(60) = SPT N value corrected and normalised for energy ratio of 60 per cent and normalised for effective overburden pressure of 1.0 tsf or 96 kPa.

zz

N = field blow count (blows/300 mm).

zz

[(ERr)/60] = rod energy ratio normalised to 60 per cent (see Table 5.5).

zz

CN = overburden pressure correction = 0.77 log10 (1920/σ v’) and σ v’ is the effective overburden pressure at the depth of the SPT N blow count.

Table 5.5

Generalised SPT energy ratios (after Seed et al, 1984 and Skempton, 1986)

Country North and South America

Japan China Italy UK

Hammer

Release

ERr (%)

ERr/60

Donut

2 turns of rope

45

0.75

Safety

2 turns of rope

55

0.92

Automatic

Trip

55 to 83

0.92 to 1.38

Donut

2 turns of rope

65

1.08

Donut

Auto-trigger

78

1.3

Donut

2 turns of rope

50

0.83

Automatic

Trip

60

1.0

Donut

Trip

65

1.08

Safety

2 turns of rope

50

0.83

Automatic

Trip

60

1.00

5.1.2 Rock The identification and description of rock for engineering purposes should be in accordance with the requirements of BS 5930:2015 and Table 5.6.

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Table 5.6

Rock identification for engineering purposes (from BS 5930:2015)

Grain size (mm)

Bedded rocks (mostly sedimentary) Grain size description

Rudaceous

20–6.3

6.3–2

Conglomerate

At least 50 per cent of grains are of carbonate

Rounded boulders cobbles and gravel cemented in a finer matrix Breccia Angular rock fragments in a finer matrix

Calcirudite

At least 50 per cent of grains are of volcanic rock

Saline rocks

Fragments of volcanic ejecta in a finer matrix. Rounded grains Agglomerate Angular grains Volcanic breccia

Coarse

0.63–0.2

Medium Quartzite Quartz grains and siliceous cement Arkose

0.063– 0.002 SLS characteristic movements

Action If Approach A is adopted: continue with most probable sequence and associated assumptions. If Approach B is adopted: change to alternative most probable approach may be possible. If Approach A is adopted contingency (in the form of reverting to the characteristic design construction sequence) may be required. If Approach B is adopted minor modifications to characteristic construction sequence may be possible. Approach D additional measures required.

The application of the OM to a multi-stage excavation is illustrated in Figure 7.9 with three possible scenarios: 1

Where the actual monitored movements at a particular stage are less than those predicted adopting most probable parameters. Under these circumstances the construction sequence based on the most probable design can be implemented.

2

Where the actual monitored movements at a particular stage are within the ‘amber’ zone. Under these circumstances the construction should be completed according to the characteristic design construction sequence.

3

Where the actual monitored movements at a particular stage are more than those predicted adopting ‘characteristic’ parameters. Under these circumstances Approach D additional measures will be necessary to prevent breach of a limit state.

There are many eventualities that need to be carefully considered by the designer to have a full understanding of how the embedded retaining wall is performing compared to the range of behaviours considered during the planning stages.

Figure 7.9

210

Trigger limits for multi-staged excavation – ab initio OM (Approaches A and B)

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Step 4: Monitoring system The parameters that most readily provide the designer with feedback on the behaviour and performance of an embedded retaining wall during construction are the deflection of the wall, the forces generated in the props/anchors and the movement of the retained ground and nearby structures. The monitoring methods for measuring these parameters are summarised in Table 7.4. It is important in the specification that the hierarchy for the various monitoring systems in place is created. Trigger limits should be applied to the ‘primary’ monitoring systems with the other systems introduced as back-ups to ensure there is redundancy in the system. Table 7.4

Summary of most commonly adopted monitoring systems

Parameter to be measured

Instrumentation examples

Notes

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Inclinometers can be: Retaining wall movement

Inclinometer

zz

manual

zz

in-place

zz

ShapeAccelArray (SAA).

Global movement of the wall should be measured by either extending the inclinometer below the toe of the wall or surveying the capping beam. Prop forces

Strain gauges attached to prop

Vibrating wire strain gauges are the most commonly used and should be used at four locations around the prop and corrected for temperature effects.

Ground and building movement

Surveying points

Movements can be measured by precise levelling studs, levelling pins, prisms or reflective targets.

Groundwater

Piezometers

Piezometers can comprise traditional standpipes or vibrating wire.

The designer of the monitoring system should choose between an automated monitoring system where readings are taken at regular short intervals, or a manual system that requires visits from a surveying team. The choice will depend on a number of considerations including: zz

accuracy of the system required – manual systems tend to achieve better accuracy

zz

rate of change of movements that is expected – automated systems will be able to provide more frequent data

zz

number of visits required and duration of the monitoring – automated systems have higher capital costs, but lower operational costs compared to manual systems

zz

accessibility – during a complex construction process it may not be possible for a survey team to safely access monitoring points. Under these circumstances an automated system may be more appropriate.

It is now common for the monitoring data to be available to the entire project team through an internetbased portal so all involved parties can access and interpret the data. Instrumentation and monitoring is discussed further in Chapter 9.

Step 5: The construction phase During the construction phase, it is important that the designer of an embedded retaining wall remains integral to the construction process. At each stage of construction, the monitoring data should be interpreted and evaluated against the trigger limits that are based on the characteristic and most probable assumptions. At each construction stage, the designer should assess the monitoring data relative to these predictions, and from this recommend what, if any, contingency measures should be implemented. An example of the typical decision-making process used (with a traffic light system) is shown in Figure 7.10.

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Design and planning (details not shown)

Identify next excavation stage

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Excavate next stage

Consider modifications to construction sequence/wall lateral movement

Monitor deflection and report excavation progress and geology encountered

Review against trigger criteria

Green

Amber

Red

Extra readings

Replace excavated soil immediately

Review contingency with designer

Figure 7.10

Inform designer, implement contingency and/or emergency plans

Example of a traffic light system for a multi-staged excavation (after Nicholson et al, 1999)

At each construction stage, the designer should take the opportunity to carry out back analysis of the current construction stage using the SSI analysis (typically numerical analysis) that was set up at the design stage. From this calibration exercise, the designer should be able to gauge whether the behaviour of the ground and the wall are relative to the design assumptions. Any trends that are developing at this stage should be assessed taking the opportunity to consider whether possible modifications or contingencies are likely. The earlier these changes are brought to the attention of the project team, the more effectively they can be implemented to the overall benefit of the project.

Application of the ipso tempore approach When the ipso tempore approach to the OM is applied to the design of an embedded retaining wall, it is initiated after the installation of the wall and the completion of some initial excavation stages. Up until this point, the design of the wall has been completed in accordance with the design by calculation method described in Section 7.3 (where calculations using DA1C1 and DA1C2 and SLS and, where applicable, accidental design situation/progressive failure check calculations have all been undertaken using characteristic parameters) without the intention of applying the OM. The wall has been designed and installed to the characteristic wall embedment depth with a structural capacity/strength based on the design effects of actions (wall bending moments, shear and prop/anchor forces) resulting from calculations that adopt characteristic parameters.

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The project team decides to initiate the change to the OM either due to the monitored performance of the wall being much better than expected and taking the proactive step to reduce programme duration, or due to the wall not performing as well as anticipated and taking the reactive step to apply additional measures to prevent a SLS or ULS from occurring. For either of these circumstances, the key steps in the application of the ipso tempore approach are shown in Figure 7.11 and described in detail as follows. Step 1

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Evaluation of actual wall performance, recalibration of ground and analytical models and identification of recalibrated parameters

Recalibrated parameters

ULS calculation by DA1C1 and DA1C2 for transient and persistent design situations and (where applicable) the accidental design situation/progressive failure check (see Section 7.3)

Recalibrated parameters including structural and geotechnical parameters and site use

Recalibrated design effect of actions to confirm adequacy of installed wall structure

Recalibrated calculations with all partial factors of unity

Recalibrated parameters, groundwater and actions

Recalibrated movements

Step 2 Calculations

Green Less than recalibrated movements Trigger limits

Amber Less than characteristic movements Red

Step 3

Greater than characteristic movements

Determination of trigger limits and contingency measures

temporary propping/anchoring berm dimensions zzexcavation depths zzblinding struts zz

Contingency measures

zz

wall deflections ground movements zzmovement of adjacent structures zzprop/anchor forces zzwater pressures zz

Step 4

zz

Monitoring system

Excavate next stage

Step 5 Remaining construction phase

Review monitoring data against trigger criteria

Identify next excavation stage

Enact contingency plan if required Note DA1C1, DA1C2, SLS (and, where applicable, accidental design situation/progressive failure check) calculations using characteristic parameters are already undertaken before Step 1.

Figure 7.11

Ipso tempore OM applied to embedded retaining walls

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Step 1: Evaluation of wall performance and recalibration of analytical models for the ground and the structures to identify ‘recalibrated’ parameters

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The monitored movements of the wall and the surrounding ground at the current stage of excavation (as assessed from the available monitoring data) should be compared with those predicted during the design process before the start of construction (from the characteristic wall movements and associated ground movements estimated from the SLS calculations described in Section 7.3.2). This evaluation should carefully consider and address the following: zz

Has the construction sequence on site followed the construction sequence assumed in design or have decisions been made on site that could have made the situation significantly better or worse?

zz

Is the pattern of wall behaviour similar to that predicted with a different magnitude, or is the wall profile a different shape?

zz

Has the excavation revealed different ground conditions to those expected based on the findings of the site investigation?

zz

Is the groundwater level higher than anticipated or the ground more or less permeable than expected? Is the excavation geometry (ie corners) having a significant effect on the wall behaviour?

zz

Is the propping/anchoring system different from that assumed in design?

zz

Have any of the assumed structural behaviours been in accordance with that assumed in the analysis, for example the connections between the props and the wall or the connection between individual wall elements, such as diaphragm wall panels?

The designer should undertake a detailed audit of the construction to establish the actual construction sequence and site operations up to the time of the assessment of the performance of the wall and its support system. From this understanding, the designer should carry out back analysis of the observed (monitored) wall behaviour and recalibrate the analytical model in terms of the excavation geometry and the behaviour of the ground and the structural elements. The designer should systematically vary the input parameters until a reasonable match is obtained between the observed (monitored) behaviour and the analytical model. This recalibration exercise may require variation of the following: zz

ground stiffness profile

zz

ground strength profile

zz

groundwater level

zz

excavation depths and levels

zz

softening assumed beneath excavation formation level

zz

assumed drained/undrained behaviour of the ground

zz

wall stiffness

zz

prop stiffness

zz

structural connections

zz

applied surcharges.

The revised ‘recalibrated’ parameters derived from the recalibrated analytical model should be compared to the site investigation data and site records to ensure that they are reasonable.

Step 2: Calculations Using the revised analytical soil/structure models recalibrated from Step 1, the designer should undertake calculations to confirm the adequacy of the installed wall’s embedment depth and structural capacity/strength from the results of DA1C1 and DA1C2 calculations (Section 7.3.1). Where applicable, the accidental design situation/progressive failure check (Section 7.3.3) using the ‘recalibrated’ parameters determined from Step 1 for the proposed construction sequence should be verified.

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Using the revised ground and analytical models recalibrated from Step 1, the designer should update the predictions of wall behaviour for the upcoming further excavation stages by undertaking an additional calculation adopting the recalibrated parameters, groundwater assumptions and actions with partial factors of unity applied throughout. The output from this additional calculation will be the designer’s updated recalibrated prediction of anticipated/expected wall movements and associated ground movements for the proposed construction sequence.

Step 3: Trigger limits and contingency measures The designer should identify appropriate trigger limits for each construction stage, considering the range of possible ground behaviours and including any proposed modifications (where the wall is performing better than anticipated) or contingencies (where the wall is performing worse), see Table 7.5 and Figure 7.12.

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Table 7.5

Identification of trigger limits at each construction stage – ipso tempore OM (Approach C)

Trigger

Value chosen for limit δ

Green

δ ≤ proportion of predicted SLS characteristic movements Modifications possible. (see Figure 7.12).

Amber

Proportion of SLS characteristic movements defining ‘green’ trigger ≤ δ ≤ SLS characteristic movements.

Modifications not possible.

Red

δ > SLS characteristic movements.

Approach D additional measures required.

Action

The application of these to a multi-stage excavation is illustrated in Figure 7.12 with three possible scenarios: 1

Where the actual monitored movements at a particular stage are within the ‘green’ zone (ie much less than the SLS characteristic predictions). Under these circumstances, modifications are possible to the construction sequence to make savings.

2

Where the actual monitored movements at a particular stage are within the ‘amber’ zone after the initiation of the OM. Under these circumstances the construction should be completed according to the characteristic design construction sequence.

3

Where the actual monitored movements at a particular stage are more than those predicted adopting characteristic parameters. Under these circumstances Approach D additional measures will be necessary to prevent breach of a limit state.

Figure 7.12

Identifying trigger limits for multi-staged excavation – ipso tempore OM

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The trigger limits should be identified for each construction stage to enable appropriate and timely decisions and interventions to be made by the project team in relation to how the wall is actually performing and how movements are developing compared to the recalibrated and SLS characteristic predictions.

Step 4: Monitoring system As Step 4 of the ab initio OM approach.

Step 5: Remaining construction stages

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During subsequent excavation stages, monitoring of the wall and the surrounding ground (including any identified structures close to the wall) should be continued. Wall behaviour should be compared with the range of predictions from the progressively updated recalibrated ground and analytical models, and modifications or contingencies applied, as required.

7.4.4

Roles and responsibilities of project team

The adoption of the OM in the design of embedded retaining walls offers potential savings in both cost and programme. However, it does require a greater level of sophistication and co-operation among the project team than the conventional design by calculation approach. Before embarking upon a design using this method, it is imperative that all members of the design team have a good understanding of the overall objectives and key criteria and that each member of the project team is aware of individual and collective responsibilities. Only in this way can the OM result in a successful outcome for the project (Nicholson et al, 1999).

Procurement The OM can be incorporated into any contract, however it is likely to be most effective when used as part of a contractual arrangement that enables strong designer/contractor interaction from an early stage. In this way the contractor has committed to the methodology from the beginning and has a strong commercial interest in making it work.

Client The client has the most to gain from the use of the OM. In addition to the potential reduction in material and construction costs associated with reduced programme duration, the structure/facility can be brought into use more quickly with obvious fiscal benefits in terms of generating income streams, reduced interest payments on loans etc. Whichever procurement methodology is adopted, the client should ensure that the designer and contractor employed to deliver the project have the technical expertise and organisational capability and experience to complete the more demanding requirements of the OM. The design fees associated with this will be more than a conventional wall design due to the calibre of designer required and the extended period of their involvement in the project. However, potential savings on construction cost alone typically outweigh these extra design fees.

Designer The designer should be confident in their understanding of the ground behaviour at the site and the particular requirements of applying the OM. The correct application of the method requires the designer to have understood the range of potential behaviours of the embedded retaining wall and to have a set of contingency measures fully developed if any of these arise. The designer should remain an integral part of the project team until after completion of construction and should continue to work closely with the client and the contractor to ensure safety of the wall is maintained at all times.

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Contractor To implement the OM successfully, the contractor should: zz

remain engaged with the wall designer throughout the project programme

zz

be prepared to implement the contingency measures developed by the designer for the range of expected wall behaviours

zz

ensure the necessary instrumentation and monitoring equipment is protected and accessible at all times.

Successful implementation requires a strong interactive and professional relationship between the contractor and the designer.

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Approving and checking bodies When considering the design of an embedded retaining wall using the OM, it is imperative that any checking body (eg independent assessors, supervisors, or an organisation) has a level of technical understanding commensurate with that of the designer. The technical demands placed on them will be equal to those on the designer. The checker should be confident regarding the effectiveness of the contingency measures developed by the designer and adopted by the contractor, the movement trigger levels and response plan, and should remain involved throughout the construction period.

7.4.5

Summary of OM

The definition and use of the OM applied to embedded retaining walls is discussed in Sections 7.4.1 and 7.4.3 and summarised in Table 7.2.

Ab initio As stated in Section 7.4.1, the ab initio OM applies at the start of the project. Each step in its application is illustrated in Figure 7.7 and discussed in detail in Section 7.4.3. For clarity, these steps are summarised here. 1

2

The ground investigation data should be assessed to determine: a

characteristic parameters (Section 5.10)

b

most probable parameters representing a statistical average of the dataset with a 50 per cent probability of exceedance (Figure 7.8).

Carry out the following for each of the optimistic and the cautious approaches: Approach A a

Undertake calculations using DA1C1 and DA1C2 (Section 7.3.1) and, where applicable, the accidental design situation/progressive failure check (Section 7.3.3) adopting most probable parameters for the assumed construction sequence and propping/anchoring arrangement. The output from this will be the most probable design effects of actions (wall bending moments, shear and prop/anchor forces).

b

From the results of the calculations in Step a, determine the wall’s embedment depth.

c

Adopting the wall embedment depth determined in Step b, undertake a further calculation using most probable parameters, groundwater assumptions and actions with partial factors of unity applied throughout. The output from this calculation will be the most probable wall movements and associated most probable ground movements.

d

Structurally design the wall and its support system for the most probable design effects of actions (wall bending moments, shear forces and prop/anchor forces) obtained from the calculations in Step a (and c, if applicable, for the case of compliance with project-specific requirements for crack width criteria, permissible stress criteria etc).

e

For this most probable wall embedment and structural capacity/strength, derive and fully develop a modified construction sequence and wall support arrangement that confirms the

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adequacy of the wall embedment depth and its structural capacity/strength from the results of DA1C1 and DA1C2 calculations (Section 7.3.1).Where applicable, apply the accidental design situation/progressive failure check (Section 7.3.3) and SLS calculation (Section 7.3.2) using characteristic parameters. f

The output from the SLS calculation in Step e using characteristic parameters will be the characteristic wall movements and associated SLS characteristic ground movements.

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Approach B a

Undertake calculations using DA1C1 and DA1C2 (Section 7.3.1) and, where applicable, the accidental design situation/progressive failure check (Section 7.3.3) adopting characteristic parameters for the assumed construction sequence and propping/anchoring arrangement. The output from this will be the characteristic design effects of actions (wall bending moments, shear and prop/anchor forces).

b

From the results of the calculations in Step a, determine the wall’s embedment depth.

c

Adopting the wall embedment depth determined in Step b, undertake the SLS calculation as set out in Section 7.3.2 adopting characteristic parameters. The output from this calculation will be the SLS characteristic movements of the wall and associated SLS characteristic ground movements.

d

Structurally design the wall and its support system for the design effects of actions (wall bending moments, shear forces and prop/anchor forces) obtained from the calculations in Step a (and c, if applicable, for the case of compliance with project-specific requirements for crack width criteria, permissible stress criteria etc).

e

For this characteristic wall embedment and structural capacity/strength, derive and fully develop a modified construction sequence and wall support arrangement that confirms the adequacy of the wall embedment depth and its structural capacity/strength from the results of DA1C1 and DA1C2 calculations (Section 7.3.1). Where applicable, apply the accidental design situation/progressive failure check (Section 7.3.3) using most probable parameters. The designer should also undertake an additional calculation using most probable parameters, groundwater assumptions and actions with partial factors of unity applied throughout to obtain most probable movements.

f

The output from the additional calculation in Step e using most probable parameters will be ‘most probable’ wall movements and associated ‘most probable’ ground movements.

3

From the results of the calculations in Step 2 and the predictions of the most probable and SLS characteristic wall movements and associated ground movements, set appropriate trigger limits (eg as illustrated in Table 7.3 and Figure 7.9). These trigger limits should be identified at key construction stages to enable appropriate and timely decisions and interventions to be made by the project team. These will be in relation to how the wall is performing and how movements are developing compared to the most probable and SLS characteristic predictions.

4

Design and install the instrumentation and monitoring system for the wall so that any feedback received will allow assessment of the retaining wall’s actual performance to be made. This is likely to comprise:

5

218

a

inclinometers in the retaining wall to inform the wall’s deflected shape

b

surveying points on the ground or on structures located behind the wall

c

strain gauges on temporary props etc.

Once construction has started, at the identified key construction stages, compare the actual monitored movements with the trigger limits set in Step 3 and reassess the wall and ground behaviour in comparison with the most probable and SLS characteristic predictions. If appropriate (see Figure 7.9), carry out modifications or apply contingency plans to the wall’s support system as construction proceeds and revise movement predictions accordingly. Continue to monitor and assess the movements as they develop to ensure that the wall and ground movements remain as expected and in accordance with the revised predictions.

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Ipso tempore As stated in Section 7.4.1, the ipso tempore OM is initiated by the project team after installation of the wall (which has been designed in accordance with the design by calculation method described in Section 7.3 where calculations using DA1C1 and DA1C2 and SLS and, where applicable, accidental design situation/ progressive failure check calculations have all been undertaken using characteristic parameters) and after initial excavation has been completed in front of the wall. The key steps are discussed in detail in Section 7.4.3 and shown in Figure 7.11. For clarity, these steps are summarised here:

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1

2

Evaluation of wall performance, recalibration of ground and analytical models and determination of recalibrated parameters: a

the movements of the wall and the surrounding ground movements at the current stage of excavation (as assessed from the available monitoring data) should be compared with that predicted during the design process before construction starts (from the characteristic wall movements and associated characteristic ground movements estimated from the SLS calculations described in Section 7.3.2)

b

undertake an audit of the construction to establish the actual construction sequence and site operations up to the time of the assessment of wall performance in Step a and carefully address the questions listed in Section 7.4.3 ipso tempore Step 1

c

from the understanding gained in Step b, carry out back analysis of the observed (monitored) wall behaviour and recalibrate the ground and analytical models (as discussed in Section 7.4.3 ipso tempore Step 1) and determine recalibrated parameters.

Using the revised ground and analytical models recalibrated from Step 1, undertake the following calculations: a

confirm the adequacy of the installed wall embedment depth and its structural capacity/strength from the results of DA1C1 and DA1C2 calculations (Section 7.3.1) and, where applicable, the accidental design situation/progressive failure check (Section 7.3.3) using the recalibrated parameters determined in Step 1

b

update predictions of wall behaviour for the upcoming further excavation stages by undertaking an additional calculation using the recalibrated parameters, groundwater assumptions and actions with partial factors of unity applied throughout. The output from this additional calculation will be the designer’s updated recalibrated prediction of anticipated/expected wall movements and associated ground movements for the proposed construction sequence.

3

Set appropriate trigger limits (eg as shown in Table 7.5 and Figure 7.12), considering a range of possible ground behaviours and including any proposed modifications (ie where the wall is performing better than anticipated) or contingencies (ie where the wall is performing worse than anticipated). These trigger limits should be identified at key construction stages to enable appropriate and timely decisions and interventions to be made by the project team in relation to how the wall is performing and how movements are developing compared to the recalibrated and SLS characteristic predictions.

4

Design and install the instrumentation and monitoring system for the wall such that feedback is provided that will allow assessment of the retaining wall’s actual performance to be made. This is likely to comprise:

5

a

inclinometers in the retaining wall to inform the wall’s deflected shape

b

surveying points on the ground or on structures located behind the wall

c

strain gauges on temporary props etc.

During subsequent excavation stages, the monitoring of the wall and the surrounding ground (including any identified structures close to the wall) should be continued. As well as the behaviour of the monitored wall and the ground compared with the range of predictions from the progressively updated recalibrated ground and analytical models with modifications or contingencies applied, as required.

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7.5

STRUCTURAL DESIGN OF THE WALL

The structural design of the wall should conform to the relevant CoP for the particular material, namely EC2-1 for reinforced concrete, EC3-1 for structural steelwork and EC3-5 for steel piling. The design of the structural members should allow for the loads generated by the temporary and permanent construction stages in addition to the installation method. For pushed, driven or vibrated sections, the installation stresses generated should also be considered. For concrete cast in situ into a pre-formed hole, the reinforcement detailing should allow for the method of placement of the reinforcement and the concrete.

7.5.1

Steel sheet pile walls

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Construction issues For driven steel sheet piling, the forces induced during the driving process should not exceed the capacity of the section. It may be necessary to use a sheet pile section of greater thickness than required from the analysis of the section in service in order to withstand the driving forces. Pre-augering may allow a reduction in section size, but the designer must consider the effects this has on the surrounding soil and the risk of increased deflections. Driveability is usually assessed based on experience of driving sections into comparable soils and to similar depths. Guidance is provided in ArcelorMittal (2016) and in Clause 7.10.2.1 of BS 8002:2015.

Durability Steel corrosion rates are generally low and steel piling may be used for permanent works in an unpainted or unprotected condition. The degree of corrosion and whether protection is required will depend upon the working environment, which can vary along the length and depth of the pile and with time. Underground corrosion of steel piles driven into undisturbed natural soils that do not comprise peat and are not chemically contaminated is negligible. This is attributed to the low oxygen levels present in such undisturbed soils. Corrosion rates are higher where steel piling is exposed to atmospheric conditions, fresh water and marine environments. ArcelorMittal (2016) and EC3-5 (Clause 4.4) give corrosion rates for each of these natural environments. These are reproduced in Table 7.6. Table 7.6 Recommended value for the loss of thickness [mm] due to corrosion for piles and sheet piles in soils, with or without groundwater (after EC3-5) Required design working life

5 years

25 years

50 years

75 years

120 years

Undisturbed natural soils (sand, silt, clay, schist)

0.00

0.30

0.60

0.90

1.20

Polluted natural soils and industrial sites

0.15

0.75

1.50

2.25

3.00

Aggressive natural soils (swamp, marsh, peat)

0.20

1.00

1.75

2.50

3.25

Non-compacted and non-aggressive fills (clay, schist, sand, silt)

0.18

0.70

1.20

1.70

2.20

Non-compacted and aggressive fills (ashes, slag)

0.50

2.00

3.25

4.50

5.75

Notes 1

Corrosion rates in compacted fills are lower than those in non-compacted ones. In compacted fills the figures here should be divided by two.

2

The values given for five and 25 years are based on measurements, whereas the other values are extrapolated.

For steel piles exposed to fresh or seawater see Table 4.3 of EC3-5 and the NA. For steel piles embedded in disturbed soils, peat or chemically contaminated ground, corrosion rates will be higher than natural undisturbed soil – protection systems should be considered in such conditions. The rate of corrosion will depend upon the aggressiveness of the ground. This will be very site specific and appropriate specialist advice should be obtained.

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The effective life of unpainted or unprotected steel piling depends upon the combined effects of imposed stresses and corrosion. Guidance on measures to increase the effective life of steel piles is given in BS 8002:2015 and in ArcelorMittal (2016). These include: zz

Use of a heavier section to allow for additional steel thicknesses as a corrosion allowance – maximum corrosion seldom occurs at the same position as the maximum bending moment. So the use of a corrosion allowance can be a cost-effective method of increasing effective life.

zz

Use of higher grade steel – all sheet pile sections are usually available in steel grades according to BS EN 10248-1:1996 and should be checked with the manufacturer. In the UK and Ireland, the main steel grades are S355GP and S430GP. Special steel grades may be available on request, such as S460GP or ASTM A572/A572M-15, and steels with improved corrosion resistance, for example, ASTM A690/A690M-13a. The higher yield steels also benefit driveability. The designer may choose to allow for a lower yield stress in structural calculations for classification of the section and verification after corrosion is taken into account.

zz

Organic coatings – steel piles should be coated under shop conditions to achieve the required coating thickness in as few coats as possible. The designer should consider the risk of damage to coating during handling and driving, and specify provision for repairs on site if required before driving the piles below ground level.

zz

Concrete encasement – this may be used to protect steel piles in marine environments. The concrete cope can be extended to below the lowest low water level to provide protection over the splash and tidal zones.

zz

Cathodic protection – the design and application of these systems requires specialist advice.

Design For the resistance of a sheet pile section to the effects of actions imposed during all the construction stages, verification of the sheet pile section in bending, shear and buckling is carried out in accordance with EC3-5. It is necessary to check the classification of the sheet pile section, after corrosion is taken into account, complying with Clause 5.2 and Table 5.1 of EC3-5.

Figure 7.13

Design of sheet pile walls to EC3-5

EC3-5 allows the full plastic material properties of the steel to be mobilised together with redistribution of earth pressures to achieve more economic design. To enable the designer to identify whether elastic or plastic section properties may be used for verification of the section, sheet pile walls are divided into four classes, as shown in Figure 7.13 and described here: zz

Class 4 – sections that fail due to local buckling within their elastic capacity.

zz

Class 3 – sections that reach their elastic moment capacity. The stress distribution across the section is elastic. The yield stress is allowed to be reached in the extreme fibres of the section.

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zz

Class 2 – sections where elastic global analysis is necessary, but plastic section properties may be used to achieve structural economy.

zz

Class 1 – sections where plastic global analysis may be carried out involving moment redistribution and using plastic section properties for verification of the section. However, for sheet pile sections, the rotation capacity for a Class 2 section requires to be carried out in accordance with Annex C in EC3-5 to enable Class 1 confirmation of the section.

For sheet pile sections in bending and shear the design moment resistance (Mc,Rd) of Class 1 and 2 sections is determined by the equation given in Clause 5.2.2 of EC3-5:

(7.28)

and for Class 3 sections:

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(7.29)

where Wel is the elastic section modulus for a continuous wall Wpl is the plastic section modulus for a continuous wall γmo is a partial safety factor given in Clause 5.1.1 (4) of EC3-5 βB is a reduction factor that takes account of a possible lack of shear force transmission in the interlocks (see Table NA2 of the UK NA to EC3-5). It is possible to save up to 15 per cent in material use for Class 2 sections especially for temporary stages of construction where corrosion effects are minimal. It is recommended that sections are chosen to remain at least Class 3 after loss of thickness due to corrosion so that the verification to check the resistance of thin walled or Class 4 sections may not be required. In the UK or where difficult driving is anticipated (eg due to generally stiff or dense ground conditions), the sections should be checked for driveability. Further guidance is given in ArcelorMittal (2016). For interlocking sheet piles the shape of the interlock requires to be compliant with BS EN 10248-1:1996 to transfer structural forces to neighbouring elements within a wall. For cold-formed piles, in accordance with BS EN 10249-2:1996, there is no requirement to transfer forces between adjacent elements that may easily declutch or separate during a rigorous installation process. For hot rolled U-piles, the designer should also consider the reduction factor (βD) required for serviceability checks in the permanent condition after corrosion effects are taken into account especially for assessment of cantilever sheet pile walls. The full stiffness of the piles should be used when calculating the effects of actions in the wall analysis using computer-based subgrade reaction or finite element modelling. Combined structural aspects of sheet piling, effects of water pressure and guidance for design of sheet pile elements in combined walls are also given in EC3-5.

7.5.2 Cast in situ concrete Constructability issues The method of constructing the concrete member below the ground can affect the structural design.

CFA piling The use of CFA piling, where the pile is bored and concreted in a single operation as the auger is drilled and extracted, restricts the depth and reinforcement density of the pile. The reinforcing cage

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is pushed into the wet concrete. Soil conditions that allow any free water to flow out of the concrete will induce a premature set in the concrete and prevent installation of the cage. High reinforcement densities, and particularly links, will also restrict the installation depth. Typical installation depths vary from 12 m to 15 m although in some cases depths of up to 20 m can be achieved. Specialist advice regarding depth of cage and construction should be sought from the piling contractor. A small vibrator attached to the top of the cage may ease the installation, but the limitations of this method should be recognised by the designer.

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Bored piles and diaphragm walls The concrete used for piles and diaphragm walls is not usually placed with the use of a vibrator and should be self-compacting with the ability to flow around the reinforcement cage (see ICE, 2007). When a drilling fluid is used to provide temporary support for the bore, the concrete should displace the support fluid. Good quality concrete should be ensured throughout the section and particularly in the cover zone between the reinforcement and the ground. It is important to note that the concrete has to flow out from the centre of the section through the reinforcement cage to the cover zone and it is this concrete that is usually important for maintaining the protection to the reinforcement in the long term. BS EN 1536:2010+A1:2015 provides guidance on the size of tremie pipes and annuli for different pile diameters and aggregate sizes. Bundles and large diameter bars tend to be used more often in embedded wall concrete sections because of the need to provide a large clear space between bars to allow the concrete to flow into the cover zone. The reinforcement cage is fabricated above ground and then lifted and lowered into the bore, so it should be designed to allow for this lift. Long cages may need to be spliced over the bore due to lifting restrictions. The designer must also give consideration to the design of the pile cage reinforcement for the lifting and installation loads during cage handling and installation.

Pile cages BS EN 1536:2010+A1:2015 recommends that the minimum clear spacing between the vertical bars, or bundles of bars, should be 100 mm to ensure an adequate flow of concrete through the reinforcement cage. Provided the maximum size of the aggregates does not exceed 20 mm, the clear space between longitudinal bars or bundles of bars of one layer may be reduced to 80 mm over the lap length. However, reducing the clear spacing below 100 mm increases the risk of poor quality concrete in the cover zone. Multiple layers of reinforcement should be avoided. Steel sections (eg universal columns or hollow circular sections) may be used to achieve the required area of reinforcement, provided there is sufficient space to install these sections based on the construction tolerances that are applicable.

Diaphragm wall cages Space should be maintained around the tremie pipe positions to allow the pipe to be installed and withdrawn without snagging on the reinforcement. Typically a minimum clear spacing of 500 mm is given at the tremie positions. Large panels may require more than one set of tremie pipes to reduce the distance that the concrete has to flow within the section. The minimum link spacing required by the structural design codes may have to be compromised in order to insert the tremie pipe. The tremie pipe should be accommodated by providing the total number of links required by the code and adjusting the spacing locally to provide the 500 mm minimum clearance. There should be an unreinforced length of wall between adjacent panels to allow for tolerances when excavating them and to allow space for the joint detail and the waterbar where this is provided, typically 400 mm to 550 mm. The steel detailing should be adjusted to allow for the unreinforced section of the panel, which may be up to 25 per cent of the panel length for a single bite panel. BS EN 1538:2010+A1:2015 gives guidance on the clear spacing between bars to ensure an adequate flow of concrete through the diaphragm wall reinforcement cage. The final clear horizontal distance between vertical bars in a single layer should be 100 mm. This figure may be reduced to 80 mm over the lap length (although this should be avoided where possible, maintaining the 100 mm minimum clear spacing).

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Where two layers of reinforcement are required on a wall face, the bars in the inner layer should be aligned behind those in the outer layer to allow concrete to flow between them. The clear vertical distance between horizontal bars should be at least 200 mm where the distance between the vertical bars is 100 mm. The vertical distance between horizontal bars can be reduced to 150 mm if the spacing of the vertical bars is increased to give a window area of at least 0.02 m2 between the horizontal and vertical bars or 0.16 m2 over lap lengths. Where links are required the horizontal distance between legs of the links should be at least 150 mm.

Reinforcement design

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Design combinations of actions Where the wall piles are meant to carry vertical loads from the superstructure and/or basement slab levels the various actions should be given to the designer of the wall in the form of permanent (Gk) and variable (Qk) actions. The limitations of designing to Eurocodes based on safe working loads have been highlighted by Selemetas and Bell (2014). The wall designer should consider the combinations of actions as described in EC0 in order to establish the ultimate actions under DA1C1 and DA1C2. Equation 6.10 of EC0 (reproduced as Equation 7.30, without the pre-stressing component for clarity) can be used to express the combinations of actions for ULS design (see Clause 6.4.3.2 of EC0):

(7.30)

Design recommendations for the diameter of cast in situ piles EC2-1 Clause 2.3.4.2(2) recommends a reduction in the diameter of cast in situ piles without permanent casing for structural calculations. This reduction is only applicable, in the absence of ‘other provisions’, but for economy in design, the piling contractor may implement controlled construction processes, such that this diameter reduction is not required. Examples of these controls, which can be considered as other provisions include rig instrumentation, measurements of drilling tools, checks that the concrete over-break is positive ie the average diameter can be verified, use of spacers in piles, and the execution control measures recommended in BS EN 1536:2010+A1:2015.

Design recommendations for combination of shear and tension If there is any tension induced in the wall piles, through uplift pressures from connecting basement slabs, the designer must consider the combination of shear and tension at the slab level, where the maximum shear would be expected. EC2 uses a strut and tie model for the transverse shear in reinforced concrete, as explained in Clause 6.2 of EC2-1. When the member is in compression, EC2 allows the designer to vary the angle θ between the compression strut and the section axis perpendicular to the shear force, such that 1≤ cot θ ≤ 2.5. The designer needs to select an appropriate angle of θ depending on the loading conditions and must check that VRd,max is not exceeded, where VRd,max is the design value of the maximum shear force that can be sustained by the member, without crushing the compression strut. When shear co-exists with tension, the user should refer to the NA of EC2-1 for guidance on the selection of cot θ. Further design guidance is given in CEB-FIP (1993), where the inclination of the compression chord (θ) is expressed as a function of the longitudinal strain in the section, giving a gradual reduction in the value of cot θ as a function of the tension in the section.

Design lap lengths Where possible, laps should be avoided at areas of high moments/forces along the length of the piled wall. The conventional practice of providing a lap length of 40 x main bar diameter should be sufficient, provided the designer can demonstrate that this is adequate for the concrete grade used and the stress in the bar at the lap location (eg a lap length of 40D for bars up to 40 mm is sufficient if the stress in the bar is less than 75 per cent of the ultimate stress, the concrete grade is C32/40 and the clear bar spacing is greater than 110 mm).

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Where it is not possible to keep the laps away from high stress areas the designer can follow the design approach given in Clause 8.7.3 of EC2-1 to calculate a full tension lap length. In most cases the lap length should be calculated assuming ‘good’ bond conditions, if the cover to the main bar is twice the diameter of the main bar (see Jones and Holt, 2004) and the walls are constructed in-line with the execution requirements of BS EN 1536:2010+A1:2015 and BS EN 1538:2010+A1:2015. For the calculation of the basic anchorage length (Clause 8.4.2 of EC2-1), the designer should note that large diameter bars are defined as bars with diameter greater than 40 mm (UK NA to EC2) (the coefficient related to the bond condition η2 =1.0 for ϕ = 32 mm and ϕ = 40 mm).

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The designer should consider, in-line with CDM 2015, the health and safety risks associated with laps in excess of 40 x bar diameter. This is particularly relevant for large diameter bars, where laps in excess of 40 x bar may require site operatives working from height. For economy in design, the designer may choose to calculate the lap length required to transfer the actual stress in the bar based on the magnitude of bending moment and axial force at the lapped section. In this case they must also consider the additional tensile force in the longitudinal reinforcement due to the ULS shear force (VEd) at the lapped section (see Clause 6.2.3 (7) of EC2-1). Clause 8.8 of EC2-1 recommends that all bars larger than 40 mm diameter be joined using mechanical couplers rather than by means of lapping, unless the section diameter is equal or greater than one metre or where the reinforcement stress is no greater than 80 per cent of the design ultimate strength. Where the design makes use of bundles of bars, Clause 8.9.1(2) of EC2-1 restricts the use of bundles to a maximum equivalent diameter of 55 mm.

Significance of cracking Cost savings are possible if a pragmatic approach is taken to crack width control. A designer concerned about a crack width of 0.3 mm may find more reasons to be concerned if excessively congested reinforcement, designed to control crack widths, creates large voids in the concrete due to insufficient flow of concrete between the bars. The designer must also consider the health and safety implications associated with specifying excessive reinforcement to control crack widths. The specification of a maximum crack width in a reinforced concrete section is a serviceability consideration and usually arises from concerns about durability, watertightness and aesthetics. Before making an assessment of what is considered to be an acceptable crack width, the designer should consider the exposure class and corrosion conditions at a given crack location, the ground conditions (effect of permeability), and the bending moment distribution (whether the section will be in compression or tension at that section). For an economic design outcome, the designer should consider crack widths as a function of construction time as well as depth, eg it may be acceptable to have a larger crack width during the transient design situation (temporary construction case), but then have a tighter limit in the persistent design situation (ie long term). Tyson (1995) states that except in extreme exposure conditions such as chemically-aggressive ground, buried steel below a standing water table is not subject to any significant corrosive activity. Above the water table, corrosion can only occur under a combination of conditions: zz

a continuous exposed metal surface at the anode

zz

a ready supply of oxygen at the cathode

zz

an electrolyte to carry current between the anode and cathode.

The absence of any of these factors, or the presence of any concrete cover that leads to alkaline conditions where oxidised solids, hydroxides or basic salts can form, will severely inhibit corrosion. Tyson (1995) argues that “in the case of a fully embedded pile, ready access to oxygen is restricted to perhaps the upper metre or so, through shrinkage cracks” and concludes that “corrosion below about 1 m below ground is therefore likely to be initially slow and, once started, quickly stopped by the deposition of solids”. The location and orientation of a crack is more important than its size. Cracks that are in-line with the reinforcement,

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eg along the lines of the links, may cause corrosion, in contrast to cracks transverse to the main reinforcement.

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Hartt (2009) considers four zones with reference to the significance of cracking on the corrosion of reinforcement in a reinforced concrete element. The application of these zones for the case of an embedded reinforced concrete piled wall is shown in Figure 7.14. These zones are briefly discussed here: zz

Zone 1 (wet) – groundwater with chlorides and sulphates can penetrate to the full depth of cracks, however the conditions are not prone to corrosion of the reinforcement as the dissolved oxygen concentration is zero in permanently submerged wet ground.

zz

Zone 2 (wet and dry) – the ground is Corrosion zones for the assessment of crack width subjected to cycles of wet period when Figure 7.14 control criteria the water table is high and dry periods when the water table is low. Chlorides may accumulate within concrete cracks during the wet period and can react with dissolved oxygen, which is available when the water table is low. This zone is the most prone to corrosion, the extent of which will depend on the concrete quality and cover to the reinforcement.

zz

Zone 3 (dry) – chlorides and sulphates from the ground may diffuse into the concrete along the crack faces. This is likely to be more pronounced when the ground is moist from rain, so the risk of corrosion here is less than for Zone 2.

zz

Zone 4 (atmospheric) – chlorides and sulphates are present on the exterior concrete surface through contact by airborne particles. The detrimental ions can then diffuse into cracks and potentially initiate corrosion. The extent of corrosion over this zone will depend on the waterproofing provision in front of the piled wall.

Durability Durability resistance is determined by the cover and mix design. The durability requirements are a function of the exposure classes given in Clause 4 of EC2-1 (see Table 7.7). The minimum cement content should be in accordance with BS EN 206:2013. Chemical analysis of the ground and groundwater should be made to assess its sulphate content and the concrete mix should be designed to the requirements of BRE (2005). The concrete cover should be in-line with Clause 4.4.1 of EC2-1. Walls subject to splashing or intermittent wetting by saline water should have adequate resistance to chloride attack and may need protection by a waterproof membrane.

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Table 7.7 Class

Exposure classes related to environmental conditions (after EC2-1)

Description

No risk of corrosion or attack X0

Unreinforced concrete or embedded metal where there is no significant freeze/thaw, abrasion or chemical attack

Corrosion induced by carbonation XC1

Dry or permanently wet

XC2

Wet, rarely dry

XC3

Moderate humidity

XC3

Cyclic wet and dry

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Corrosion induced by chlorides other than from seawater XD1

Moderate humidity

XD2

Wet, rarely dry

XD3

Cyclic wet and dry

Corrosion induced by chlorides from seawater XS1

Exposed to airborne salt but not in direct contact with seawater

XS2

Permanently submerged

XS3

Tidal, splash and spray zones

Freeze/thaw with or without de-icing agents XF1

Moderate water saturation without de-icing agent

XF2

Moderate water saturation with de-icing agent

XF3

High water saturation without de-icing agent

XF4

High water saturation with de-icing agent or seawater

Crack width calculations in accordance with EC2 Crack width control requirements and calculations principles are covered in Section 7.3 of EC2-1. Table NA.4 of the UK NA to EC2-1 recommends a crack width limit of 0.3 mm for all exposure classes, except for classes X0 and XC1, where this limit may be relaxed in the absence of appearance conditions and where the crack width has no influence on durability. Where crack width calculations are required, these should be based on the bending moment distribution derived from an SLS calculation assuming the quasi-permanent combination of actions. Quasi-permanent loading for an embedded retaining wall is the output from a typical SLS analysis that includes earth pressures plus any applied permanent surcharges and variable surcharges reduced as per Clause 6.5.3(2c) of EC0. Specific problems associated with calculations of crack widths to EC2 for circular sections have been reported by Kaethner (2011). Apart from the specific modelling difficulties when dealing with nonstandard sections (eg circular sections), complications have also been reported for rectangular sections with two rows of bars or large covers. A refined model for the calculation of crack widths for circular sections and irregular bar layouts is given in Clause 2.22 of PD 6687-1:2010.

Crack control without direct calculation in accordance with EC2 For simplicity in design, where there is a requirement for crack width control, the designer may make use of the ‘deemed to satisfy’ requirements given in Tables 7.2N and 7.3N of EC2-1. These requirements are given in the form of either maximum bar size or maximum bar spacing corresponding to various values of steel stress. The steel stress should be calculated based on the cracked section properties under the relevant combination of actions.

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Watertightness Any size of crack that passes through the section may let in water. However, wall flexure will normally cause a compression zone (provided it is not combined with significant tension), which should prevent water passage. For water-retaining structures, water leakage will affect the function of the structure and cracks of certain size may become unacceptable. EC2-3 gives specific guidance on the crack control requirement for liquid retaining and containment structures (see Table 7.8).

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Table 7.8 Crack width control criteria as a function of water tightness class for liquid retaining and containment structures (after EC2-3) Tightness class

Requirements for leakage

0

Some degree of leakage acceptable, General provisions for crack control (Clause 7.3.1 of EC2-1). or leakage of liquids irrelevant.

1

Leakage to be limited to a small amount. Some surface staining and damp patches acceptable.

2

Cracks that may be expected to pass through the full thickness of the Leakage to be minimal. Appearance section should generally be avoided unless appropriate measures (eg not to be impaired by staining. liners or water bars) have been incorporated.

3

No leakage permitted.

Measures for meeting crack width control criteria

For full thickness cracks the crack width is limited to 0.2 mm where the pressure gradient is 5 or less. The crack width limit is reduced to 0.05 m where the pressure gradient is more than 35, with interpolation for intermediate values (unless given in the UK NA to EC2-3).

Special measures (eg liners or prestress) are required to ensure watertightness.

Aesthetics There is no universal agreement as to what constitutes an aesthetically acceptable crack width as it is very subjective to human perception and the long-term use of the retaining wall. Campbell-Allen (1979) proposed a graphical aid for determining what is an aesthetically-acceptable crack width as a function of the nature of the structure and the viewing distance. For most practical applications where an internal facing wall will be in place in the long term in front of the piled retaining wall, it is expected that the design of the reinforcement will not be dictated by the perception of aesthetically-acceptable crack widths.

Autogenous healing of cracks The function of autogenous healing of cracks, due to formation of calcium carbonate crystals (CaCO3), is well recognised and it is generally accepted that a crack width of 0.2 mm or less is very likely to be selfhealing provided the pressure gradient across the section is not greater than 5. EC2-3 provides further guidance linking the extent of autogenous healing with the pressure gradient.

7.6

KEY POINTS AND RECOMMENDATIONS

1

The limit state design philosophy described in Chapter 2 and summarised in Section 7.2 should be adopted. Design calculations should satisfy the ULS of wall stability and structural strength and the required SLS. The designer should demonstrate that exceedance of ULS and SLS is sufficiently improbable in the envisaged design situations.

2

Embedded retaining walls are geotechnically designed in accordance with EC7 and the associated NA. The use of ‘prescriptive measures’ or ‘experimental models and load tests’ are not considered appropriate for the design of embedded retaining walls, and so the choice is limited to the use of calculation or the OM. See Sections 7.3 and 7.4.

3

The wall embedment depth should be the deeper of that required to satisfy:

228

a

load-bearing capacity

b

hydraulic cut-off and uplift

c

global stability

d

lateral stability.

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The determination of the required wall embedment depth and associated effects of actions (wall bending moments, shear forces and prop/anchor forces) will depend on the method of analysis adopted by the designer and the type of wall under consideration.

4

UK NA to EC7-1 requires that the design of geotechnical structures by use of calculation should be undertaken using DA1. Two combinations are used to check the ULS: a

DA1C1 – where partial factors greater than unity are applied to unfavourable variable actions and to the effects of actions, in conjunction with partial factors of unity applied to characteristic soil and rock parameters

b

DA1C2 – where partial factors greater than unity are applied to unfavourable variable actions and to characteristic soil and rock parameters, in conjunction with partial factors of unity applied to the effects of actions



Where the wall embedment depth is governed by lateral stability considerations, it will typically be determined from the DA1C2 calculation. Where SSI analysis is undertaken, conventional practice in the UK is to adopt the wall embedment depth calculated in DA1C2 (or based on the considerations listed in Step 3, if these require a deeper wall) in the DA1C1 calculation. Where simple limit equilibrium analysis is undertaken, this is not possible, as the wall would not be in equilibrium under the revised limiting lateral stresses associated with the differently factored characteristic soil and rock strengths. So, limit equilibrium analyses for the two combinations must be carried out completely separately. The DA1C1 limit equilibrium analysis will result in a smaller depth of wall embedment compared to the corresponding DA1C2 calculation. To account for this, the DA1C1 calculated wall bending moments may be modified to account for the reduction in wall length compared with the DA1C2 calculation, as shown in Figure 7.2.

5

DA1C1 and DA1C2 are both ULS calculations and so, if required, a SLS calculation and/or an accidental design situation/progressive failure check should be undertaken separately. A SLS calculation adopting characteristic parameters and similar calculations adopting most probable or recalibrated parameters will be necessary if the OM is applied.

6

The SLS for an embedded retaining wall should be explicitly considered by the designer where: a

the wall is required to satisfy criteria that necessitates undertaking SLS calculation, eg crack width criteria for reinforced concrete walls, allowable stress criteria for steel sheet pile walls

b

wall deflections and associated ground movements are of importance and their assessment is required



The wall deflections and associated ground movements that are estimated as part of a SLS calculation will be characteristic movements, not average or most probable values that might be used as part of the OM. Such predicted movements should be larger than those to be expected or measured in practice.

7

For walls that are critically dependent upon their support systems for lateral stability and where there is a possibility that part of this support may be affected by an accidental action, the designer should carry out a risk assessment, as discussed in Chapter 2. The aim should be to avoid the possibility of such an accidental occurrence through design changes, construction procedural controls etc. If this does not adequately mitigate the risk, the designer should carry out a calculation to explicitly show that progressive failure will not occur under such circumstances.



The output from the accidental design situation/progressive failure check calculation is a set of design effects of actions (wall bending moments, shear forces and prop/anchor forces) that compare with the DA1C1 and DA1C2 design effects of actions. These are used in the structural design of the wall and its support system.

8

Following consideration of the issues listed in point 3 and completion of the ULS (DA1C1 and DA1C2) and (if required) SLS calculations for each design situation (transient, persistent, and where applicable, accidental and seismic), the designer should have the following output: a

embedded depth of wall

b

ULS design effects of actions (wall bending moments, shear forces and prop/anchor forces), taken as the greater of the values of the design effects of actions calculated from DA1C1 and DA1C2 and, where applicable, the accidental design situation/progressive failure check calculations

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c 9

if required, SLS design effects of actions (wall bending moments, shear forces, and prop/anchor forces), characteristic wall deflections and associated ground movements.

Structurally, the wall should be designed in accordance with EC2 and/or EC3 adopting the greater values of the effects of actions determined from Step 8b, and from the SLS design effects of actions (Step 8c), where the wall is required to satisfy project specific criteria (eg crack width criteria for reinforced concrete walls, allowable stress criteria for steel sheet pile walls). Detailed guidance is provided in Section 7.5.

10 EC7 is the first design standard in the UK that explicitly includes provision for the use of the OM. Its use offers potentially significant savings in construction programme and costs as well as a rigorous and clear allocation and treatment of ground-related risk.

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11 The OM can be divided into two categories:



a

ab initio – where the use of the OM is planned from the start of the project before the embedded retaining wall is installed

b

ipso tempore – where the design of the embedded retaining wall is reassessed during construction after its installation

The implementation of the OM is discussed in detail in Sections 7.4.1 to 7.4.4 and summarised in Section 7.4.5. Before embarking upon a design using this method, it is important that all members of the design team have a good understanding of the overall objectives and key criteria, and that each member of the project team is aware of individual and collective responsibilities. Only then can the OM result in a successful outcome for the project (Nicholson et al, 1999).

12 The structural design of the wall should conform to the relevant CoP for the particular material (eg EC2-1 for reinforced concrete and EC3-5 for structural steelwork). The design of the structural members should allow for the loads generated by the temporary and permanent construction stages in addition to the installation method. For pushed, driven or vibrated sections, the installation stresses generated should also be considered. 13 Where steel sheet piling is installed in circumstances where difficult driving conditions are not anticipated and where corrosion effects are minimal (eg during temporary stages of construction), cost savings may be realised by mobilising the full plastic material properties of the steel (as permitted by EC3-5) together with SSI analysis (redistribution of earth pressures) to achieve more economic design. However in the UK or where difficult driving conditions are anticipated, the sections should be checked for driveability (see ArcelorMittal, 2016). 14 For reinforced concrete sections, cost savings are possible if a pragmatic approach is taken to crack width control. The specification of a maximum crack width in a reinforced concrete section is a serviceability consideration and usually arises from concerns about durability, watertightness and aesthetics. Before making an assessment of what is considered to be an acceptable crack width, the designer should consider the exposure class and corrosion conditions at a given crack location, the ground conditions (effect of permeability), and the bending moment distribution (whether the section will be in compression or tension at that section). For an economic design outcome, the designer should consider crack widths as a function of construction time as well as depth, eg it may be acceptable to have a larger crack width during the transient design situation (temporary construction case), but then have a tighter limit in the persistent design situation (long term). The location and orientation of a crack is more important than its size (see Section 7.5.2). 15 Where crack width calculations are required, these should be based on the bending moment distribution derived from an SLS calculation assuming the quasi-permanent combination of actions, as described in Clause 6.5.3(2) of EC0. Specific problems associated with calculations of crack widths to EC2 for circular sections have been reported by Kaethner (2011). Apart from the specific modelling difficulties when dealing with non-standard sections (eg circular sections), complications have also been reported for rectangular sections with two rows of bars or large covers. A refined model for the calculation of crack widths for circular sections and irregular bar layouts is given in Clause 2.22 of PD 6687-1:2010. 16 The support system to the wall should be designed in accordance with the guidance provided in Chapter 8.

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8 Design of temporary support systems This chapter is intended primarily for geotechnical and structural designers involved in the design of temporary support systems to the wall.

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This chapter aims to provide: zz

design guidance on both the geotechnical and non-geotechnical actions applied to temporary propping systems

zz

guidance on the structural design of temporary props

zz

recommendations by which earth berms may be represented in limit equilibrium and SSI analyses using subgrade reaction and pseudo-finite element methods

zz

guidance on how to estimate wall deflections due to berm removal

zz

information on the type and typical characteristics of ground anchors

zz

key considerations for the design of ground anchorages and where detailed guidance can be obtained for their design and construction, recognising that this is a highly specialised area.

This chapter should be read in conjunction with Section 3.3.

8.1

PROPPING SYSTEMS

Temporary props are used extensively in bottom-up construction sequences. Steel sections, typically tubular, box and universal columns sections are commonly used. However there are a number of highcapacity modular reusable systems available today as a more sustainable alternative to traditional site fabricated steel. Modular systems may also incorporate hydraulic actuation to accommodate on-site adjustment and pre-stressing (see Figure 8.1). In top-down construction, permanent propping is usually provided by concrete floor slabs, although steel props may also be used as part of a top-down sequence. Floor slabs are generally very stiff (except where significant openings are made in the slab) and of adequate capacity to support the temporary loads arising during construction. Where propping forces are transferred through a permanent slab, it is important to establish a robust load-path through the permanent slab, particularly where the slab changes in level. Design of these slabs will be covered in the design of the permanent works, so this section concentrates on temporary propping only. The cost of the temporary propping system is usually small in comparison with the cost of the retaining wall. However, the expense of the delay and disruption to the excavation caused by badly planned and executed temporary props may be significant, and such activities should be carefully considered Figure 8.1 Modular steel props supporting a diaphragm and explicitly included in the design, planning and wall in a top-down sequence and a piled wall in a bottom-up sequence (courtesy Groundforce) programming for the construction works.

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Guidance on the procedural control of temporary works is given in BS 5975:2008+A1:2011. While efficient and appropriate design of the propping system should be the aim, major reductions in overall construction costs will not be achieved if the construction programme overruns, particularly if props are supplied on a rental basis. Economy can be achieved by reducing the number of props or prop levels, through the application of the OM (see Section 7.4). Temporary props are usually robustly designed owing to the risks inherent in their use. Failures are rare and are generally the result of poor detailing and lack of consideration in the design of the end connections, misjudgement of ground conditions or accidental strike (Twine and Roscoe, 1999). Failure resulting in the loss of a prop can lead to progressive collapse of the wall. This should be explicitly addressed in design (see Section 7.3.3) and is considered further in Section 8.1.1 from a design responsibility viewpoint.

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8.1.1 Design responsibility As discussed in Chapters 2 and 7, clear allocation of design responsibility is essential particularly where specific design activities are carried out by subcontract or third party designers. It is current practice for the load/resistance aspects of temporary works design to be carried out by different design organisations. For example the geotechnical analysis may be done by a consultant or a contractor’s engineers with the design of the supporting framework carried out by engineers employed by specialist suppliers. So, it is important that for a full understanding of loading information, both the SLS and ULS loadings are provided to third party design organisations. It is also important that the temporary works designers are made aware of the performance requirements of the permanent works (eg the temporary wall/prop deflections do not compromise the permanent works). Temporary works propping systems must take into account buildability, ie that the site operations are not unduly constrained or hampered. The temporary works designer should take account of how the permanent works will be constructed and the preferred method of prop removal. The PD (who is a key dutyholder under CDM 2015, see Chapter 2) should ensure that propping and anchoring levels are discussed and agreed between the main contractor and the element designer and that consideration is given to installation and removal of props (and stressing and de-stressing of ground anchors). This needs to be done to ensure that props can be removed (and anchors de-stressed) safely. Where the permanent wall is used to provide temporary support during construction, the situation is more complex. The designer of the temporary propping system should consider the effects of load transfer on the permanent wall due to the installation and removal of the temporary props. In this instance, the designer of the permanent wall should inform the designer of the temporary props about the assumptions made regarding temporary propping in the design of the permanent works, eg propping levels and spacing, installation and removal sequence as well as critical wall deflection limitations that will dictate prop stiffness and pre-load. Where the designer of the temporary props is unable to fully comply with the assumptions made by the designer of the permanent wall, the lead designer (see Chapter 2), should co-ordinate the necessary interaction between the designers to ensure continuity of information. The design of individual props should be robust. For economy in design, it is advisable that control measures are implemented to prevent the accidental loss or damaging of a prop. Where these measures are not in place, the designer should consider the implications of the accidental loss of a prop in two ways: 1

Incorporate the loss of a prop in the design of the wall support system in the calculations undertaken in accordance with the guidance provided in Chapter 7.

2

Adequately mitigate the risk of accidentally damaging or removing a prop through design changes and a robust construction management strategy as discussed in Chapter 2.

The load transfer from the temporary props may induce high local stresses in the permanent works. Openings in the permanent works slabs may require temporary propping until the structure is completed.

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8.1.2 Prop stiffness

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Where spring-type models are used to model the SSI mechanics of a propped wall, the user is required to define the prop stiffness, Kp . Often, the wall designer will not be in a position to know the exact propping arrangements at the time of designing the wall so they would need to assume an effective prop stiffness based on previous experience. Where proprietary hydraulic prop systems are used, the prop stiffness is governed by the hydraulic system rather than the steel section unless a mechanical lock-off is used. Due to the variety of propping systems available, equipment suppliers should be consulted to ensure that the correct prop stiffness values are used as these may have a significant influence on the analysis of the retaining wall and resulting prop forces. The lead designer should note that there is an element of co-ordination required between the wall designer and the temporary works designer. If they are from different organisations, the bending moment distribution in the wall and prop loads may change depending on the prop stiffness used. For a prop modelled as a horizontal elastic support the spring stiffness can be expressed as follows:

(8.1)

where A is the prop cross-sectional area E is the Young’s modulus of the prop material s is the spacing between props L is the ‘free length’ of the prop, ie the distance between the wall and the point at which is the prop is rigidly supported (for symmetrical walls, the ‘free length’ is half the length of the prop). This type of analysis assumes that the waling beam spanning between props is of infinite stiffness, which effectively relies on the ground redistributing the load to the each prop. If the sensitivity of the calculation to the flexibility of the waling beam is required, elementary beam bending deflection equations can be used to estimate the reduction in stiffness in between prop locations. For example, for a simply supported waling beam, the stiffness of the beam can be defined as:

(8.2)

where Ew, Iw are the Young’s modulus and second moment of area of the beam respectively d is the length of the simply supported beam between props. The combined stiffness of the prop and waling beam system (Kc) can be expressed as:

(8.3)

where β is a factor allowing for the redistribution of earth pressure from the waling beam to the prop locations. Borin (2007) recommends a factor of 0.5 or less unless most of the load is due to water pressure and cannot be redistributed For an infinitely stiff waling beam, β = 0, which gives Kc = Kp .

8.1.3 Prop selection considerations In addition to the required structural resistance, the selection of the prop type and its subsequent design will be influenced by the following considerations: zz

span requirements

zz

on site fabrication facilities

zz

material availability and lead time

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zz

buildability and sequencing

zz

requirement for pre-stressing

zz

stiffness, resistance to deflection

zz

operational requirements such as installation, removal and possible reuse

zz

programme duration for the temporary work.

In most cases a value engineering exercise will be carried out to assess all relevant factors so as to produce the most practical and economical solution.

8.1.4 Design actions on props

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When considering a clear spanning temporary prop, there are a number of fundamental characteristic actions that also need to be checked in combination. These actions are shown in the Figure 8.2.

Notes 1 The characteristic prop self-weight is taken as its total weight applied across its entire length as a uniformly distributed load (UDL) and is considered as a permanent (Gk) action. 2

The characteristic geotechnical loading is considered as a permanent (Gk,GEO) action.

3 The characteristic thermal loading (see Box 8.1) is derived from a temperature increase, and a coefficient of wall restraint, and is treated as a variable (Qk,temp) action. 4 A characteristic accidental point load (typically 10 kN to 50 kN from Williams and Waite, 1993) should be positioned to act at the worst case location. When considering buckling, this is usually acting vertically at mid-span. Accidental loading is considered as a variable (Qk,accidental) action. 5 Unless moment-free bearing details are employed, an eccentricity of load application (e) should be considered. The magnitude of this eccentric load can be taken as equal to 10 per cent of the height of the bearing plate in contact with the wall (or waling beam), unless twin waling beams are used.

Figure 8.2

Schematic diagram of actions to be considered for prop design

The design load acting on the props will depend upon the analysis method adopted in the calculations for the design of the wall. Prop loads calculated from limit equilibrium analysis may be unconservative, as the effects of SSI are not included (Section 4.2). In such circumstances, the calculated prop loads should be increased by 15 per cent to allow for the effects of stress redistribution and arching behind the wall in the case of multiple props (this is introduced as a model factor γSd equal to 1.15 discussed in Section 8.1.6). Where the wall is propped by a single prop, due to its criticality in supporting the wall it is recommended that the prop loads calculated from limit equilibrium analysis are increased by 30 per cent (this is introduced as a model factor γSd equal to 1.3 discussed in Section 8.1.6). SSI methods (undertaken as described in Section 4.2) that allow stress redistribution to model more realistically the non-linear pressure profile behind the wall should provide calculated values of prop loads that better represent the particular project circumstances modelled. Irrespective of the type of analysis undertaken, the calculated prop loads should be checked for adequacy by comparing them with those derived from comparable experience. Where possible, this should be based on reliable field measurements from case study data in comparable conditions. In situations where the calculated prop loads differ significantly from those derived from experience of comparable construction, the designer should carefully investigate and understand the reasons for the calculated values. Typically, this will involve carrying out a detailed review of the assumptions made in the calculations as well as sensitivity analyses. The outcome of such investigations should enable the designer to adopt appropriate values for use in the design of the props.

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8.1.5 Eurocode methodology for the structural design of props The structural design resistance of the props should be calculated generally in accordance with Sections 6.2 and 6.3 of EC3. Unless the prop span is short, resistance to buckling will generally be the critical failure condition. However, local effects should be considered carefully to verify adequacy of end connection details and in cases where proprietary hydraulic struts are used, the axial resistance of the hydraulic ram itself must be verified. Where Class 4 (or slender) sections are used as props (as is often the case with proprietary equipment), effective section properties for cross-sectional area and section modulus should be used in the analysis.

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Prop design should be carried out in accordance with Section 6.3.3 of EC3 (members resisting both compression and bending forces) using either Annex A or B to determine the relevant interaction factors. Note that this method makes allowances for initial bow imperfection in the strut and includes second order analysis. If this methodology is adopted, no explicit check on second order effects needs to be carried out.

8.1.6 Combination of actions for prop design Permanent and variable loads can be combined in the following two load cases (LC1 and LC2). Note that thermal loading is considered to be the leading variable load in all cases. Where applicable, an accidental load combination (LC3) is also considered. The most onerous load case is used to design the prop: zz

LC1 considers fully factored permanent loading combined with a reduced factored thermal load.

zz

LC2 considers reduced factored permanent loading with a fully factored thermal load.

zz

LC3 considers reduced factored permanent and thermal loads with an unfactored accidental load.

These combinations of actions can be represented as follows: LC1: γg . Gk + γg . Gk,GEO + γQ . ψ0 . Qk,temp (8.4) LC2: γg . Gk . ξ + γg . Gk,GEO . ξ + γQ . Qk,temp (8.5) LC3: Gk + Gk,GEO + Qk,temp . (ψ1,1 or ψ2,1) + Qk,accidental (8.6) Note values for the γ, ξ and ψ factors are given in the NA to EC0. The component of ULS structural prop actions (Ed) derived from the wall design calculations should be determined to be the greater of Equations 8.7 and 8.8:

(8.7)

where PSLS is the SLS action derived from either SSI analysis or other methods (limit equilibrium or distributed load method) γG is the factor for permanent unfavourable actions (γG =1.35) γSd is the model factor for stress redistribution effects (γSd =1.0 for numerical element analysis and pseudo-finite element analysis where stress re-distribution is modelled and γSd =1.15 for limit equilibrium analysis). Where the wall is supported by a single prop and analysed by limit equilibrium methods, the recommended value of γSd is 1.30.

(8.8)

where PULS,d is the maximum design effect of actions from DA1C1 or DA1C2 calculations (Section 7.3.1) (and where applicable, the accidental design situation/progressive failure check calculations, Section 7.3.3) using either SSI analysis or other methods (limit equilibrium or distributed load method).

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If the prop forces are provided by the wall designer to another party to design the propping system, it is important that the results from the DA1C1, DA1C2 (and where applicable, the accidental design situation/progressive failure check calculations) and SLS analyses are provided separately and unambiguously to avoid confusion. The designer may also use the distributed prop load (DPL) method for the design of temporary props (see Appendix A8).

8.1.7

Temperature effects on props

An increase or decrease in the temperature of a prop from its installation temperature will cause the prop to expand or contract according to the relationship:

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(8.9)

where ΔL = change in prop length a = thermal coefficient of expansion for the prop material Δt = change in prop temperature from the installation temperature L = prop length. If the prop is restricted or prevented from expanding freely, an additional load is generated in the prop. For a uniform cross section steel prop the magnitude of this additional load is:

(8.10)

where E = Young’s modulus of the prop material A = cross-sectional area of the prop β = percentage degree of restraint of the prop (based on back analysis of field case studies (Richards et al, 1999, Batten and Powrie, 2000, Ivanova, 2012, Chambers et al, 2016) it is recommended that 50 per cent is used for stiff walls in stiff ground and 30 per cent is used for flexible walls in stiff ground). The designer should select the appropriate value of β to suit particular project circumstances. Consideration should also be given to the fact that different levels of props are likely to be exposed to different temperature variations. The temperature range is expected to reduce with depth, as the distance from the ground surface increases, but the value of β may increase with depth (see Powrie and Batten, 2000b). Values other than those generally recommended here may be applied where the designer is confident that such values can be justified, eg based on comparable experience. For example, the use of hydraulic proprietary props could justify a lower value of Qk,temp due to their lower inherent stiffness when compared to a fabricated steel prop. Thermal loading is considered as a variable load and is factored according to the relevant NA to EC0.

8.1.8 Sway effects The designer should allow for any imbalance in horizontal loading across the excavation. This imbalance will cause the entire retaining wall and its support system to sway towards the side with the lower external load. Props may be inclined downwards from the high load side – the resulting vertical component of force should also be taken into account in design. Sway will increase ground movements on one side and reduce them on the other side (see Figure 8.3). The designer should ensure that the walls are designed to accommodate the associated pressures. The use of simple analytical techniques can grossly overestimate effects, as highlighted by Loveridge et al (2008). Simple calculations involving the application of larger predicted prop load from one wall to the opposite wall take no account of the movement at the prop level, which takes place in the form of prop shortening and by sway, as shown in Figure 8.3. For structures where significant sway movements are anticipated, the use of a finite element analysis would result in more realistic predictions of the interaction between the two walls.

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Figure 8.3

Definition of sway (after Loveridge et al, 2008)

Box 8.1 Back analysis of temporary prop forces from a deep excavation in Paddington Station, London (after Chambers et al, 2016) Measurements of prop loads capturing the effects of temperature were carried out during construction of the Paddington station box in London. The box is 260 m long, 25 m wide and 23 m deep and was constructed with 1.2 m thick diaphragm walls. The excavation took place following a top-down sequence with 50 proprietary hydraulic props. Forty-two MP500 props spanned the width of the excavation and two MP250 props were used in a raking configuration in each corner. All of the props were connected to 900 mm × 600 mm fabricated plate girder walers (see figure).

Hydraulic props and plate girder walers used at Paddington station box The box was constructed using the following top down method (mTD = mOD + 100 m): 1

Install southern diaphragm wall from level +124.000 mTD to +85.5 mTD.

2

Install northern diaphragm wall from level +123.175 mTD to +85.5 mTD.

3

Excavate to roof slab at level +120.0 mTD.

4

Install roof slab at +120.0 mTD.

5

Excavate to level +113.6 mTD.

6

Install temporary propping at level +114.5 mTD.

7

Continue with excavation to +111.125 mTD.

8

Construct base slab at +112.85 mTD.

9

Remove props.

10 Excavate beneath base slab to +102.56 mTD to create platform. The props were designed for a DA1C2 action of 12 000 kN, including a characteristic action of 3200 kN, due to a temperature variation of +30°C. The manufacturer designed the props to a characteristic action of 6250 kN and an ultimate resistance of 12500 kN.

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Temperature effects

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The lowest prop temperature measured was 5°C in early December, and the highest was 22°C in early June, when the temperature monitoring was removed. This would indicate a thermal variation of 17°C compared to 30°C used at design stage. The external ambient temperature, during this period, varied from -2°C to 31°C. As the box was significantly covered by a roof slab this would indicate a thermal damping effect of the box. Similarly, Powrie and Batten (2000b) report that the thermal effects in the upper props at Canada Water were more significant compared to the lower props. Although the thermal loads at Paddington were small, they did have a noticeable effect on the works. The thermal load was typically at its lowest around 8.00am (see figure), and the site team discovered that the props were difficult to remove from the walers after midday without having to use additional measures. This left a four hour window for the props to be released from the walers. Another consideration is the time of installation. If the props were installed in the summer months, it is likely that the thermal loads would decrease through winter and vice versa.

Typical daily temperature-induced load variation

System stiffness Four of the props were installed with surface temperature monitors, which enabled the system stiffness (β) to be backcalculated (see Section 8.1.7). The Young’s Modulus for steel was taken as 210 GPa kN/m2, the cross-sectional area was 0.0605 m2, α = 1.2 × 10 -5 and the change in load due to thermal effects was extracted from the load pins. At design stage, the system stiffness (β) was taken as 0.7 as per Gaba et al (2003) recommendations as the propping system comprised stiff mechanical props, attached to 900 × 600 walers and 1.2 m thick reinforced concrete diaphragm walls installed in stiff London Clay. In reality the measured system stiffness averaged 35 per cent, with the range of values suggesting that a system stiffness of 50 per cent was more appropriate. The design thermal characteristic load was 3200 kN, however the measured thermal load was 1136 kN, ie 35 per cent of the expected value. This study shows that the discrepancy in system stiffness, combined with the lower than expected temperature variation in the box led to the measured thermal loads being considerably smaller than the design thermal loads.

Construction considerations The continuous monitoring of the prop loads with time gave a valuable insight into the effects of casting a slab near a prop (see figure). In this case, prop 11 is directly above slab C3 (base slab). Initially the hydrostatic pressure of the concrete and the slab’s expansion as it heats during curing caused a decrease in prop load. When the slab continues to cure, it contracts and pulls the diaphragm walls in with it, causing an increase in prop load. This process takes around eight days. A drop in load of 100 tons was typical for props immediately above slab pours. Also noted was the general trend of load increasing at a higher rate after the slab pour. This would Effects of casting a slab on prop load. ‘Expected load’ = SLS design load be relevant if the prop load was near a trigger value before a slab was cast. Similar findings are reported by Batten and Powrie (2000b) at Canary Wharf Station, where the casting of the concrete base slab resulted in a significant reduction in the prop loads due to the expansion of the slab. During the subsequent cooling stage, about 60 per cent of the prop load was recovered within 30 days.

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8.2 BERMS

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Berms can be used to help stabilise embedded retaining walls and to reduce their movement. Easton and Darley (1999) describe four case studies where berms were successfully used for this purpose.

Figure 8.4

Definition of berm geometry

In given ground conditions, the degree of support offered by a berm will depend on the height, H, the bench width, B, and the slope, S (Figure 8.4). The maximum S will be governed by soil and groundwater conditions, while H and B may be limited by considerations of space and access. In soils of low permeability, the drainage conditions assumed in design and the length of time for which the berm is required to remain effective will be important. Most methods of representing the effect of a berm in a limit equilibrium or SSI analysis using subgrade reaction and pseudo-finite element methods are semiempirical, even in the ideal case where conditions on site equate to plane strain. If the berm is removed in sections along its length to allow permanent supports to be installed, a 3D analysis may be required to assess stability and wall deflection. This may explain why berms have often been used in conjunction with the OM (Tse and Nicholson, 1993, Powrie et al, 1993, Gourvenec et al, 1996).

8.2.1 Modelling earth berms In a limit equilibrium or SSI analysis using subgrade reaction or pseudo-finite element methods, it is necessary to estimate separately or make some assumptions about: zz

the lateral stresses exerted by the berm on the wall above formation level

zz

the influence of the berm on the lateral stresses in front of the wall below formation level

because these are not calculated explicitly in the analysis. Descriptions and a discussion of the relative merits of some common methods of representing an earth berm in an undrained limit equilibrium or SSI analysis are given by Daly and Powrie (2001). Powrie and Daly (2002) describe the results of a series of plane strain centrifuge model tests of embedded cantilever retaining walls of various embedment depths supported by berms of different sizes. These, together with supporting analyses presented by Daly and Powrie (2001), demonstrated that providing or increasing the size of a berm is more efficient in enhancing wall stability than increasing the depth of wall embedment.

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8.2.2 Recommended method of modelling earth berms

Figure 8.5

Representation of a berm by means of a raised effective formation level (after Fleming et al, 2008)

The recommended method of modelling an earth berm in an undrained limit equilibrium or pseudofinite element analysis is the raised effective formation level approach (Fleming et al, 2008). This is shown by Daly and Powrie (2001) to offer a reasonable and slightly conservative balance between ease of application and strict analytical rigour. In undrained conditions, the raised effective formation level approach models the berm as being equivalent to a rise in the level of the excavation in front of the wall (Figure 8.5). For an undrained analysis, the original berm profile is reduced to a design berm profile with a slope of 1:3, but the base width of the berm, b, remains unaltered. The height of the design berm becomes b/3 and the increase in effective formation level is taken as half of the design berm height, ie b/6. Any portion of the actual berm above the effective formation level and the design berm (shown shaded in Figure 8.5) is then treated as an equivalent surcharge S acting at the revised formation level, over the width of the critical passive Coulomb wedge:

(8.11)

where w is the weight of the shaded area of the berm shown in Figure 8.5 θc is the angle of the critical passive Coulomb wedge to the horizontal. The raised effective formation level approach takes partial account of the lateral pressure exerted by the berm, and is conservative. For an effective stress analysis, Smethurst and Powrie (2008) showed that the application of the raised effective formation method as described above could overestimate the effect of the berm by typically 10 to 15 per cent. They proposed a modified raised effective formation level approach for use in an effective stress analysis, with the increase in effective formation level y given by:

(8.12)

where W is the total weight of the berm

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d the wall embedment below formation level ϕ′ is the effective angle of shearing resistance γ is the unit weight of the soil from which the berm is made (as used in the calculation of W).

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Smethurst and Powrie (2008) investigated soils having an angle of shearing resistance ϕ′ in the range of 21° to 26° with a groundwater level in front of the wall at or below the main excavation formation level, assuming a stable berm in which pore pressure effects could be neglected. The same principles, with an appropriate treatment of water pressures and pore water pressures, would apply in the case of a submerged berm within a flooded excavation. In a finite element analysis, a berm can be modelled directly without prejudging its effect on the lateral stresses in front of the wall. However, careful consideration should be given to the internal stability of the berm. For example, in an effective stress analysis in which the berm slope is steeper than the angle of shearing resistance of the soil, negative pore water pressures may need to be specified and maintained within the berm for the duration of the analysis (eg Gourvenec and Powrie, 2000 and Powrie and Daly, 2002). If berm stability does depend on the maintenance of negative pore water pressures, it would be prudent to take steps to ensure that these can be relied on in practice, such as blinding the surface of the berm with concrete or covering it with an impermeable membrane. In summary, it is recommended that for routine limit equilibrium or simple SSI analysis in undrained conditions, the raised effective formation level approach illustrated in Figure 8.5 is used to model the effect of an earth berm. In an effective stress analysis, the modified effective formation level approach, with the rise in formation level given by Equation 8.12, may be used. In any limit equilibrium analysis of a bermsupported retaining wall, potential failure mechanisms that arise because of the wall and berm geometry and soil stratigraphy (eg possible sliding on a weak horizontal layer) should also be considered explicitly. Where there is scope for achieving economy with more design effort, a multiple Coulomb wedge approach (NFEC, 1986) summarised by Daly and Powrie (2001) for total stress analysis and by Smethurst and Powrie (2008) for effective stress analysis could be adopted in SSI analysis using pseudo-finite element or subgrade reaction methods, or a full finite element or finite difference analysis may be carried out.

8.2.3 Deflections of walls supported by berms The analyses described in Section 8.2.2 assume conditions of plane strain, ie that the berm remains intact over the entire length of the wall throughout the excavation and construction period. In reality, it may be necessary to remove the berm in sections so that the permanent support, eg formation level props or a basement slab, can be installed. Removal of a long berm in sections is a 3D problem. Wall movement due to the removal of a section of a long berm would be expected to increase in proportion to the length of the section removed. Gourvenec and Powrie (2000) carried out a series of 3D finite element analyses to investigate the effect on wall movements of the removal of sections of an earth berm supporting a long embedded retaining wall in overconsolidated clay. The results of their analyses are presented in Figure 8.6. The main practical implication is that wall movement resulting from the removal of a number of berm sections simultaneously can be minimised by maintaining a separation of at least one to three times the length of the berm section removed (β = 25 to 50 per cent in Figure 8.6, where β = B/(B + B′) and B′ is the length of wall section supported by the berm and B is the length of the unsupported section). Easton et al (1999) used 3D finite element analysis to develop a relationship between berm height and the effective uniform increase in formation level in front of the wall to give the same wall movement. Their analyses considered a berm supported retaining wall having the cross-sectional geometry shown in Figure 8.7, with berms of different height within the profile envelope indicated.

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Note The data points and solid lines represent confirmed findings, and the broken lines conjecture.

Figure 8.6

Normalised wall top displacement at the centre of the unsupported section against degree of discontinuity,

β, for different excavated bay lengths, B

Soil type

Ko

E′ (MPA)

mE′ (MPa/m)

φ′(o)

c′ (kPA)

k x (m/sec)

Stiff to very stiff clay

2.0

32

8.4

22

0 and 20

5 × 10 -10

1 × 10 -10

Firm to stiff clay

1.0

16

4.2

28

0 and 10

1 × 10 -5

1 × 10 -7

k y (m/sec)

Notes zz

Soil assumed as elastic perfectly plastic with Mohr-Colomb failure criterion.

zz

Retaining wall wished in place.

zz

Groundwater table assumed at one metre below existing ground level.

The analyses started with the retaining wall already (‘wished’) in place, ie wall installation effects were not modelled explicitly.

Figure 8.7

242

3D finite element mesh, wall and excavation geometry and assumed soil parameters (after Easton et al, 1999)

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Along the wall, the berm was divided into three central bays, each five metres in length, and two outer bays, each 30 metres in length. Two sets of 3D finite element analyses were carried out. In one, a construction sequence using berms of varying height was modelled. In the other, a construction sequence using a uniform effective excavation level (ie no berm) was modelled. In both cases, excavation to final formation level for permanent prop slab installation was carried out using the same sequence of bay excavation. The construction sequence in Box 8.2 was analysed. Charts presenting the results of the Easton et al (1999) analyses are given in Figure 8.8. For comparable ground conditions, wall and excavation geometries may be used to rationalise the design of soil berms in temporary works design. Box 8.2

Construction sequence analysed by Easton et al (1999)

Stage 1: Excavation to final formation level in the centre with perimeter berms or to an effective ground level (as required) over a period of 30 days.

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Stage 2: Excavation to final formation level in the central (5 m wide) bay over a period of 30 days. Stage 3: Installation of the prop slab in the central bay over a period of one day. Stage 4: Excavation to final formation level in the first adjacent bay over a period of 30 days. Stage5: Installation of prop slab in the first adjacent bay over a period of one day. Stage 6: Excavation to final formation level in the second adjacent bay over a period of 30 days. Stage 7: Installation of prop slab in the second adjacent bay over a period of one day. Stage 8: Excavation to final formation level in remaining bays and installation of prop slab over remaining bays over a period of 30 days. Consolidating element models were used for the soil strata, with the soil stiffness and permeability given in Figure 8.7. Non-consolidating element models were used for the structural elements.

a

b

Figure 8.8 Relationship between berm height and effective uniform ground level, stiff to very stiff clay, ϕ = 22° (a), firm to stiff clay, ϕ′ = 28° (b)

For any given wall depth, the design approach presented in Chapter 7 may be adopted to calculate the effective uniform ground level above final formation level, which is required to satisfy ULS wall stability. Cantilever wall deflections can then be assessed (Chapter 6). The height of berm above final formation level corresponding to the effective uniform ground level obtained from the ULS calculations may be obtained from Figure 8.8.

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8.3 ANCHORS As an alternative to propping systems and support from earth berms within the excavation, passive deadman anchors or ground anchorages may be installed behind the wall on the retained side. Irrespective of whether a passive deadman anchor or ground anchorages are installed behind the wall, it is important to carefully consider the locations of the critical failure planes for the whole support system to ensure satisfactory global stability. The distance behind the wall should be sufficient to position the anchors such that they extend beyond any such failure planes and are not obstructed by existing substructures, basements, utilities etc.

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Depending on particular excavation geometry, it is common for anchors installed for such an excavation to clash with one another (Figure 8.9). In such circumstances, the designer must ensure that the anchors are designed to avoid intersecting one another and that there is sufficient clearance between passing anchors, for example, by varying their vertical inclinations and skew (horizontal angle to the wall).

Figure 8.9

Typical intersection of anchors at re-entrant corners (after Turner, 2012)

8.3.1 Deadman anchors A passive anchor typically comprises a tie that derives its resistance from a deadman (an anchor block or anchor back pile) to which it is connected (Figure 8.10). The design of deadman anchors should comply with the requirements of BS 8002:2015 Annex A. The available passive resistance should be calculated based on net available resistance, ie passive pressure less active pressure. No advantage should be taken of any surcharge loading on the ground surface in front of the deadman, but surcharge loading immediately behind it should be allowed for in the Figure 8.10 Passive deadman anchor (after Williams and Waite, 1993) calculations as this would increase the active force and therefore the anchor’s capacity. Analysis of deflections should also include an assessment of the elastic extension of tie bars, deflection of the deadman etc. It may be necessary to preload the tie bars and the deadman anchors to reduce the potential deflections and to ensure load/displacement compatibility between the retaining wall and the deadman anchor. Depending upon project-specific circumstances, it may also be necessary to provide protection to tie bars and the deadman from near surface, overburden and compaction forces.

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8.3.2 Ground anchorages

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A ground anchorage is a structural element that is capable of transmitting an applied tensile load to a load bearing stratum. It mainly consists of an anchor head, a free anchor length and a fixed anchor length that is bonded to the ground by grout (Figure 8.11). Loads are applied by tensioning the anchor tendon at the anchor head. This load is then transferred along the tendon, through the de-bonded free tendon length and into the ground through the fixed anchor length. The anchor head transfers the applied force onto the wall and the free anchor length spans the gap between the anchor head and the fixed anchor length. Ground anchorages are likely to be favoured in preference to passive deadman anchors where the groundwater level is above the tie rod level or where the ground level rises steeply behind the wall. In most other applications, where anchors are required at one level near the top of the wall, passive anchors will usually be more economical, provided there is sufficient space available behind the wall. Ground anchorages have the advantage that they can be installed at more than one level. The advantages and limitations of ground anchorages are discussed in Chapter 3 (Section 3.3.3 and Table 3.10). BS 8081:2015 categorises ground anchorages into four groups, based primarily on the method of borehole drilling and the type and pressure of the grouting (Table 8.1 and Figure 8.12). Table 8.1 Type of ground anchorage Type A Figure 8.12a

Type B Figure 8.12b

Type C Figure 8.12c

Type D Figure 8.12d

Types of ground anchorages Typical characteristics

Typical ground conditions and design assumptions

Straight shafted borehole temporarily lined or unlined depending on borehole stability. Grout placed under gravity by tremie, packer or cartridge.

Rock or very stiff to hard consistency fine-grained soils. Resistance from side shear at ground/grout interface.

Low pressure grouting (typically < 1 MPa) via lining tube Weak fissured rocks and fine to coarse-grained or in situ packer at the top of the fixed anchor length. soils. Resistance primarily from side shear, but Effective diameter of the borehole increased as an end bearing component may be assumed in grout permeates/compacts ground locally along design when calculating ultimate capacity. fixed anchor length. High pressure grouting (typically > 2 MPa) via tube-à-manchette systems (or similar) allowing multiple phased injections (if required) along fixed anchor length. Ground hydro-fractured resulting in a grout fissure system extending beyond the nominal borehole diameter.

Borehole is enlarged by bells or under-reams along fixed anchor length. Grout placed under gravity by tremie.

Fine-grained soils. Design assumes uniform shear along fixed anchor length.

Can be used in coarse-grained soils and cohesive soils in conjunction with local ground improvement (pre-injection of cement or chemical grout in the ground around the fixed anchor length prior to borehole drilling) to improve side wall stability over the enlarged length. Resistance from side shear and end bearing.

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a Bond-type grouted anchor – details of anchor head and head protection omitted

b Compression-type grouted anchor – details of anchor head and head protection omitted

L A length of anchor Le external length of tendon measured from the tendon anchorage in the anchor head to the anchorage point in the stressing jack Lfixed fixed anchor length free anchor length Lfree Ltb tendon bond length tendon free length Ltf

m m m m m m m

Key 1

Anchorage point at jack during stressing

9

2

Anchorage point at anchor head in service

10 Borehole

3

Tensioning element at anchor head

11 Debonding sleeve

4

Bearing plate

12 Tendon

5

Load transfer block

13 Fixed-length grout body

6

Structural element

14 Free-length filling, where appropriate

7

Trumpet or anchor head tube

15 Compression element

8

O-ring

Figure 8.11

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Key

Figure 8.12

a

Type A anchor

b

Type B anchor

c

Type C anchor

d

Type D anchor

Classification of ground anchor types A, B, C and D (after BS 8081:2015)

The design of ground anchorages should conform to Clause 8 of EC7-1, which stipulates the limit states, design situations and actions and the design and construction considerations that must be addressed, and to BS 8081:2015. The reader is referred to these standards and to Turner (2012) and Judge (2012) for more detailed design and construction considerations. Reference should be made to ArcelorMittal (2016) and to EAU (2012) for guidance on the design of unbalanced anchorage systems. BS EN 1537:2013 defines temporary ground anchorages as having a design life of two years or less. Permanent ground anchorages are those that have a design life in excess of two years. This distinction establishes the requirements of corrosion protection that should be provided and there is a good discussion of this in Turner (2012). A flowchart for the design, construction and maintenance of ground anchorages is given in Figure 8.13. Although guidance on the design, construction and maintenance of ground anchorages is provided in Turner (2012), Judge (2012) and BS EN 1537:2013, this is a highly specialised area and appropriately experienced expert guidance should be sought. It is important that the construction of ground anchorages is carried out so that the validity of design intent and assumptions is maintained. The design responsibilities for the planning, design, installation, testing and maintenance of ground anchorages are often spread among several parties, including the client, the project designer and specialist designers and subcontractors engaged by one or more of the contractors. So it is important that the relative responsibilities of all parties involved in the design, execution and maintenance of such ground anchorage systems are appropriately defined at the start and specialist ground anchorage contractors are consulted at an early stage to advise on construction issues which may influence the proposed design. This is discussed further in Section 8.3.3.

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Identify ground anchorages as a feasible option Assemble all ground data available. Carry out desk study, and field study to assess geological, loading, cost and legal implications of grouted anchor option Do grouted anchors continue to be a viable option? Yes

No

Find alternative solution Refused

Seek clients’ approval for the use of grouted anchors Approved

Ensure adequate ground data available for the design and construction of grouted anchors Establish the overall stability and interaction required of the anchor system, and assess the loads for which the anchors are to be designed Assess various options of anchor capacity, number, free length, inclination and reaction arrangements and determine most suitable option Appoint a suitably-qualified geotechnical specialist to supervise the design and execution of grouted anchors in accordance with BS EN 1997-1:2004+A1:2013, NA+A1:2014, BS EN 1537:2013 and BS 8081:2015 Select level of protection required to suit intended life of anchors, having assessed the degree of aggressivity of the ground, based on results of analysis of soil and groundwater samples

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Will anchors encroach under neighbouring property? Yes

No

Obtain necessary consent

Specify monitoring and maintenance requirements and obtain owners’ agreement

Proceed with the management of the detailed design and execution of the grouted anchors Design fixed anchor (see Clause 11, BS 8081:2015) Determine the fixed anchor length dimensions by calculation/testing Is long-term monitoring required of some or all anchors and the supported structure or slope? (see Annex H, BS 8081:2015) Yes Implement Annex G.5.1, G.5.3, G.5.4, BS 8081:2015

No

Design and/or specify testing of trial anchors: a Ground/grout bond behaviour and capacity (see Annex B.3, BS 8081:2015). b Grout/tendon bond behaviour and capacity (see Annex B.5, BS 8081:2015). c Materials and components (see Clause 12, BS 8081:2015). d Corrosion protection (see Annex F, BS 8081:2015). e Testing (see Annex G, BS 8081:2015). Specify and implement: a Stressing equipment. b Construction, including method statement, programme, staffing and quality controls (see Clause 14, BS 8081:2015). c Plan for remedial meaures (see Clause 16.2, BS 8081:2015). d Health and safety plan (see Annex I, BS 8081:2015). Construct and test trial anchors (see Annex G, BS 8081:2015) Revise fixed length design

No or

Investigation tests are not undertaken as part of the design process

Do results confirm design assumptions? (see Annex G, BS 8081:2015) Yes

Revise, in conjunction with the support designer, anchor configuration to accomodate reduced capacity

Construct and test on-site suitability of anchors (see Annex G, BS 8081:2015) Confirm performance satisfies design requirements (see Annex G, BS 8081:2015) Construct works anchors and carry out acceptance tests (see Annex G, BS 8081:2015)

In conjunction with the support structure designer, agree and implement remedial measures

No

Do anchors pass the acceptance criteria? (see Annex G, BS 8081:2015) Yes Agree and issue: a geotechnical design report b geotechnical feedback report Accept anchors into the works No

Is monitoring to be carried out? Yes Appoint anchor monitoring advisor and anchor monitoring specialist Take initial readings. Decide intervals between subsequent readings and determine limiting criteria (see Annex G, BS 8081:2015) which, if exceeded, require the implementation of a further investigation and/or remedial measures (see Annex H, BS 8081:2015) Agree a maintenance programme in accordance with Clause 16, BS 8081:2015 Proceed with monitoring and maintenance programmes as agreed. Proceed with investigation and remedial works where necessary

Arrange for records including geotechnical design report and geotechnical feedback report to be placed in the custody of the owner and/or appropriate authorities

Dismantle after service, or destress and abandon in the ground

Figure 8.13

248

Flow chart for the design and construction of ground anchorages (after BS 8081:2015)

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8.3.3

Design and execution: wall and anchor designers’ responsibilities

There are two distinct roles: 1

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2

The wall designer – responsible for the overall design of the supported structure (ie the retaining wall). They are expected to identify the full range and combination of actions and issues affecting the design, construction and performance of the overall retaining structure supported by the ground anchorages. It is the wall designer’s responsibility to identify: a

the magnitude of the horizontal restraint required to be provided per metre run of the wall

b

its level or levels of action.

The anchor designer – responsible for the selection and design of the specific anchor type and its component parts, including details of individual anchor loads and their spacing and orientation, taking full account of the ground conditions at the site.

Table 8.2 lists the relative responsibilities of the wall and anchor designers and highlights the key interfaces between them. It also presents a simplified division of relative responsibilities between the wall and anchor designers. Typically, a project designer (for example an architect or structural engineer) will be employed by either the client or the main contractor. The project designer may identify the need for an anchored-retaining structure within the overall project scheme. A specialist contractor or subcontractor may either be appointed by the client or the main contractor for the design and construction of the retaining wall. The specialist wall subcontractor may then appoint a specialist geotechnical contractor for the design and execution of the ground anchorages, who may also employ a specialist anchor consultant to design them on its behalf. Alternatively, the project designer may undertake the design and specification of the retaining wall and the requirements of anchors to provide lateral support, identifying the levels and spacing of such support. In this scenario, the wall may be constructed by a specialist subcontractor with the ground anchorages being designed and installed either by the subcontractor or by an anchor specialist to provide the loadings specified by the project designer. Many variations are possible for the design and execution of the anchored structure. The key point is that, for any project specific arrangement, the responsibilities should be explicitly identified and understood and accepted by all parties engaged on that project.

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Wall and anchors designers’ relative responsibilities

Detailing of the specific corrosion protection system. Supply and install ground anchorage monitoring system. Quality control of works on site to ensure compliance with specified requirements. Maintenance of ground anchorages as specified.

Definition of anchor life (permanent or temporary) and requirement for corrosion protection.

Specification for monitoring ground anchorage behaviour and for interpretation of the results.

Technical supervision of the works on site to ensure compliance with overall design intent of supported wall structure.

Maintenance specification for ground anchorages or removal (if required).

5

6

7

8

Supply and install ground anchorage system. Execution and assessment of on-site anchor load testing.

Determination of minimum free anchor length for overall wall stability.

Specification of anchor spacing and orientation, anchor loads (SLS and ULS) for overall stability of the supported wall structure, including load transfer mechanism from anchors to the wall structure and any sequence of anchor loading required by the supported wall structure.

4

Determination of individual anchor spacing and orientation and anchor loads (if not specified by the wall designer).

Common agreement on key components of the overall design philosophy of the supported wall structure to which special attention should be directed.

Common agreement on monitoring frequency, trigger levels, reporting requirements and contingency measures (as appropriate).

Common agreement that specific anchor arrangement satisfies design requirements of supported structure for overall vertical and lateral stability, recognising that anchor force inclinations will exert a vertical component of load on the supported wall structure. The free anchor length may be increased by the anchor designer to ensure that the fixed length is satisfactorily embedded in suitable founding strata.

Selection of ground anchorage type, components and Common agreement on ground model at site and appropriateness details, including determination of fixed anchor dimensions. of selected ground anchorage type and testing regime.

Decision to use ground anchorages, required trials and testing and provision of a specification (see also point 4).

3

Overall design of anchored structure, including calculations of horizontal restraint required and its level(s) of action.

Assessment of site investigation data with regard to design Planning, scope and details of additional site investigation assumptions. established in consultation between wall and anchor designer.

Provision of site investigation data for the design and construction of ground anchorages – borings near fixed anchor locations and outside the site working area, as appropriate.

2

Key interfaces between the wall and the anchor designer

Acquisition of legal authorisation and entitlement to encroach on third party property (if required) including party wall agreement (if required).

Anchor designer’s responsibilities: specialist activities

1

Activity Wall designer’s responsibilities: overall design activities

Table 8.2

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8.3.4

Load transfer into supported wall structure

The head assembly for a ground anchor typically comprises an anchor block and a bearing plate (Figure 8.11) which transfers the anchor load into the supported wall structure. The bearing plate often bears directly on steel or concrete waling beams to distribute the anchor loads more evenly onto the supported wall structure.

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Figures 8.14 and 8.15 show typical details of the use of steel walings for sheet pile walls and king post walls and the use of reinforced concrete walings typically adopted for contiguous or secant piled walls.

a

Internal waling with external angled bracket assembly

b

External waling with angled bracket assembly

c

External waling with angle bracket assembly

Figure 8.14

Typical use of steel walings with ground anchorages (after Turner, 2012)

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Figure 8.15 Turner, 2012)

Typical use of reinforced concrete walings or load transfer assemblies with ground anchorages (after

For diaphragm walls, an appropriate angled pocket is usually cast into the wall panel together with a guide tube through the wall section to locate the drill string clear of the reinforcement.

8.3.5

Ground anchor testing and maintenance

Ground anchorages are greatly underused in the UK compared to elsewhere in Europe. However, where they are used, they are probably the most tested component in geotechnical works (Judge, 2012). Ground anchorages providing support to retaining walls should be subjected to the regime of investigation, suitability and acceptance testing set out in BS EN 1537:2013 and BS 8081:2015. Unlike props or berms, by their nature, the structural elements of ground anchorages are largely hidden from sight. Anchor failure may be evident through either a loss of anchor pre-stress or, in some cases, an increase in the anchor load. Consequently, it is important to monitor loads in selected anchors to confirm satisfactory performance of the supported wall structure as part of the ongoing anchor maintenance regime. As recommended in BS 8081:2015, such a regime must consider the de-stressing and dismantling of temporary anchors and remedial measures where replacement of defective permanent anchors may be required or where re-stressing of the tendon may be necessary.

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8.4

KEY POINTS AND RECOMMENDATIONS

1

The cost of a temporary propping system is usually small in comparison with the cost of the retaining wall. However, the delay and disruption to the excavation while the temporary props are installed may be a significant expense and such activities should be considered carefully and explicitly included in the overall programme for the construction works. Economy can be achieved by reducing the number of props or prop levels, eg through the application of the OM.

2

Clear responsibility for the design of the propping system to the wall is essential. Where the permanent wall is used to provide temporary support during construction, the designer of the permanent wall should inform the designer of the temporary props of the assumptions made about temporary propping in the design of the permanent works. Where the designer of the temporary props is unable to comply fully with the assumptions made by the designer of the permanent wall, the lead designer should co-ordinate the necessary interaction between the two designers.

3

The design load acting on a prop will depend on the analysis method used in the calculations for the design of the wall. Prop loads calculated from limit equilibrium calculations may be unconservative, as the effects of SSI are not included.

4

The designer should allow for any imbalance in horizontal loading across the excavation. Conventional analytical methods can overestimate effects due to stress path reversal.

5

Earth berms represent an effective means of temporary support, for example before installation of permanent props at formation level. Increasing the size of the berm is likely to be a more efficient way of enhancing stability than increasing the depth of embedment of the wall.

6

Earth berms can be represented in plane strain limit equilibrium and simple SSI analyses by means of the raised effective formation level approach. This method is conservative, but adequate for routine design. In cases where more design effort will produce significant economy, the multiple Coulomb wedge analysis approach (NFEC, 1986) summarised by Daly and Powrie (2001) for total stress analysis and by Smethurst and Powrie (2008) for effective stress analysis could be adopted in SSI analysis using pseudo-finite element or subgrade reaction methods. Alternatively, a finite element or finite difference analysis could be carried out.

7

Removal of a long berm in sections, for example to construct a permanent prop at formation level, is a 3D problem. Wall movement due to the removal of a section of a long berm increases in proportion to the length of the section removed. Further wall movement resulting from the removal of several sections simultaneously can be minimised by maintaining a separation of at least one to three times the length of the section removed.

8

Careful consideration should be given to the locations of the critical failure planes behind the wall to ensure satisfactory global stability of the whole support system. The distance behind the wall should be sufficient to position ground anchors such that they extend beyond any such failure planes.

9

Ground anchorages are greatly underused in the UK compared to experience elsewhere in Europe. Their increased use may result in significant savings over propping schemes where programme time is available for the construction of ground anchorages and the space is available to locate them. The potential to remove the anchor following completion of the works would remove possible objections from neighbouring property owners.

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9 Inspection, monitoring and maintenance 9.1 INTRODUCTION

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This chapter provides guidance to designers, but will also be of interest to asset owners and those providing a service that could be affected by performance of a nearby embedded retaining wall. Throughout the construction stage, monitoring data can be widely used to highlight movement trends, compare actual observations against predictions and inform current and future design work. As discussed in Chapter 7, monitoring against appropriate trigger levels is vital for the successful implementation of the OM. After assets have been built, or for existing assets, planning for regular inspection, maintenance and monitoring can help to maintain optimum condition, prolong life and achieve best value or performance. Strategic, planned interventions can reduce risk and enable an asset owner to manage their assets, and their performance, in a cost-effective manner. Data can then be interrogated to help inform future decisions, modelling and selection or timing of interventions. These principles apply at individual asset and portfolio level, although the type of asset and its use may define the extent to which this is carried out. This chapter sets out the following: zz

designer’s responsibilities

zz

the business case for inspecting, maintaining and monitoring embedded retaining walls

zz

consideration of whole life cycle and whole life cost

zz

inspection

zz

monitoring

zz

maintenance.

9.2

THE ROLE OF THE DESIGNER

EC7-1 requires a geotechnical design report to be produced, at a level of detail that is appropriate to the design. This report should contain information on items to be checked during construction, or requiring maintenance or monitoring. It should also include a plan of supervision, monitoring and clear identification of requirements for ongoing maintenance. EC7-1 Clause 2.8(6)P states that “an extract from the geotechnical design report containing the supervision, monitoring and maintenance requirements for the completed structure, shall be provided to the owner/client”. The responsibility for the designer to provide this guidance for new constructions is evident. The designer’s responsibilities under CDM 2015 are discussed in detail in Chapter 2 and in relation to the implementation of the OM, in Chapter 7.

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9.3

COST/BENEFIT ASSESSMENT

It is important for a designer to ensure that the client understands the need for the inspection programme, maintenance regime or monitoring methodology proposed. This is particularly the case when monitoring new constructions or where the OM is proposed. In this case the whole team and the client need to be fully engaged with the plan and understand the possible cost savings. Regular monitoring can provide information about how the asset is performing and whether it is working as intended. If not, this can be brought to the attention of the designer/asset owner for action.

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Monitoring is one of the most crucial requirements when implementing the OM and can be used to realise cost and programme savings by back analysis of a combination of wall movement data, prop/ anchor forces and movements of nearby structures. These can be used to revise parameters and construction sequences etc, upon which material and programme savings may be made and that can inform future phases of work (see Section 7.4). Maintenance is nearly always necessary to maintain or prolong asset life and ensure the wall design assumptions remain valid (for example unclogging weep holes/cleaning drainage channels). Regular, planned maintenance can provide an opportunity to achieve cost savings by preventing or delaying the need for any major repairs. Inspections are crucial in checking that maintenance has been carried out adequately, design assumptions are still valid and that the wall is performing as it was designed. A well thought out inspection, maintenance and monitoring schedule is a powerful tool that can be used to achieve optimal performance of the asset and reduce risk of failure. This is particularly important for assets such as embedded retaining walls as these are not often designed with the intention of renewal or replacement at the end of their design life. Reliable case study data (see Chapter 6) are vital to the industry and publication of such data should be encouraged where possible. The reader is referred to Hooper et al (2009) which provides further detail on the business case for good infrastructure asset management.

9.4

DESIGN APPROACH

9.4.1

Design strategy

As discussed in Chapters 2 and 3, before committing to a particular wall type or design, there are a number of important questions or considerations that may influence the decision made. The later these questions are raised in the design process, the smaller the opportunity for modifications or cost savings. These considerations should include (but are not limited to): 1

Will there be an ongoing requirement for maintenance for the particular type of wall chosen? Will this lead to problems for ease of access, requirement for possessions etc?

2

Who has responsibility for the wall?

3

Are there any key stakeholders?

4

Is the wall located in a high-risk area or on a strategic part of a route or network? For example, would there be a high consequence of non-performance, would failure present a safety risk to the public? Would regular interventions cause widespread disruption?

5

Is the type of wall chosen appropriate for the life cycle of the asset?

6

What is the cost/penalty of non-performance (ie would poor condition result in an adjacent road being closed that would have a knock-on effect on service levels or availability targets?)

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9.4.2

Whole life assessment

An important part of the decision-making process is to consider the whole life cycle cost of the asset (see Chapter 2). This ranges from ‘birth’ of the asset (ie creation or acquisition) through to operating and maintaining that asset over its useful life and eventually deciding whether the asset will be renewed or disposed of. This can be expressed diagrammatically in a number of ways. A typical illustration of an asset life cycle is shown in Figure 9.1.

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Consideration should be given to each stage of this life cycle as early as possible: 1

Create/acquire: the cycle ‘starts’ either with design of the asset or at the point where an asset has been handed over or purchased from another party. Figure 9.1 If this is an initial design phase, the designer should consider the following three steps as a whole.

Whole life cycle

2

Use/operate: the designer should consider whether any regular inspections or planned maintenance activities are required for usual operation of the asset. This could include, for example, clearing of drains, de-vegetation and routine inspections. The cost of these can depend on many factors and may require, for example, night-time track or road possessions, lane closures, power outages or mobilisation of specialist teams. The frequency of inspections required can also be an important factor. In some cases, these operational costs are significant in comparison to the cost of installing the wall, or the ‘create’ cost, and should not be overlooked.

3

Maintain/repair: a distinction should be made between maintenance (which should be planned and preventative) or repair, which can be either an active intervention to resolve a potential problem or reactive intervention to resolve a defect. The balance between renewing or repairing an asset or element, or replacing it altogether, is an important one, particularly for assets that may be in service beyond their design life. For these assets, there will be a trade-off between cost of replacement and the level of acceptable risk.

4

Renew/reuse/dispose: a plan for end of life should be made at the earliest opportunity and where applicable, the cost and impact (eg environmental, health and safety) of demolition at that stage considered.

9.4.3

Design life

The design life for an embedded retaining wall will vary between projects and depending on the client’s requirements. However, as a general guide, the UK NA to EC0 suggests indicative working lives as listed in Table 9.1. Table 9.1

Suggested design lives (after Table NA2.1, UK NA to EC0)

Design working Indicative design Examples life category working life (years) 1

10

Temporary structures (not including structures that are dismantled and reused).

2

10 to 30

Replaceable structural parts, eg gantry girders, bearings.

3

15 to 25

Agricultural and similar structures.

4

50

Building structures and other common structures, not listed elsewhere in this table.

5

120

Monumental building structures, highway and railway bridges, and other civil engineering structures.

For the structures considered in this publication, a design life of 100 to 120 years would be appropriate.

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With an understanding of the proposed design life of the structure, decisions can be made early to either maintain the condition of the asset before it is replaced or to begin the process of managed degradation. This may depend predominantly on whether the asset still has a purpose and whether it would pose a safety risk if left to degrade. There may also be a plan to hand the asset over to another party, for example at the end of construction or after a lease agreement has expired, where a particular requirement may exist for the condition of the asset at that stage.

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9.5 INSPECTION Inspections are required to check both the condition of the structure, and to confirm that the assumptions upon which the wall was designed are still valid, and if not, undertake further assessment. Many walls are designed assuming that a baseline amount of maintenance takes place regularly (eg unclogging weep holes/cleaning drainage channels) and their performance depends on this. Some existing structures, for example bridge abutments or retaining walls along highways or railway lines, may not have been designed to account for today’s increased traffic loads and may be showing signs of distress because of this. So walls that previously would have a minimal maintenance regime may now require additional attention due to changes in loading, ground movements or change in ground conditions (for example groundwater level increase or drainage path changes). Some structures may have deteriorated due to the weather, and some of these defects may be hidden and undetected.

9.5.1

Type and frequency

Inspections should be carried out regularly, in accordance with the designer’s specified requirements. Inspection requirements should be set out in the design report which is provided to the client/owner. The report should state the type (eg safety, general, visual, detailed), frequency (eg every year, every five years) and who should carry out each type of inspection and report back. Many organisations develop their own standards that determine the frequency and type of inspection required, particularly where they own many infrastructure assets. The type and frequency of examinations usually depends on how important the asset is and its location, for example coastal defences may be examined more frequently than other assets, commonly once a year, or again following an extreme event. Usually, an inspection regime will consist of a range of different types of inspection to address different needs, for example visual inspections on a more regular basis than a detailed inspection. Highways England (2007) define inspections in terms of safety, general, principal, special or inspection for assessment, all of which have separate requirements. Chapman et al (2000) also sets out possible inspection regimes. Where there is no defined approach/ regime set out by an asset-owning organisation, the decision on how frequently to inspect a structure may depend upon an acceptable trade-off between the risk of not inspecting the structure, cost of the inspections and Figure 9.2 the asset condition (see Figure 9.2).

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9.5.2 Access/visibility The designer needs to bear in mind that during inspections, only the face of the exposed portion of the embedded retaining wall will be accessible. The designer should clarify, for the particular type of wall, the expected signs of distress to be looked for during such inspections. The requirement for inspection at design stage should be carefully considered and any inspection hatches incorporated into the design, as appropriate. Walls that are clad with panels or brick faced may hide defects.

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Other particular constraints can include traffic management or possessions that may be required to access the wall, and health and safety considerations, for example where working in confined spaces, close to live rails or live overhead line equipment (Iddon and Carpenter, 2009).

9.5.3

Risk-based approach

It is important to determine whether inspection is necessary in the first place and what information is required. The purpose of the inspection should be clear from the start, for example to verify condition, expected life, assign a risk score to the asset, or to determine maintenance activities required. Where the asset is part of a larger group, a risk score may be attributed that does not necessarily have to be a function of the age or condition of the asset, but could be a combined effect of its location, importance to the surrounding infrastructure or the consequence of it not performing. Risk scores allow the asset owner to prioritise interventions and control spend more effectively. Longer term inspections may be necessary for ageing assets that could pose a risk to the public or nearby buildings or to the smooth running of a service.

9.5.4

During the inspection

Inspections should take into account the conditions either side of the wall and over the full zone of influence of the wall. Table 9.2 lists a number of points to be addressed during an inspection. Table 9.2

Points for inspection

In front of the wall

At the wall

Behind the wall

zz

new excavation/trenches

zz

cracks

zz

new structures

zz

signs of ground movement

zz

bulges

zz

changes in surcharge

zz

leaking or seepage.

zz

shifts

zz

signs of ground movement

zz

leaning/tilt

zz

ponding of water.

zz

spalling concrete/corrosion

zz

missing blockwork/defects on facing

zz

destructive vegetation growth

zz

blocked drainage.

In addition to these, the inspection should consider whether the maintenance regime currently in place is sufficient to maintain condition as much as practicably possible and whether any changes to that regime need to be made.

9.5.5

Post inspection

A record should be made of the inspection to document the condition at a specific point in time. The weather at the time of the inspection should also be noted. Any actions resulting from the inspection should be captured and documented in a plan. This record will provide a reference for future inspections and a baseline from which degradation or condition can be compared (see also Section 9.8).

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9.6 MAINTENANCE Maintenance generally involves one (or both) of planned maintenance or reactive maintenance, discussed in the following sub-sections.

9.6.1

Planned maintenance

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This should be undertaken as standard and it should address all usual items to confirm that the original assumptions made in the design of the structure are still valid. This might include regular drainage cleaning, crack repairs/filling, removal of vegetation or debris that could clog drains or widen cracks and cause structural damage etc. This type of maintenance can include both preventative maintenance activities and condition-based maintenance.

9.6.2

Reactive maintenance

Reactive maintenance aims to resolve or minimise an existing defect. Activities can be prioritised, depending on the asset’s criticality or importance.

9.6.3

Choosing what to do

The approach to maintenance may either be ‘worst-first’ or risk based. Risk-based maintenance schedules are likely to be more effective because they address defects in high-risk structures, with a higher consequence of failure, first. Design advice should be sought for larger maintenance tasks such as removal and replacement. Where replacement is an option, either replacement of components or of the wall, modern materials should be considered for longer term works rather than like-for-like replacement.

9.6.4

Timing of maintenance activities

Maintenance activities may be outlined on a routine maintenance schedule and any specific requirements noted at design stage should be stated in the geotechnical design report. Minor/major works should be scheduled as necessary and in some cases may be prioritised eg due to an extreme event (heavy rainfall, impact etc). Many walls are not exposed (eg basement walls) and so are not subject to routine maintenance. If this is the case, they should be designed with this in mind and their gradual deterioration allowed for in design assumptions (eg partial loss of section) as discussed in Chapter 7. The overall optimum time to intervene is where the total business impact of the maintenance is lowest, and may not be immediately obvious. This will not necessarily be where the risk cost of ‘doing nothing’ equals the cost of intervention, as demonstrated in Figure 9.3.

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Figure 9.3

Optimum intervention timing (from IAM, 2015)

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9.7 MONITORING A good overview and summary of the principles of geotechnical monitoring is provided by Dunnicliff (2012) and Dunnicliff et al (2012). This guidance includes aspects to consider when planning a monitoring programme and types of instrumentation available for different purposes.

9.7.1

Purpose and extent

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EC7-1 Clause 4.5(1)P states that: “monitoring shall be applied: zz

to check the validity of predictions of performance made during the design

zz

to ensure that the structure will continue to perform as required after completion.”

Monitoring has many useful benefits. The purpose of the monitoring and the extent to which it is needed should be the focus from the start. Measurement techniques do not have to be expensive, but good quality data are valuable at all stages of construction to understand behaviour of the wall and nearby structures. Monitoring of movements should start as early as possible in the project cycle to establish whether there might be any background movements from nearby sites, or other activities that could affect results. This baseline monitoring should encompass the enabling works period, temporary works phase, the construction period and beyond completion. It is good practice to apply a systematic approach when designing a monitoring regime. Start with clearly defined goals or a list of questions and from these develop a regime that will provide the data to answer them. The extent/scale of the monitoring needs to be appropriate to the issue. Dunnicliff et al (2012) assert a ‘golden rule’ that “every instrument on a project should be selected and placed to assist in answering a specific question: if there is no question, there should be no instrumentation.” The designer should be able to answer the following questions and record answers in the geotechnical design report, as required by EC7-1 Clause 2.8(5)P: zz

the purpose of each set of observations or measurements

zz

the parts of the structure, which are to be monitored and the locations at which observations are to be made

zz

the frequency with which readings are to be taken

zz

the ways in which the results are to be evaluated

zz

the range of values within which the results are to be expected

zz

the period of time for which monitoring is to continue after construction is complete

zz

the parties responsible for making measurements and observations, for interpreting the results obtained and for maintaining the instruments.

The duration of post-construction monitoring should be established following observations made during construction (EC7-1 Clause 4.5(6)). In the case of an existing asset, its condition and/or maintenance requirements will determine the duration and extent of the monitoring required, as discussed in Section 9.6.

9.7.2

Choice of instrumentation

The choice of instrumentation should be selected to provide information specific to the job, ie defined by what should be known about the structure and how it is behaving, or the ground conditions. Table 9.3 provides an overview of some commonly used instrumentation for monitoring embedded walls retaining excavations.

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Table 9.3 Some instruments to consider for monitoring propped/anchored embedded walls retaining excavations (after Dunnicliff, 2012) Geotechnical questions Measurement

Instruments to consider zz

Groundwater pressure What are the initial site conditions?

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Are the props/ struts being installed correctly?

surveying methods

Widths of cracks in structures

zz

crack gauges

Load in struts

zz

calibrated hydraulic jacks

zz

load cells

zz

dial indicators

zz

surface mounted strain gauges

Load in ground anchor

zz

calibrated hydraulic jacks (load cells)

Deformation at head

zz

dial anchors

Settlement of ground surface, structures and top of supporting wall

zz

surveying methods

Horizontal deformation of ground surface, structures, and exposed part of supporting wall

zz

surveying methods (convergence gauges)

Change in width of cracks in structures and utilities

zz

crack gauges

zz

inclinometers

zz

in-place inclinometers

zz

Subsurface settlement of ground and utilities Load in props/struts

Load in ground anchors

Groundwater pressure

Is an individual prop/ strut being overloaded?

zz

(fixed borehole extensometers) (fibre-optic instruments) probe extensometers (fixed borehole extensometers)

zz

surface-mounted strain gauges

zz

load cells

zz

(calibrated hydraulic jacks and load cells, lift-off tests)

zz

surface mounted strain gauges

zz

open standpipe piezometers

zz

vibrating wire piezometers installed by the fully grouted method (pneumatic piezometers)

Base heave

zz

probe extensometers

Load in prop/strut

zz

surface-mounted strain gauges

zz

open standpipe piezometers

Is the groundwater table Groundwater pressure being lowered?

Is excessive base heave occurring?

vibrating wire piezometers installed by the fully grouted method (pneumatic piezometers)

zz

Subsurface horizontal deformation of ground Is the excavation stable, and are nearby structures being affected adversely by ground movements?

open standpipe piezometers

Vertical deformation

Load in ground anchor What is a suitable design for ground anchorages (by Deformation at head constructing and testing test anchors)? Load transfer in grouted zone Are the ground anchors being installed correctly (by performance and proof testing)?

zz

Base heave Subsurface horizontal deformation

zz

vibrating wire piezometers Installed by the grouted method (pneumatic piezometers)

zz

probe extensometers

zz

inclinometers

zz

in-place inclinometers

Note that the instrumentation will also be subject to calibration and maintenance and that greater accuracy implementation may be needed where the OM is adopted (Section 7.4).

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Other monitoring techniques that could be used include: zz

SAA

zz

optical/digital photogrammetry

zz

electro-acoustic monitoring

zz

liquid level sensors.

Careful consideration should be given to what information will come from the instrumentation and monitoring and how it will be processed and used. Advances in technology and the ability to cope with large volumes of data have led to a range of sophisticated monitoring techniques being available, with real time data presented remotely through user-friendly interfaces that are accessible via the internet. Box 9.1 provides an example of this.

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Box 9.1

Prince of Wales pier, Dover (courtesy ITM Monitoring)

a

b

This project demonstrates the design and installation of a monitoring system for the Prince of Wales pier at the Port of Dover. Dover Harbour Board required an automated monitoring system to continuously monitor the pier structure as part of its asset management programme and commitment to maximise the operational life of its assets. The steel sheet pile wall in question has been in place for over 40 years. The approach was to use MEMS (microelectromechanical) tilt sensors fixed to the face of the sheet pile wall, just below the capping beam (Figures a and b). In a highly-corrosive environment, there was a need to upgrade the sensor enclosures to marine grade stainless steel. The sensors are connected to a data-logger for remote data acquisition. The verticality of the wall is monitored using web-based data visualisation software. The client can access data and view tilt due to tidal influence to assess whether there are any long-term movement trends. An example plot of data over a two week period is shown in Figure c. The installation was completed in December 2014 and monitoring is intended over a three year period. In the future, the same monitoring approach could be used for other piled pier structures around the port.

c

Trigger levels The designer should select instrumentation that has appropriate accuracy for the type and range of movements/loads/levels to be expected. Appropriate trigger levels should then be selected based on expected response and the avoidance of identified limit states. For example, detailed discussion on the selection of green, amber and red trigger levels relating to the implementation of the OM is provided in Section 7.4.

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9.8

RECORDING INFORMATION

Many companies that own multiple infrastructure assets actively record asset information through bespoke management systems in order to manage their portfolio. The benefit of this can be realised in reducing risk and optimising intervention cycles to increase performance in a cost effective manner. Over an extended period, data can be used to build relationships and trends, for example, degradation profiles that then help with future planning.

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Building Information Modelling (BIM) is becoming a vital tool for safety and risk management and is being used more in monitoring systems to reduce time and errors associated with manually collecting and processing regular monitoring data. Instead, vast amounts of data are downloaded or collected in a database and integrated in a BIM system. This provides the ability to upload and access information from tablets or computers while on site. This technology supports better information management and provides a platform for data to be investigated and visualised. The user has the ability to compare field monitoring values to limit values and use coloured data markers to symbolise where limit values have been exceeded. The project teams can then be alerted of the increased risk level quicker than if data was processed manually.

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10 Areas of further work and research

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The eight areas of further work and research identified in Gaba et al (2003) remain valid and are repeated as follows. 1

The contractual environment within which embedded retaining walls are designed and constructed is fragmented. It is recommended that a lead designer be appointed to review and oversee all stages of the design and construction process to ensure that the client’s requirements are met. This is important to ensure consistency and certainty of outcome. The role of principal designer introduced under CDM 2015 may satisfy this role, provided they have the technical understanding and project oversight. If not, a lead designer for the embedded retaining wall should be identified within the project team.

2

Significant cost savings can be achieved by adopting: a

an appropriate method and sequence of construction, selection of wall type and optimisation of the temporary and permanent use of the retaining structure

b

a risk-based approach to design and construction through the use of the OM

c

an appropriate method of analysis. SSI analyses can result in economies of wall structure compared to limit equilibrium design methods (see point 4).



Ground anchorages are greatly underused in the UK. Their increased use may result in significant savings over propping schemes. This should be seriously considered.

3

The literature contains insufficient good-quality data on the performance of walls. Case study data in the UK are mainly limited to:



4

264

a

bored pile and diaphragm walls installed in stiff clays

b

short-duration measurements of wall deflections and, occasionally, ground surface movements behind such retaining walls

c

rare measurements of stresses around retaining walls

There is an urgent requirement for more case study data to provide high-quality measurement of the actual behaviour of different types of retaining wall installed in a range of ground conditions. In particular, there is a need for short-term and long-term measurements of: a

stress changes and displacements in the ground due to the installation and subsequent performance of the retaining wall during its working life so that stress changes due to wall installation and in the long term may be better understood

b

vertical and horizontal movements of the wall and the ground around the wall to establish appropriate relationships between wall deflections, depth of excavation and ground movements behind and in front of the wall (not only at ground surface level, but also with depth and distance from the wall)

c

stress changes, prop or anchor loads, wall performance and ground movements around 3D excavation geometries so that 3D effects, eg corner effects, and behaviour of berms may be better understood



Greater reliance on more advanced computers and associated software (point 4) will inevitably increase the risk of erroneous results due to a lack of fundamental understanding. It is important that the data are continually gathered, interpreted and fully understood.

Advances in computer software and hardware will continue. This will enable greater use of finite element and finite difference methods of analysis, particularly 3D. This should lead to the development of more complex soil constitutive models that are based on laboratory studies validated by field monitoring, particularly through the application of the OM, and model testing, eg centrifuge testing. There is still much research to be undertaken in this area.

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5

Much research work has been undertaken over recent years to understand better the small-strain stiffness of soil. However, the stiffness-strain behaviour of the material comprising the wall is not well understood and requires further research. In particular: a

the process of shear transfer and slippage at sheet pile interlocks/clutches and the associated effect on the mobilised section modulus

b

the relationship between cracked/uncracked and the short/long-term values of wall flexural stiffness (EI) for reinforced concrete walls



The analysis of SSI has progressed sufficiently for these to be a significant issue. This will become more urgent in view of the likely future trend in SSI analyses (point 4).

6

The use of plastic methods of design for steel sheet pile walls can lead to significant savings in material costs. The development and application of such methods to routine design requires further work and research.

7

Research should be undertaken in developing new methods of construction to achieve overall economy and ease of construction, for example:

8

a

the use of material other than steel, eg carbon or glass-fibre, as reinforcement to concrete

b

the development of different construction sequences, eg installing slabs before excavation in a top-down sequence.

Research should be undertaken to streamline methods of routine design to achieve economy, eg the development of simple rules on stress redistribution behind propped or anchored retaining walls for use in simple limit equilibrium calculations.

In addition to these points, it is recommended that the next iteration of EC7 should consider the following changes: 9

Replace the requirement for the DA1C1 calculation with a SLS calculation with partial factors applied to the calculated effects of actions, as set out in Section 7.3.2.

10 The significant benefits of an appropriately focused good quality site investigation are discussed in Appendix A7 to promote and reward better quality of site investigation, with encouragement offered through the CoP by lower partial factors due to greater certainty of parameters. For example, this could include better measurement of angle of shear resistance and providing a more logical and informed choice of design water levels (see Section 7.3). 11 Adopt a minimum unplanned excavation tolerance of 0.1 m, in preference to zero. 12 Include more precise guidance on the use of the OM, as set out in Section 7.4. Similarly, it is recommended that the next iteration of EC2 should consider the following changes: 13 For cast in situ piles without a permanent casing, it is recommended that the partial material factor for concrete (γc) should be 1.5, ie kf = 1.0 under Clause 2.4.2.5(2) of EC2-1. Currently the UK NA recommended value is kf = 1.1, which results in a material factor of γc = 1.5 × 1.1 = 1.65. It can be shown that the use of a concrete material factor of 1.65 reduces the moment capacity of a pile section at elevated axial stress levels compared with BS 8110-1:1997. In BS 8110, the ultimate design stress of reinforced concrete in flexure was limited to 0.67fcu /1.5 = 0.45fcu , compared with a limit of 0.85fck /1.65 = 0.41fcu under EC2 ( fck = 0.8fcu). The use of an increased material factor for concrete in EC2 has also implications on other calculations including the ultimate reinforced concrete bond stress (Clause 8.4.2 of EC2-1) and the maximum concrete stress limit for plain concrete (Clause 12.3.1(1) of EC2-1). If the same reliability with BS 8110 is deemed acceptable, the designer should consider applying for a concession to use a concrete material factor of 1.5. 14 EC2 does not include specific formulations for the capacity of circular sections. There are inherent complexities associated with the non-constant width and height of a circular section. While it is not practical to follow an iterative procedure to establish the effective depth and the lever arm between compression and tension chords along the length of a pile, the industry would benefit from a simple analysis model using the basic definitions proposed by Feltham (2004). 15 Where there is combination of shear and tension in reinforced concrete piles, the UK NA proposes a gradual reduction of cot(θ) as a function of tensile load, as per CEB-FIB (2013) recommendations. The current UK NA recommendation of cot(θ) = 1.25, irrespective of the amount of tension in the

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section, requires considerably more shear reinforcement in piles, compared with BS 8110. This is particularly relevant where there is only nominal amount of tension in the pile. If the same reliability with BS 8110 is deemed to be acceptable, the designer should consider a gradual reduction of cot(θ) as a function of axial stress in the pile.

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16 It is recommended that the material factor for reinforcing steel is revised to γs = 1.05. Currently, the EC2 material factor is γs = 1.15. It is hoped that advances in the quality control systems and testing would allow this change, such that the material factor for steel can be identical to the material factor used in BS 8110.

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SIMON, P, HILLSON, D and NEWLAND, K (1997) Project risk analysis management guide (PRAM) guide, second edition, Association for Project Management, High Wycombe (ISBN: 978-1-90349-412-7) https://www.apm.org.uk/PRAMGuide SIMPSON, B (1992) “Development and application of a new model for prediction of ground movements”. In: Proc of the Wroth memorial symposium. Predictive soil mechanics, St Catherine’s College, Oxford, 27–29 July 1992, Thomas Telford, London (ISBN: 0-72771-916-5) pp 628–643 SIMPSON, B (2000) “Partial factors: where to apply them?” In: Proc LSD2000: International workshop on limit state design in geotechnical engineering, Melbourne, Australia, 18 November 2000 SIMPSON, B (2012) “Eurocode 7 – fundamental issues and some implications for users”. In: Proc 12th Nordic geotechnical meeting, DGF Bulletin 27, Dansk Geoteknisk Forening (Danish Geotechnical Society), Denmark, May 2012 SIMPSON, B and DRISCOLL, R (1998) Eurocode 7 – a commentary, BRE Press, Building Research Establishment, Garston (ISBN: 978-1-86081-226-2) SIMPSON, B and POWRIE, W (2001) “Embedded retaining walls: theory, practice and design”. In: Proc of the 15th int conf on soil mechanics and geotechnical engineering, (XV ICSMGE), Istanbul, August 2001, Taylor and Francis, London (ISBN: 978-9-02651-838-6) pp 2505–2524 SIMPSON, B and YAZDCHI, M (2003) “Use of finite element methods in ultimate limit state design”. In: Proc LSD 2003: International workshop in limit state design in geotechnical engineering practice, Cambridge, Massachusetts, USA, 26 June 2001 SIMPSON, B and HOCOMBE, T (2010) “Implications of modern design codes for earth retaining structures”. In: Proc earth retaining conference (ER2010), American Society of Civil Engineers, Reston VA, USA, August 2010, pp 786–803 SIMPSON, B, CALABRESI, G, SOMMER, H and WALLAYS, M (1979) “Design parameters for stiff clays”. In: Proc of the seventh European conference on soil mechanics and foundation engineering, Brighton, England September 1979: the measurement, selection and use of design parameters in geotechnical engineering, vol 5, British Geotechnical Society, London (ISBN: 978-0-72770-080-3) pp 91–125 SIMPSON, B, VOGT, N and SETERS, A J (2011) “Geotechnical safety in relation to water pressures”. In: Proc thid int symp on geotechnical safety and risk (ISGSR11), Munich, Germany, 2–3 June 2011. N Vogt, B Schuppener, D Straub, G Brau (eds) Geotechnical safety and risk, Geotechnical Safety Network (ISBN: 978-3-93923-001-4) pp 501–517 SKEMPTON, A W (1957) “Discussion on planning and design of the new Hong Kong airport” Proceedings of the Institution Civil Engineers, vol 7, 2, Institution of Civil Engineers, London, pp 305–325 SKEMPTON, A W (1961) “Horizontal stresses in an over-consolidated Eocene clay”. In: Proc of the fifth international conference on soil mechanics and foundation engineering, 17–22 July 1961, Paris, Dunod, pp 351–357 SKEMPTON, A W (1986) “Standard penetration test procedures and the effects in sands of overburden pressure, relative density, particle size, ageing and overconsolidation” Géotechnique, vol 36, 2, ICE Publications, Institution of Civil Engineers, London, pp 425–447 SKEMPTON, A W and NORTHEY, R D (1952) “Sensitivity of clays” Géotechnique, vol 3, 1, ICE Publications, Institution of Civil Engineers, London, pp 30–53 SMETHURST, J A and POWRIE, W (2008) “Effective-stress analysis of berm-supported retaining walls” Proceedings of the Institution of Civil Engineers – Geotechnical Engineering, vol 161, 1, Institution of Civil Engineers, London, pp 39–48 SOGA, K, CHAU, C, NICHOLSON, D and PANTELIDOU, H (2011) “Embodied energy: soil retaining geosystems” KSCE Journal of civil engineering, vol 15, 4, Springer, UK, pp 739–749

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Statutes Acts Climate Change Act 2008 (c.27)

Directives Directive 92/57/EEC Temporary or mobile construction sites of 24 June 1992 on the implementation of minimum safety and health requirements at temporary or mobile construction sites (eighth individual Directive within the meaning of Article 16 (1) of Directive 89/391/EEC)

Regulations Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

The Construction (Design and Management) Regulations 2015 (CDM 2015) (S.I. 2015/51)

Standards British BS 5228-1:2009+A1:2014 Code of practice for noise and vibration control on construction and open sites. Noise BS 5930:1999 Code of practice for site investigations BS 5930:2015 Code of practice for site investigations BS 5975:2008+A1:2011 Code of practice for temporary works procedures and the permissible stress design of falsework BS 6031:2009 Code of practice for earthworks BS 6349-1:2000 Maritime structures. Code of practice for general criteria BS 6349-2:2010 Maritime works. Code of practice for the design of quay walls, jetties and dolphins BS 6349-1-3:2012 Maritime works. General. Code of practice for geotechnical design BS 6349-4:2014 Maritime works. Code of practice for design of fendering and mooring systems BS 8002:2015 Code of practice for earth retaining structures BS 8004:1986 Code of practice for foundations BS 8004:2015 Code of practice for foundations BS 8006:1995 Code of practice for strengthened/reinforced soils and other fills (superseded) BS 8008:1996+A1:2008 Safety precautions and procedures for the construction and descent of machine-bored shafts for piling and other purposes BS 8081:2015 Code of practice for grouted anchors BS 8102:2009 Code of practice for protection of below ground structures against water from the ground BS 8500-1:2002 Concrete. Complementary British Standard to BS EN 206-1. Method of specifying and guidance for the specifier BS 8500-1:2006+A1:2012 Concrete. Complementary British Standard to BS EN 206-1. Method of specifying and guidance for the specifier BS 8500-2:2015+A1:2016 Concrete. Complementary British Standard to BS EN 206. Specification for constituent materials and concrete BS 8110-1:1997 Strcutural use of concrete. Code of practice for design and construction BS 10175: 2001+A1:2013 Investigation of potentially contaminated sites – code of practice

Eurocodes BS EN 1990:2002 Basis of structural design BS EN 1990:2002+A1:2005 Eurocode Basis of structural design BS EN 1991 Eurocode 1. Actions on structures BS EN 1991-1-1:2002 Eurocode 1. Actions on structures. General actions. Densities, self-weight, imposed loads for buildings BS EN 1991-1-2:2002 Eurocode 1. Actions on structures. General actions. Actions on structures exposed to fire BS EN 1991-1-3:2003 Eurocode 1. Actions on structures. General actions. Snow loads BS EN 1991-1-4:2005 Eurocode 1. Actions on structures. General actions. Wind loads BS EN 1991-1-5:2003 Eurocode 1. Actions on structures. General actions. Thermal actions BS EN 1991-1-6:2005 Eurocode 1. Actions on structures. General actions. Actions during execution BS EN 1991-1-7:2006 Eurocode 1. Actions on structures. General actions. Accidental actions BS EN 1991-2:2003 Eurocode 1. Actions on structures BS EN 1991-3:2006 Eurocode 1. Actions on structures. Actions induced by cranes and machinery

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BS EN 1991-4:2006 Eurocode 1. Actions on structures. Silos and tanks BS EN 1992-1-1:2004 Eurocode 2. Design of concrete structures. General rules and rules for buildings BS EN 1992-1-2:2004 Eurocode 2. Design of concrete structures. General rules. Structural fire design BS EN 1992-2:2005 Eurocode 2. Design of concrete structures. Concrete bridges. Design and detailing rules BS EN 1992-3:2006 Eurocode 2. Design of concrete structures. Liquid retaining and containment structures BS EN 1993-1-1:2005 Eurocode 3. Design of steel structures. General rules and rules for buildings BS EN 1993-1-1:2005+A1:2014 Eurocode 3. Design of steel structures. General rules and rules for buildings BS EN 1993-1-2:2005 Eurocode 3. Design of steel structures. General rules. Structural fire design BS EN 1993-1-3:2006 Eurocode 3. Design of steel structures. General rules. Supplementary rules for cold-formed members and sheeting BS EN 1993-1-4:2006 Eurocode 3. Design of steel structures. General rules. Supplementary rules for stainless steels BS EN 1993-1-5:2006 Eurocode 3. Design of steel structures. Plated structural elements BS EN 1993-1-6:2007 Eurocode 3. Design of steel structures. Strength and stability of shell structures BS EN 1993-1-7:2007 Eurocode 3. Design of steel structures. Plated structures subject to out of plane loading BS EN 1993-1-8:2005 Eurocode 3. Design of steel structures. Design of joints

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BS EN 1993-1-9:2005 Eurocode 3. Design of steel structures. Fatigue BS EN 1993-1-10:2005 Eurocode 3. Design of steel structures. Material toughness and through-thickness properties BS EN 1993-1-11:2006 Eurocode 3. Design of steel structures. Design of structures with tension components BS EN 1993-1-12:2007 Eurocode 3. Design of steel structures. General. High strength steels BS EN 1993-2:2006 Eurocode 3. Design of steel structures. Steel bridges BS EN 1993-3-1:2006 Eurocode 3. Design of steel structures. Towers, masts and chimneys. Towers and masts BS EN 1993-3-2:2006 Eurocode 3. Design of steel structures. Towers, masts and chimneys. Chimneys BS EN 1993-3-2:2006 Eurocode 3. Design of steel structures. Towers, masts and chimneys. Chimneys BS EN 1993-4-1:2007 Eurocode 3. Design of steel structures. Silos BS EN 1993-4-2:2007 Eurocode 3. Design of steel structures. Tanks BS EN 1993-4-3:2007 Eurocode 3. Design of steel structures. Pipelines BS EN 1993-5:2007 Eurocode 3. Design of steel structures. Piling BS EN 1993-6:2007 Eurocode 3. Design of steel structures. Crane supporting structures BS EN 1994-1-1:2004 Eurocode 4. Design of composite steel and concrete structures. General rules and rules for buildings BS EN 1994-1-2:2005 Eurocode 4. Design of composite steel and concrete structures. General rules. Structural fire design BS EN 1994-2:2005 Eurocode 4. Design of composite steel and concrete structures. General rules and rules for bridges BS EN 1995-1-1:2004 Eurocode 5.Common rules and rules for buildings BS EN 1995-1-1:2004+A2:2014 Eurocode 5. Design of timber structures. General. Common rules and rules for buildings BS EN 1995-1-2:2004 Eurocode 5. Design of timber structures. General. Structural fire design BS EN 1995-2:2004 Eurocode 5. Design of timber structures. Bridges BS EN 1996-1-1:2005+A1:2012 Eurocode 6. Design of masonry structures. General rules for reinforced and unreinforced masonry structures BS EN 1996-1-2:2005 Eurocode 6. Design of masonry structures. General rules. Structural fire design BS EN 1996-2:2006 Eurocode 6. Design of masonry structures. Design considerations, selection of materials and execution of masonry BS EN 1996-3:2006 Eurocode 6. Design of masonry structures. Simplified calculation methods and simple rules for masonry structures BS EN 1997-1:2004 Eurocode 7. Geotechnical design. General rules BS EN 1997-1:2004+A1:2013 Eurocode 7. Geotechnical design. General rules BS EN 1997-2:2007 Eurocode 7. Geotechnical design. Ground investigation and testing BS EN 1998-1:2004 Eurocode 8. Design of structures for earthquake resistance. General rules, seismic actions and rules for buildings BS EN 1998-2:2005+A1:2009 Eurocode 8. Design of structures for earthquake resistance. Bridges BS EN 1998-2:2005+A2:2011 Eurocode 8. Design of structures for earthquake resistance. Bridges BS EN 1998-3:2005 Eurocode 8. Design of structures for earthquake resistance. Assessment and retrofitting of buildings BS EN 1998-4:2006 Eurocode 8. Design of structures for earthquake resistance. Silos, tanks and pipelines BS EN 1998-5:2004 Eurocode 8. Design of structures for earthquake resistance. Foundations, retaining structures and geotechnical aspects BS EN 1998-6:2005 Eurocode 8. Design of structures for earthquake resistance. Towers, masts and chimneys BS EN 1999-1-1:2007+A1:2009 General structural rules BS EN 1999-1-1:2007+A2:2013 Eurocode 9: Design of aluminium structures. General structural rules BS EN 1999-1-2:2007 Eurocode 9. Design of aluminium structures. Structural fire design BS EN 1999-1-3:2007 Eurocode 9. Design of aluminium structures. Structures susceptible to fatigue BS EN 1999-1-4:2007 Eurocode 9. Design of aluminium structures. Cold-formed structural sheeting BS EN 1999-1-5:2007 Eurocode 9. Design of aluminium structures. Shell structures

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Eurocodes – National Annexes NA to BS EN 1993-1-12:2007 UK National Annex to Eurocode 3. Design of steel structures. Additional rules for the extension of EN 1993 up to steel grades S700 NA to BS EN 1995-1-1:2004+A1:2008 UK National Annex to Eurocode 5: Design of timber structures. General. Common rules and rules for buildings NA+A1:2014 to BS EN 1997-1:2004+A1:2013 UK National Annex to Eurocode 7. Geotechnical design. General rules

Execution standards BS EN 1536:2010+A1:2015 Execution of special geotechnical work. Bored piles BS EN 1537:2013 Execution of special geotechnical works. Ground anchors BS EN 1538:2010+A1:2015 Execution of special geotechnical works BS EN 12063:1999 Execution of special geotechnical works. Sheet pile walls BS EN 12699:2015 Execution of special geotechnical works. Displacement piles BS EN 12715:2000 Execution of special geotechnical work. Grouting

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BS EN 12716:2001 Execution of special geotechnical works. Jet grouting BS EN 14199:2015 Execution of special geotechnical works. Micropiles BS EN 14475:2006 Execution of special geotechnical works. Reinforced fill BS EN 14490:2010 Execution of special geotechnical works. Soil nailing BS EN 14679:2005 Execution of special geotechnical works. Deep mixing BS EN 14731:2005 Execution of special geotechnical works. Ground treatment by deep vibration BS EN 15237:2007 Execution of special geotechnical works. Vertical drainage

Material and product standards BS EN 206:2013 Concrete. Specification, performance, production and conformity BS EN 10248-1:1996 Hot rolled sheet piling of non alloy steels. Technical delivery conditions BS EN 10249-1:1996 Cold-rolled sheet piling of non alloy steels BS EN 10249-2:1996 Cold formed sheet piling of non alloy steels. Tolerance on shape and dimensions

German DIN 4126 Stability analysis of diaphragm walls

International (ISO) BS EN ISO 14688-1:2002+A1:2013 Geotechnical investigation and testing. Identification and classification of soil. Identification and description BS EN ISO 14688-2:2004+A1:2013 Geotechnical investigation and testing. Identification and classification of soil. Principles for a classification BS EN ISO 14689-1:2003 Geotechnical investigation and testing. Identification and classification of rock. Identification and description BS EN ISO 22475-1:2006 Geotechnical investigation and testing. Sampling methods and groundwater measurements. Technical principles for execution BS ISO 5667-11:2009 Water quality. Sampling. Guidance on sampling of groundwaters

USA ASTM A572/A572M-15 Standard specification for high-strength low-alloy columbium-vanadium structural steel ASTM A690/A690M-13a Standard specification for high-strength low-alloy nickel, copper, phosphorus steel h-piles and sheet piling with atmospheric corrosion resistance for use in marine environments

PAS PAS 2080:2016 Carbon management in infrastructure

PD PD 6687-1:2010 Background paper to the National Annexes to BS EN 1992-1 and BS EN 1992-3

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A1

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A2 Example forms: CDM risk assessment

Figure A2.1

Risk assessment – decision justification

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Figure A2.2

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Risk assessment – record of selection

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Figure A2.3

Risk assessment register

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A3 Wall types For a more extensive coverage of different embedded wall types see Gaba (2012). The following is a brief summary of the most commonly-used embedded wall types with reference to their selection and constructability considerations. This appendix covers sheet pile walls, concrete piled walls and diaphragm walls.

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A3.1 SHEET PILE WALL Sheet pile walls should be hot rolled to BS EN 10248-1:1996 or cold formed to BS EN 10249-1:1996. For structural interlocking and resistance to de-clutching during installation hot rolled sheet piling should be used. There is no requirement for interlocking capability for cold formed products to BS EN 10249-1. The type of wall chosen will depend on the stiffness required for the design. Standard U-pile and Z-pile sections can be chosen for wall EI values up to 250 000 kNm2/m and are comparatively flexible compared with much stiffer steel combi and high modulus systems, which can be designed for stiffness EI values up to 10 000 000 kNm2/m. ArcelorMittal (2016) contains information on the various available sections and steel grades. The most commonly used basic shapes to BS EN 10248-1:1996 are the U or Z profiles shown in Figure A3.1. Profile selection depends on the design requirements, but it is important that the designer considers the driving conditions and the effects of installation on the design. In addition to the installation tolerances given in Table 3.2, the designer must consider the manufacturing tolerances of a sheet pile wall (see Table A3.1) and also the effect of the shape on structural properties (see Section 7.5.1).

Figure A3.1

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Typical steel sheet pile shapes (courtesy ArcelorMittal)

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Table A3.1

Manufacturing tolerances for steel sheet piles to BS EN 10248-1:1996 (courtesy ArcelorMittal)

Tolerances

U

Z

±5 %

±5 %

±200 mm

±200 mm

h≤200 mm:±4.0 mm

h≤200 mm:±5.0 mm

h> 200 mm:±5.0 mm

200 mm 8.5 mm :± 6 %

Mass Length

Height

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Thickness Width

±2 %

Double pile width

±3 %

Straightness

0.2 % of the length

Ends out of square

±2 % of pile width

ArcelorMittal (2016) describes several methods to inhibit water seepage through sheet pile walls, concentrating on sealing the interlocks. The systems include non-swelling sealants, hydrophilic (waterswelling) sealants, combination systems and welded interlocks. For the systems that rely on pre-applied sealants, the integrity of the vertical joints is highly dependent on the driving conditions and the installation techniques used. Site welding is the only impervious solution, but cannot be carried out below the excavation level except for piles driven in pairs, where the joint between the pair can be welded full length before installation. One of the attractive features of sheet pile walls is that in some cases they can be used as both temporary and permanent retaining structures. Guidance for the selection of appropriate values of elastic and plastic section properties is given in manufacturer’s literature. Durability and corrosion resistance of sheet pile wall sections is discussed in Section 7.5.1.

Figure A3.2

Sheet pile wall for an underground car park in Belgium (courtesy ArcelorMittal)

A3.2 HIGH MODULUS AND COMBI WALLS Where higher stiffness and strength are required, high modulus sections or combi steel walls may be used. Guidance for the design and detailing of combined walls is found in EC3-5, BS EN 12063:1999 and from ArcelorMittal (see Websites). High modulus sheet walls are formed by interlocking steel elements that have the same geometry. Combi steel walls are defined in EC3-5 as retaining walls composed of primary and secondary elements. The primary elements are normally steel tubular piles, I-sections or built-up box types, spaced uniformly along the length of the wall. The secondary elements are generally steel sheet piles of various types installed in the spaces between the primary elements and connected to them by interlocks (see Figure A3.3). The interlocks for combi walls should comply with BS EN 102481:1996 for structural integrity and to resist declutching. For stiffer high modulus walls the interlocks may be hot rolled or, in the case of pipe walls, may be structurally fabricated to accommodate installation tolerances required. Websites ArcelorMittal: http://corporate.arcelormittal.com

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a

HZM = high modulus wall Note Provides high stiffness straight face

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Typical EI range: 250 000 to 4500 000 kNm2/m

Pipewall Note Can also be driven by Japanese silent press (eg Canary Wharf station box) Typical EI range: 750 000 to 8500 000 kNm2/m

b

All hot-rolled connecting system Note Combi wall – HZM system Typical EI range: 250 000 up to 2500 000 kNm2/m

Tube combi wall – fabricated tube and hot rolled sheets Note Facilitates boring through tube Typical EI range: 500 000 up to 7000 000 kNm2/m

Figure A3.3

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Examples of high modulus (a) and combi steel walls (b) (from EC3-5)

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b

Box combi wall – fabricated box pile and hot rolled sheets Note Special grades possible Typical EI range: 300 000 to 700 000 kNm2/m

Figure A3.3

Examples of high modulus (a) and combi steel walls (b) (from EC3-5) (contd)

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A3.3 KING POST WALL Isolated steel beams or posts are installed along the line of the wall, either driven into place or placed in bored cast in situ piles at centres typically between one and three metres. The space between the posts is filled-in over the retaining height as the excavation proceeds, using either the traditional solution of timber railway sleepers (with the king post piles at 2.4 m centres), precast concrete elements, in situ or spray concrete. An example of a king post wall with precast concrete panels is shown in Figure A3.4. King post walls are not suitable for retaining coarse-grained soils below the water table and the excavation process relies on Figure A3.4 King post wall in City Park, Aberdeen (courtesy Cementation Skanska) some short-term cohesion in the retained material to avoid any significant over-break and collapse. Control of displacements in the retained material is largely dependent on the workmanship and the ability of the retained soil to be selfsupporting in the short term (Chapter 6).

A3.4 CONTIGUOUS PILE WALL A contiguous pile wall consists of a series of bored cast in situ concrete piles constructed along the line of the wall, without intersecting each other (see Figures A3.5 and A3.6). The gap between the piles is typically 150 mm, but this can be varied to suit site dimensions and the specific ground conditions within a typical range of 100 mm to 200 mm. Construction is by means of CFA or rotary bored piling rigs with temporary casings as required. The use of CFA piles limits the depth of installation of the reinforcement cage, as noted in Table 3.2.

Figure A3.5

Contiguous bored pile wall

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The maximum achievable wall depths for rotary bored piles are limited by the length of the kelly bar of each rig. Verticality tolerances should be considered to ensure either that the potential gap between piles does not increase unacceptably with depth, and that the piles do not overlap. In practice, it is unusual for pile walls to be constructed in excess of 25 m depth, but isolated piles may be extended deeper to provide vertical load capacity.

Figure A3.6

Contiguous bored pile wall (courtesy Cementation Skanska)

Contiguous pile walls as a standalone solution are not suitable for retaining water-bearing coarsegrained soils and are usually only specified as temporary walls. A permanent wall may be created with a structural facing applied to the piles to fill in the gaps. This may either take the form of a structural concrete facing wall, tied as necessary to the contiguous piles or sprayed concrete, which fills the gaps to beyond the pile centreline to form a positive key between the contiguous piles and the spray concrete. Any lining wall that is required to resist groundwater should be designed to resist these pressures and transfer the load either back to the contiguous piles or directly to the permanent propping system.

A3.5 SECANT PILE WALL Secant pile walls are formed by a series of interlocking primary and secondary bored cast in situ piles. Primary piles are those constructed first and may compromise a cement/bentonite/sand mix, unreinforced low-strength concrete, unreinforced or reinforced structure concrete. Secondary piles are those constructed second and these always comprise reinforced structural concrete. Secant wall piles are very versatile retaining wall systems, as they offer flexibility in plan shape leading to efficient use of basement space (see Figure A3.7). Depending on the make-up of the primary pile, secant walls are classified as hard/soft, hard/firm or hard/hard. This difference in the type of primary mix influences the design life of the structure (eg temporary works or permanent works), the structural capacity of the wall (primary piles are not typically reinforced, but they can be) and the minimum and maximum cut/ spacing between primary and secondary piles. Irrespective of the secant wall type, all secant walls are limited by the capabilities of the piling rig Figure A3.7 Secant wall Crossrail, Bond Street Station, London (courtesy used, which affects the verticality Cementation Skanska)

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that can be achieved and the depth over which interlock between primary and secondary piles can be maintained. A brief description of the different secant wall types is given in Sections A3.5.1 to A3.5.3.

A3.5.1 Hard/soft secant

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A hard/soft secant pile wall consists of overlapping piles, as shown on Figure A3.8. The primary piles are cast first and consist of a soft pile mix, typically cement and bentonite or cement, bentonite and sand with a characteristic compressive strength of 1 to 3N/mm2. The soft piles are unreinforced.

Figure A3.8

Hard/soft secant pile wall

The secondary piles are subsequently installed to intersect the primary piles as shown. An example of a completed hard/soft secant wall is shown in Figure A3.9. CFA piling rigs are often used to install the wall, but rotary bored piling rigs may be used. As for contiguous pile walls, the reinforcement depth may be limited by the installation technique. The depth of the hard/soft secant wall is limited by the ability to control the verticality tolerances in order to maintain the interlock. The primary and secondary piles may have different diameters and may extend to different depths (Beadman and Ward, 1998). For small projects it may be preferable to retain the same diameter for both the secondary and primary piles to avoid either duplicating piling rigs or the time taken to change the drilling diameter for a single rig operation. When ground conditions allow, the primary piles may be curtailed after penetrating finegrained strata below water-bearing coarse-grained strata. Soft piles have been used to retain up to eight metre head of groundwater. Typically, the minimum cut between hard and soft piles is 150 mm and the maximum cut should be selected to ensure adequate pile overlap at the lowest level to which a groundwater cut-off is needed, with due allowance for construction tolerances (see Table 3.2). In addition, pile spacing should be such that the installation of the secondary piles will not compromise the integrity of the primary piles (this often means that the spacing should be selected to leave at least 50 per cent of the primary pile intact). The soft pile mix is usually a short-term solution to retain water, due to the shrinkage and cracking characteristics of the mix when it dries out. For permanent works, a structural lining wall is installed at the face of the hard/soft secant piles rendering the soft piles redundant in the long term. Hard/soft walls are only occasionally used because hard/firm pile walls (see Section A3.5.2) are equally economic, more robust and do not require special mobilisation of bentonite mixing and handling facilities. Figure A3.9 Hard/soft secant pile wall (courtesy Cementation Skanska)

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A3.5.2 Hard/firm secant

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A hard/firm secant pile wall comprises interlocking primary and secondary piles as shown in Figures A3.10 and A3.11. The primary piles have a characteristic compressive cube concrete strength of 10 to 15 N/mm², which is retarded to reduce the strength of the mix while the secondary piles are installed between the primary piles. The primary piles are unreinforced. Typically, the characteristic strength of the primary pile mix is specified as a 56-day strength rather than the more usual 28-day strength. This enables such walls to be installed with CFA piling rigs, although cased CFA and segmental cased rotary systems can be employed to achieve better verticality tolerances (see Table 3.2).

Figure A3.10

Hard/firm secant pile wall

The primary piles are typically installed to a lesser depth than the secondary piles, which is dictated by the level where the interlock for water cut-off is required. This level would usually be a minimum of one metre into a low permeability layer or one metre below formation level, whichever is the lower. In some cases, where the low permeability layer is above excavation level and a permanent lining wall is provided, it is economic to terminate the primary piles one metre into the low permeability layer giving rise Figure A3.11 Hard/firm secant pile wall (courtesy Cementation Skanska) to a secant wall, which becomes a contiguous wall below the level of the primary piles. The low strength ‘firm’ mix may not meet the longterm durability requirements set out in BS 8500:2012 and BS EN 206:2013. In such cases additional measures to provide a permanent works solution may be necessary. Quillin et al (2005) provides useful guidance for the selection of appropriate firm mix in hard/firm secant piles. The spacing of the secondary piles is calculated by ensuring that there is an overlap at the depth where water cut-off is required when the most onerous verticality tolerances are applied (see Table 3.2). Typically, the cut between hard and firm piles will vary between 125 mm and 225 mm, although larger cuts may be possible for larger diameter piles. The designer should consult the specialist piling contractor in those cases. Pile spacing should be limited such that the installation of the secondary pile will not compromise the integrity of the primary pile (this often means the spacing should be selected to leave at least 50 per cent of the primary pile intact). Guidance on the selection of required cut and pile spacing to achieve the required interlock for water cut-off is shown in Figure A3.12 for CFA piling rigs and Figure A3.13 for segmental cased rotary piling rigs. The nomograms shown in Figures A3.12 and A3.13 can be used to establish the required interlock and pile spacing (secondary to secondary) as follows: 1

Establish ground profile by drawing ground stratigraphy.

2

Establish groundwater table level.

3

Establish maximum excavation level.

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5

Connect the point showing the value of required cut with the point showing the proposed secondary pile diameter. The line will intersect the secondary to secondary pile spacing axis showing the required spacing to achieve the required interlock. Determining minimum cut and spacing required for hard/firm piles constructed with CFA piling rigs (verticality tolerance 1:100, positional tolerance ±25 mm)

Establish level where interlock for water cut-off is required and draw a perpendicular line to the interlock depth axis to find the required cut. In the figures, this level signifies where there is [theoretically] zero interlock between primary and secondary piles, if the positional and verticality tolerances are considered in the worst case orientation for primary and secondary piles. The designer should consider what is minimum interlock required based on the ground and groundwater conditions.

Figure A3.12

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Figure A3.13 Determining minimum cut and spacing for hard/firm piles constructed with segmental cased rotary piling rigs (verticality tolerance 1:200, positional tolerance ±25 mm)

For practical purposes it is advantageous to use primary piles in internal corners to avoid the use of sacrificial piles, which may be used to prevent corner secondary piles taking the direction of least resistance and deviating with depth (see Figure A3.14). Sacrificial piles may be used depending on the length of the wall between the two corner points. However, walls are often very close to existing buildings, which may prevent their use.

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Ideal case: primary pile at the corner Figure A3.14

Less preferred: sacrificial pile

Corner detail for hard/firm secant wall

A3.5.3 Hard/hard secant The secondary and primary piles of a hard/hard secant pile wall are both cast with full-strength concrete and both may be reinforced, although the primary piles are typically unreinforced. After casting the primary piles, the secondary piles are formed by drilling into the primary piles using a casing rotated by a hydraulic rig or an oscillator. A thick-walled casing, typically 40 mm thick, is used to resist the high torque generated during the cutting process. As shown in Figure A3.15, the primary pile reinforcement should be detailed and placed to avoid being cut during the installation of the secondary piles, with due allowance for pile construction tolerances.

Figure A3.15

Hard/hard secant pile wall

The primary pile reinforcement comprises either a rectangular cage, carefully spaced and orientated in the pile bore before concreting, or a steel section. Reinforcement is commonly prefabricated offsite (especially in busy city centre sites with limited working space). The cages are then delivered in sections for splicing or coupling over the bore. Note that the designer must consider the health and safety related issues associated with splicing and couplers. The limiting depth for hard/hard secant pile walls is about 25 m and is limited by the piling rigs’ ability to rotate the casing. In suitable ground greater depths can be achieved, but will require increasingly powerful rigs and space to operate heavy duty casing oscillators, making such walls less economical. Where oscillators are used to remove casings there is a risk that some reinforcement cages will be twisted beyond normally specified tolerances. Specialist contractors offer a wide range of thick-wall casing sizes and thicknesses, and may also construct the wall using either cased-CFA or rotary-cased methods. It is important that the final design (including pile spacing and cage detailing) reflects the available casing sizes (see Table 3.3). Reinforcement cage sizes must be designed to achieve appropriate cover throughout the length of the pile and are normally based on the uncased diameter (see Figure 3.2). As with hard/firm pile walls the characteristic strength of the primary pile mix is commonly specified as a 56-day strength rather than the more usual 28-day strength. This enables such walls to be installed using less energy to cut the primary piles. The verticality tolerances and depth of cut are dictated by the plant used in pile construction. The pile design, including spacing and primary pile reinforcement detailing should reflect these practical constraints.

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Hard/hard secant pile walls may be used for circular shafts with no waling beams, using hoop compression to resist the ground and groundwater forces. The width and alignment of the interlock for the most onerous tolerances should be designed at maximum excavation level to ensure that it is capable of transmitting the compression forces. The maximum achievable depth of shaft, designed and constructed in this way, is dependent on the ground conditions, shaft diameter and the pile construction tolerances, but is commonly limited to about 20 m to 25 m with secant piling.

A3.6 DIAPHRAGM WALL Diaphragm walls are formed by sequenced excavation under a support fluid followed by the lowering of the prefabricated reinforcement cage and concreting using tremie pipes to displace Figure A3.16 Hard/hard secant pile wall (courtesy the fluid. Excavation is carried out by grab Cementation Skanska) (mechanical or hydraulic), using chisels to break up obstructions, or using reverse circulation mills to cut through harder materials. Reverse circulation mills rely on the spoil from the trench being held in suspension by the support fluid, which is pumped out and cleaned to extract the spoil. This requires an extensive site installation for the handling of the fluid. Fine-grained material may block the cutters and reduce the efficiency of the cutter. The disposal of the support fluid can be costly as it is treated as a contaminated material and requires disposal in a suitably licensed site. The reinforced concrete for diaphragm walls is normally cast in situ, but occasionally post-tensioned or precast panels may be used, pre-tensioned if required. The precast panels are lowered into the fluidfilled trench and sealed into the ground with in situ concrete or grout, tremied into position. Precast panels provide a preformed high-quality surface finish. However, often the weight of the panels makes this solution impractical.

Figure A3.17

Typical diaphragm wall panels and joints

Figure A3.17 shows a typical diaphragm wall panel. Panel widths are a function of the grab and cutter widths available and are typically 600, 800, 1000, 1200 or 1500 mm wide. Specialist contractors offer different joint systems for use with walls constructed with grabs. These include permanent precast concrete joints and temporary, vertically pulled or peel-off steel systems, some of which enable a vertical water-bar to be included. Joints in milled walls are formed by cutting into the concrete of the adjacent completed panel and casting directly against the milled face. Figure A3.18 shows a typical diaphragm wall construction sequence. The panels are most commonly excavated in one, two or three vertical bites. Flat panel lengths commonly vary between about three and seven metres. The final panel lengths are a complex function of: zz

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zz

grab/cutter length (vary between specialists – 2.2 m to 3 m, typically 2.8 m)

zz

joint type and dimensions (vary between specialists)

zz

panel type (starter/intermediate/closure, and tee-panel, corner panel)

zz

interfaces with the internal substructure.

Figure A3.18

Typical diaphragm wall construction sequence

Key

I = Intermediate panel

S = Starter panel

C = closure panel

In addition to variations between specialist contractor systems, the detailed planning of the site works will often affect the panel layout, for instance dictating where the starter and closure panels need to be located. So it is important that the panel layout is agreed between the designer, principal contractor and specialist contractor before being finalised. The installation of any form of embedded retaining wall results in some ground movement. The stability of a bentonite-filled temporary trench may be checked for adequacy using the formulations proposed by Huder (1972) or advanced 3D numerical methods. Where the induced ground movements may critically affect adjacent assets, shorter ‘singlebite’ panel lengths may be used to minimise the construction cycle time and corresponding ground deformations. The maximum depth of the diaphragm wall is limited by the depth of reach of the mill, which relates to the length of hose available to connect the bentonite pumps at the mill end to the surface. For a rope-operated grab, the limit is the length of the rope, which may be considerable. Diaphragm walls have been constructed to depths of 100 metres in the UK. The designer should consider the impact of verticality tolerances at the specified depths. Deep walls introduce other practical difficulties including the concrete mix design (for placing under high pressure, over a lengthy concreting period), installation of deep reinforcement cages with multiple cage splices, the length and weight of the cage, and the formation of deep Figure A3.19 Diaphragm wall attendances (courtesy Cementation Skanska) panel joints.

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Reinforcement is commonly prefabricated offsite and the cages are then delivered in sections for splicing or coupling over the trench. Owing to transport constraints the reinforcement for two and three-bite panels is generally formed in two distinct vertical cages placed with a gap between them. This gap, and the clearance between each cage and panel joint, must be sufficient to allow for the verticality tolerances of placing the stop-ends and cages, if clashes are to be avoided and cover maintained.

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The available space on site will govern the feasibility of a diaphragm wall solution, which requires a number of attendances including bentonite station, grabs, cranes and cage layout and handling areas (see Figure A3.19). Diaphragm walls may be used to form circular shafts, or peanut-shaped shafts with no waling beams, using hoop compression to resist the ground and groundwater forces (see Figures A3.20 and A3.21). The width and alignment of the panels and joints should be designed for the most onerous tolerances at maximum excavation level to ensure that the wall is capable of transmitting the compression forces. The maximum achievable depth of shaft, designed and constructed in this way, is dependent on the ground conditions, shaft diameter and the achievable wall construction tolerances. Circular diaphragm wall shafts up to 90 m deep and 30 m diameter have been successfully constructed in the UK.

Figure A3.20 Circular diaphragm wall shaft (courtesy Cementation Skanska)

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Figure A3.21 Peanut-shaped diaphragm wall shaft (courtesy Bachy Soletanche)

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A4 Geomechanics

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This appendix summarises the basic principles of soil and soft rock mechanics essential to retaining wall design, which are: zz

the concepts of total, effective and shear stress

zz

the representation of the stress state within the cross sectional plane of a long retaining wall using the Mohr circle construction

zz

the distinction between undrained (short term) total stress analysis and drained (long term) effective stress analysis

zz

key aspects of ground behaviour relevant to embedded retaining walls, including the effect of stress history, strength and stiffness.

It also: zz

Discusses the effects of concrete diaphragm and bored pile wall installation on the in situ lateral earth pressure coefficient and how this may be taken into account in analysis.

zz

Provides charts from which numerical values of active and passive lateral earth pressure coefficient may be obtained.

zz

Presents analyses of idealised embedded walls to illustrate the differences between the results obtained using limit equilibrium and SSI methods.

A4.1 STRESS ANALYSIS A4.1.1 Introduction The loads and forces applied to a solid body (such as a soil or rock mass) are distributed within that body as stresses. Provided there are no inhomogeneities to interrupt the transfer of stress, it is usually assumed that the stresses vary smoothly throughout the body, which is then described as a continuum. Any plane within a solid body in a general state of stress will be subject to both shear stresses acting parallel to the plane, and normal or direct stresses acting perpendicular to the plane. In ground mechanics, it is usual to take compressive direct stresses and strains as positive. This is in contrast to structural mechanics, in which tensile direct stresses and strains are conventionally taken as positive. The compression positive sign convention is adopted in ground mechanics because most soft ground is soil, which is a particulate material and cannot sustain tensile stresses unless the particles are cemented together. Also, the stresses arising from the self-weight of the material are significant, and generally compressive. So stresses in ground mechanics are frequently compressive. A stress increment, however, can be tensile if the cumulative stress remains compressive. Tensile strains are also permissible, again if the overall stress remains compressive.

A4.1.2 Principal stresses In a 3D body, and under a general state of stress, there will be three orthogonal planes on which the shear stress is zero. These three planes are known as principal planes, and the normal stresses on them are the principal stresses. By definition, the shear stress associated with a principal stress is zero. The largest principal stress is termed the major principal stress, the smallest principal stress is the minor principal stress, and the remaining principal stress is the intermediate principal stress.

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A4.1.3 Plane strain Many retaining walls are long in comparison with their height and the excavation width. So any crosssectional plane must be identical to any other in all respects, including the stresses acting on and within it. Considerations of equilibrium and symmetry require that the plane of the cross-section is a principal plane with zero strain in the direction normal to it.

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The cross-sectional plane is almost always the plane on which the intermediate principal stress acts, so that it contains the major and minor principal stresses. As the failure of a soil or a soft rock is governed by the major and minor principal stresses, analysis of a retaining wall that is long in comparison with its other dimensions need consider only the plane of the cross-section. Also, the longitudinal (intermediate) principal stress takes up whatever value is necessary to ensure that the strain in the longitudinal direction is zero. So, all deformation takes place within the cross-section of the structure – a condition known as plane strain. In reality, geotechnical structures are of finite extent and there may be differences in geometry and/or soil conditions along their length. However, the plane strain assumption is a useful and generally reasonable approximation that is made in nearly all simple retaining wall analyses.

A4.1.4 Total and effective stress A saturated soil comprises two distinct material phases, the soil particles and the pore water, which have very different shear strengths (ie ability to resist shear). The shear strength of the pore water is zero. The soil skeleton can resist shear partly because of particle interlocking and structure, but mainly because of inter-particle friction. The strengths of the soil skeleton and the pore water are very different, so it is necessary to consider the stresses acting on each phase separately. As the pore water cannot take shear, all shear stresses must be carried by the soil skeleton. The normal total stress (denoted σ) applied to a soil element may be separated into the effective stress (σ′) carried by the soil skeleton and the pore water pressure (u, measured relative to atmospheric pressure), using the principle of effective stress (Terzaghi, 1936). For a saturated soil: σ′ = σ – u

(A4.1)

The effective stress σ′ controls the (volumetric and shear) stiffness, strength and failure of the soil. However, the equilibrium of a retaining wall is governed by the total stress acting on it: σ = σ′ + u

(A4.2)

Unsaturated soils, containing air in the pores as well as water, are more complicated and are beyond the scope of this guide. In the design of embedded walls, soils are generally assumed to be either saturated or effectively dry. Similar considerations apply in soft rocks, although bonding or cementing between individual grains gives potentially significant strength to the solid phase, while the pore water pressure may act primarily in fissures between intact lumps of solid material.

A4.1.5 Mohr circle of stress The normal and shear stresses σ and τ acting on a plane projecting as a line in a given direction within the cross-section will depend on the orientation of the projected plane with respect to the major and minor principal stress directions (Figure A4.1).

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If the projected plane is perpendicular to either the major or the minor principal stress, the shear stress acting in the direction of the projected plane will be zero. However, in general there will be a shear stress acting along the projected plane, the magnitude of which increases as the plane is rotated away from the direction of the planes of principal stress. The stress state within a plane containing any pair of principal stresses is represented by means of a graphical construction known as the Mohr circle of stress (Figure A4.2). This is a circle, plotted on a graph of shear stress τ against normal stress σ. The circle may be plotted for either Figure A4.1 Normal and shear stresses acting on an imaginary plane total normal stresses, σ, or effective within the cross-section plane (after Powrie, 2014) normal stresses, σ′. The total and effective shear stresses are the same, as all shear stress must be carried by the soil skeleton. The Mohr circle passes through the points representing the major and the minor principal stresses, whose co-ordinates are (σ′1, 0) and (σ′3, 0) (for effective stresses) and (σ1, 0) and (σ3, 0) (for total stresses) respectively. The centre of the circle of effective stress is at ([σ′1 + σ′3]/2, 0), and the centre of the circle of total stress is at ([σ1+ σ3]/2, 0). Recalling that σ = σ′ + u (where u is the pore water pressure), the centres of the circles of effective and total stress are separated by a distance equal to u along the normal stress axis. (σ′1 + σ′3)/2 is the average of the major and minor principal effective stresses and is conventionally given the symbol s′. Similarly, (σ1+ σ3)/2 is the average of the major and minor principal total stresses and is given the symbol s. The radius of the circle of effective stress is [σ′1 – σ′3]/2, while the radius of the circle of total stress is [σ1 – σ3]/2. These are identical, because the pore water pressure u is cancelled out in the subtraction of the two principal effective stresses. [σ′1 – σ′3]/2 (or [σ1 – σ3]/2) is equal to the maximum shear stress acting within the principal plane represented by the Mohr circle and is conventionally given the symbol t.

Figure A4.2

Mohr circles of stress (after Powrie, 2014)

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The stresses acting on a projected plane at an angle θ anticlockwise from the plane on which the major principal stress acts are found by drawing a line from the centre of the Mohr circle to the circumference, which makes an angle 2θ (measured anticlockwise) with

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the normal stress (σ or σ′) axis. The stress state on the projected plane (in effective stress terms, [σ′, τ]) is given by the point where this diameter meets the circumference of the circle (Figure A4.2). The Mohr circle of stress shows that, unless the major and minor principal stresses are equal, shear stresses must act somewhere within the plane for which the Mohr circle has been drawn. The maximum shear stress within the plane represented by the circle is equal to the radius of the Mohr circle, [σ′1 – σ′3 ]/2 = [σ1 – σ3 ]/2. It occurs at angles of ±90° to the normal stress axis on the Mohr diagram, indicating that in reality, the shear stress is largest on planes that are at ±45° to the planes on which the major and minor principal stresses act.

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The maximum ratio of shear to normal effective stress, (τ/σ’)max , within the plane represented by the Mohr circle occurs where the tangents through the origin touch the circle. If the soil has no cohesion and is at shear failure, this occurs at angles of ±(90° ± ϕ′) to the normal stress axis on the Mohr diagram, indicating that in reality, the shear stress is largest on planes that are at ±(45° + ϕ′/2) to the plane on which the major principal stress acts, and ±(45° – ϕ′/2) to the plane on which the minor principal stress acts.

A4.2 DRAINED AND UNDRAINED CONDITIONS One of the key differences between the engineering behaviour of clays and sands is the rate at which the effective stresses within the soil respond to a loading or unloading, or a change in pore water conditions, at a boundary. Application or removal of a load at the surface of a saturated soil results initially in a change in the pore water pressure. This leads to a hydraulic gradient, in response to which pore water flows out of or into the soil and the soil compresses or swells. As the water flows out of the soil, the pore water pressures gradually move to their long-term equilibrium values and the soil deforms in the timedependent process of consolidation. The time taken for consolidation to occur decreases with increasing soil stiffness (because the ultimate volume change is reduced) and permeability (because the pore water can flow more easily through the soil skeleton). The consolidation behaviour of a soil is characterised by the consolidation coefficient, cv cv = k.Mo /γw

(A4.3)

where k is the coefficient of permeability of the soil in the direction of drainage Mo is the 1D modulus (constrained modulus) in the direction of compression or swelling γw is the unit weight of water In sands that are often both stiff and permeable, consolidation is effectively instantaneous. Volume changes are small, and pore water pressures move rapidly to their equilibrium long term or drained values. The term drained is used to indicate that the pore water pressures have reached their longterm, steady-state values, ie they will not usually be zero. As the steady-state pore water pressures can be calculated by means of an appropriate seepage analysis such as a flownet, the effective stresses can be determined and the behaviour of the sand can always be analysed using effective stress parameters. In clays, consolidation (or swelling) in response to a change in boundary loading or water pressure conditions can take many years or even decades. The changes in pore water pressure that occur in a clay during consolidation or swelling can significantly affect the stability of a geotechnical structure. In general, unloading processes will promote swelling and softening of the soil (which can make failure more likely in the long term), while loading processes will cause long-term consolidation (so that failure is more likely in the short term, immediately after loading). So in clays, it is usually necessary to investigate separately the possibility of failure in both the short term (ie immediately after loading/unloading) and in the long term. The long-term calculation must be carried out in terms of effective stresses and pore water pressures, and is often referred to as a drained analysis. A short-term calculation in terms of effective stresses can

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be problematic using simple analytical tools, as the non-equilibrium pore water pressures following loading/unloading and during consolidation/swelling [and the effective stresses] in a clay soil are difficult to predict. However, it is possible to carry out an analysis in terms of total stresses, using a different strength criterion, for deformations that occur rapidly in comparison with the time it takes for changes in specific volume to occur. This is termed an undrained analysis, as the underlying assumption that there is no volume change is equivalent to assuming that there is no drainage of pore water into or out of a saturated soil.

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Excavation in front of an embedded retaining wall in a stiff clay is also an unloading process. It is likely to generate negative excess pore water pressures, so that stability would be expected to be more critical in the long term than in the short term. However, it also brings about changes in the hydraulic boundary conditions (for example, a lowering of the groundwater level in front of the wall) that make the longterm equilibrium pore water pressures different from the initial groundwater regime. In addition, there may be a degree of vertical reloading as basement floors are constructed and soft clays can be more problematic in the short term. Each case must be assessed based on its own facts. While soil behaviour is governed by the effective stress, the wall structure will respond to the total stress acting on it so changes in pore water pressure and effective stress must both be considered in retaining wall design.

A4.3 STRESS HISTORY The current volumetric state (void ratio, specific volume or water content) of a clay is related to its previous loading or unloading history. The geological stress history of a stiff clay deposit is likely to comprise 1D compression, as further material was deposited on top, followed by 1D swelling as overlying material was removed by erosion. For example, it is estimated that the London Clay at Bradwell, Essex, has in the past been subjected to an overburden about 1450 kPa greater than that at present (Skempton, 1961). The stress history of a typical overconsolidated clay deposit may be represented on a graph of the specific volume v against the natural logarithm of the vertical effective stress, ln σ′v , (Figure A4.3), or on a graph of ρ′v against σ′h as shown in Figure 4.1. During deposition of a clay, there is a unique, straight-line relationship between the specific volume v (=1+e) and ln σ′v . This is known as the 1D normal compression line. During first compression, deformations occur due to particle distortion and particle Figure A4.3 Schematic stress history of an overconsolidated clay slip or breakage, as the soil skeleton (after Powrie, 2014) rearranges itself to support the increased stress. The first of these is recoverable on unloading, but the second is not. So on unloading (and also on subsequent reloading should it occur), the soil response in terms of the change in specific volume for a given change in vertical effective stress will be much stiffer. In unloading and reloading, the soil will follow a hysteresis loop on the graph of v against ln σ′v , which is usually idealised as a straight line. Unlike

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the normal compression line, there is no unique unload/reload line, which can begin from any point on the normal compression line where the soil starts to be unloaded. A soil whose state lies on the normal compression line has never before been subjected to a vertical effective stress higher than the current value. Such a soil is termed normally consolidated. A soil that has previously been consolidated to a higher vertical effective stress than that which currently acts is overconsolidated, with an overconsolidation ratio (OCR) given by: OCR = σ′v(max prev) /σ′v(current)

(A4.4)

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A rise in the groundwater level will reduce the vertical effective stress, resulting in overconsolidation without physical overburden removal. The overconsolidation ratio (Equation A4.4) is a crude but important indicator of the stress state of a clay, in relation to its previous stress history. An overconsolidated clay is likely to be much stiffer than a normally consolidated clay at the same effective stress σ′v , and stiffness generally increases with overconsolidation ratio. A further important distinction is that heavily overconsolidated clays (ie with a high OCR) will often dilate when sheared, while normally consolidated or lightly overconsolidated clays (ie with an OCR of less than about 3) may compress. The tendency of heavily overconsolidated clays to dilate when sheared will lead to the development of a peak strength, which is unlikely in a normally consolidated or lightly overconsolidated material. When sheared in undrained conditions, in which volume change is prevented, heavily overconsolidated clays will generate reduced pore water pressures that may become negative (ie suction), while lightly overconsolidated or normally consolidated clays will generate increased pore water pressures.

A4.4 SHEAR STRENGTH The shear strength of a soil may be defined in terms of effective stress as an angle of shearing resistance ϕ′ and a further component, generally small or zero, known as effective cohesion, c′. In terms of total stress, for an undrained analysis of a clay, soil strength may be defined as an undrained shear strength cu .

A4.4.1 Effective stress When sheared (and, in the case of a clay, allowed to drain during shear), a loose or lightly overconsolidated soil will gradually compress until it reaches a constant volume (critical state), at which shearing may continue without any further change in shear stress τ, normal effective stress σ′ and specific volume v. A dense or heavily overconsolidated soil will initially compress and then dilate to achieve the critical state (Figure A4.4). For a loose soil, the critical state is relatively easy to identify and, if the specimen does not rupture, may reasonably be determined on the basis of measurements made at the boundaries of a test specimen by assuming that the specimen deforms as a continuum with uniform stresses and strains. To define fully the state of a soil, three variables are required quantifying the specific volume v, the shear stress τ and the normal effective stress σ′. Critical states are combinations of these three variables at which steady, continued shear deformation can take place. On a 3D plot of specific volume against shear stress against normal effective stress, the critical states form a unique line known as the critical state line. On a graph of shear stress τ against normal effective stress σ′ on the plane of maximum stress ratio, the critical state line is straight with Equation A4.5: τ = σ′.tanϕ′crit

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(A4.5)

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Figure A4.5

Critical state line (after Powrie, 2014)

On a graph of specific volume v against the logarithm of the normal stress, ln σ′, the critical state line is idealised as straight, as indicated in Figures A4.5 and A4.6.

Key L = loose sample D = dense sample

For a dense or heavily overconsolidated soil, the stress-strain behaviour is more complex. The shear stress rises to a peak, at or near which a rupture surface develops. The shear stress then falls rapidly, and failure is brittle. Once the rupture has formed, it governs the overall behaviour of the soil element being tested.

Figure A4.4 Typical stress-strain data for a loose (lightly overconsolidated or normally consolidated) soil and for dense (heavily overconsolidated) soil (after Powrie, 2014)

a

b

Figure A4.6 Undrained state paths for clay specimens having the same specific volume v against ln σ’ (a) and τ against σ’ (b). Note that A is heavily overconsolidated, while B is lightly overconsolidated (after Powrie, 2014)

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Compression between the ends of a triaxial test specimen is due to relative sliding along the rupture surface rather than a uniform, continuous axial strain. The axial load that the specimen can sustain depends on the stress state of the soil in a thin rupture zone, which is likely to soften and swell preferentially and be very different from the remainder of the specimen. So, it is inappropriate to convert loads and displacements measured at the boundaries of the test specimen to equivalent uniform stresses and strains after a rupture surface has developed.

Figure A4.7

Ring shear test data for undisturbed London Clay (from Bishop et al, 1971)

Figure A4.7 shows data from a ring shear test on an undisturbed sample of unweathered London Clay. As shearing continues after the peak, the clay particles in the rupture zone gradually become aligned with the direction of shear, resulting in a gradual polishing and loss of strength on the rupture surface. The angle of shearing resistance ϕ′ (or the maximum stress ratio τ/σ′) falls through the critical state, until eventually a residual strength is reached. When quoting a value of soil strength, it is important to state whether it refers to the peak, critical state, or residual. A further point is that the peak angle of shearing resistance ϕ′p occurs when the stress ratio τ/σ′ is greatest, which in a conventional triaxial compression test (in which both the shear stress and the normal effective stress increase with the axial load) does not generally coincide with the peak shear stress. Traditionally, effective stress strength parameters were often determined by carrying out three conventional triaxial compression tests on similar specimens at different cell pressures – plotting Mohr circles of effective stress at peak stress ratio, and drawing the best fit tangent that defines a failure envelope of the form: τ = c′ + σ′tanϕ′tgt

(A4.6)

where ϕ′tgt is the slope of the failure envelope and c′ its intercept with the shear stress axis. If the data used to plot the Mohr circles of stress represent critical states, the slope of the failure envelope will be equal to the critical state (constant volume) angle of shearing resistance ϕ′crit (= ϕ′cv) and the failure envelope will pass through the origin giving c′ = 0. This is because the only component of soil strength still operating at the critical state is due to interparticle friction, ie if the effective stress is zero, then so is the shear strength. In general, however, treatment of peak strength data in the way indicated by Equation A4.6 is unsatisfactory, because: 1

Equation A4.6 has no direct physical interpretation – it is merely a fit to the data over the range for which data are available. c′ is not necessarily indicative of a real cohesion (ie an ability to withstand shear stresses at zero effective stress), and ϕ′tgt is not a true angle of shearing resistance, because when it is defined according to Equation A4.6 it may be smaller than the value of ϕ′crit (or ϕ′cv) for the same soil.

2

The approach takes no account of differences in stress history and specific volume between individual specimens, which would be expected to alter the potential for dilation and so the peak strength is achieved.

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3

The scatter in the values of c′ and ϕ′tgt obtained from similar sets of specimens can be very wide (Muir Wood, 1991).

A more satisfactory interpretation of peak strength data may be obtained by normalising the values of shear stress and normal effective stress on the plane of maximum stress ratio with respect to the equivalent consolidation pressure σ′e (the value of σ′ on the normal compression line at the current specimen specific volume). In this way, the dependence of peak strength on stress history and specific volume or water content is to some extent taken into account, and the scatter in the results is reduced. This is demonstrated by Muir Wood (1991) using triaxial test data. When plotted on a graph of τ/σ′e against σ′/σ′e , the peak strength data should lie on a straight line of equation, known as the Hvorslev line:

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τ/σ′e = he (g + σ′/σ′e)

(A4.7)

The Hvorslev line is limited at its left hand end by the point where it forms a tangent to a Mohr circle of stress passing through the origin (due to the inability of the soil to carry tension). At its right hand end, the Hvorslev line intersects the critical state line, which represents a soil insufficiently overconsolidated to generate peak strength. This behaviour may be idealised as a tri-linear failure envelope, representing tensile failure at very low stresses, rupture at intermediate stresses (the Hvorslev line), and yield towards a critical state at higher stresses (Schofield, 1980 and Figure A4.8). The key point is that to understand the peak strengths of clay, it is necessary to take account of both the normal effective stress and the specific volume at failure. Both of these will influence the potential for dilation and the peak strength actually developed. A bonded or cemented soil may have a capacity to carry a shear stress at zero normal effective stress, in which case the c′ component of the strength in the generalised Mohr-Coulomb failure envelope represented by Equation A4.6 will be non-zero. Otherwise, the peak strength is probably best defined by the line joining the stress state at the peak to the origin on the Mohr diagram, giving a stress-dependent peak angle of shearing resistance: Figure A4.8

Normalised failure behaviour regimes for soils (after Powrie, 2014)

ϕ′peak = tan-1 (τ/σ′) peak

(A4.8)

Alternatively, a non-linear envelope of the form: τ peak = A.σ′b

(A4.9)

where A and b are soil-specific parameters, may be used to represent peak stress states. For most soils, the difference between the critical state (constant volume) and the peak angles of shearing resistance is related to the angle of dilation, ψ. It is important to determine the peak strength over a stress range applicable to the retaining wall under consideration. This is discussed in Section 5.5.5. The strength of rocks is complex and depends on a range of factors including the rock components (grains, cement, voids and discontinuities) and their interaction, but also, as with soils, on the relative stress conditions at which failure occurs. Weak rocks exhibit three different modes of failure – tensile failure at low stresses, shear failure at intermediate stresses and pore collapse at high stresses, as discussed by Sanderson (2012). These are illustrated in Figure A4.9, which is analogous to Figure A4.8 for soils.

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Differential stress (q) against mean effective stress (p’) showing the three main types of failure mechanism: 1

To = tensile strength.

2

Co = uniaxial compressional stress (UCS).

3

Pc = pre-consolidation pressure.

Figure A4.9

Normalised failure behavioural regimes for rocks (from Sanderson, 2012)

In the context of embedded retaining walls, the failure mode of most interest is usually shear. So the strength of weak rock masses in retaining wall analyses is generally represented by the Mohr-Coulomb failure criterion, Equation A4.8, or variants of it proposed by Byerlee (1978) or Hoek and Brown (1980). Experiments of sliding on planar surfaces cut in rock show a linear relationship between the shear stress τ and the normal effective stress σ′ at low stresses, of the form: τ = σ′tanϕ′

(A4.10)

where tanϕ′ is the coefficient of sliding friction, up to a normal effective stress σ′ of 200 MPa. Results reported by Byerlee (1978) indicated an average of tanϕ′ = 0.85, irrespective of rock type at least for well consolidated and crystalline rocks (Sanderson, 2012). Cohesionless friction of the form given in Equation A4.10 is often used to characterise failure in many fractured rocks, with coefficients of sliding friction (tanϕ′) generally between 0.65 and 0.85. This has become known as Byerlee’s Law (Sanderson, 2012). However, the presence of a weak material in the fault plane can considerably reduce the effective angle of friction. As with soils, the actual shear resistance is determined by the effective normal stress. Pore water pressures within the fractures will reduce the effective stress according to Equation A4.1, and the actual sliding resistance. The potential for failure by frictional sliding on pre-existing rock fractures will depend on the orientation of the fractures. Only if there is a sufficient variation in fracture orientation within a rock mass will shear failure on optimally oriented fractures dominate (eg Zhang and Sanderson, 2001). So, the potential for sliding along a dominant fracture set should be considered (Richards et al, 2004). In reality, the transition between the tensile and shear modes of failure illustrated in Figure A4.9 is smooth rather than abrupt. This can be taken into account by means of the failure condition proposed by Hoek (1966), and later modified by Hoek and Brown (1980). The generalised Hoek-Brown failure criterion is:

(A4.11)

where σ′1 and σ′3 are the major and minor principal effective stresses at failure

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σ′ci is the unconfined compressive strength of the intact rock mb is the Hoek-Brown constant s and a are constants that depend on the rock mass characterisation (see Chapter 5).

A4.4.2 Total stress (undrained shear strength) The critical state model illustrated in Figure A4.5 describes the state of a soil at failure in terms of the specific volume and the effective shear and normal stresses. It has already been mentioned that clays will consolidate or swell in response to a change in loading or in hydraulic boundary conditions, during which time the pore water pressures – and the effective stresses – can be difficult to predict.

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The critical state framework provides an alternative way of characterising the strength of a clay soil in the short term, assuming that the clay is sheared quickly in comparison with the time it takes for changes in specific volume to occur. Shear failure at constant specific volume must follow a horizontal path on the graph of v against ln σ′ from the initial condition to the critical state (see Figure A4.6a). The position on the critical state line is fixed by the specific volume of the specimen as sheared, which also defines the shear stress at undrained failure (see Figure A4.6b). The effective stress state on the critical state line is independent of the external changes in stress (loading or unloading) that cause failure. The pore water pressure will take up the difference between the total and effective normal stresses, but the shear stress at failure is a function of the specific volume alone.

Note See Figure A4.6 for undrained state paths for clay samples A and B.

Figure A4.10 Mohr circles representation of undrained shear strength failure criterion in terms of total stresses for shearing at constant specific volume (after Powrie, 2014)

Figure A4.10 shows that, for a specimen of clay sheared undrained to the critical state, there is only one possible Mohr circle of effective stress at failure. The radius of this Mohr circle – and so its position on the σ′ axis, given that it must touch the failure envelope τ = σ′ tanϕ′crit – depends on the specific volume of the clay as sheared. The position of the Mohr circle of total stress on the normal stress axis depends on the applied loading, with the pore water pressure (which may be negative or positive) being equal to the distance between the centres of the circles of total and effective stress (Figure A4.10). However the radius of the Mohr circle of total stress must be the same as the radius of the Mohr circle of effective stress. So, the envelope to all possible Mohr circles of total stress is given by: τ max = ±cu

(A4.12)

where τmax is the maximum shear stress within the specimen cu is the undrained shear strength of the clay.

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The cu model for failure in terms of total stresses is unique, and is applicable only to clay soils sheared at constant volume. A further problem is that the value of undrained shear strength can be particularly sensitive to sample or specimen size, sample disturbance and the test apparatus used, as discussed in Section 5.5.

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The undrained shear strength of a clay of a given specific volume is usually determined in the laboratory by carrying out unconsolidated-undrained triaxial tests at different cell pressures. When the Mohr circles of total stress are plotted from such tests, they should all have the same radius cu provided that the specimens all had the same initial specific volume. Their common tangent should be horizontal, intercepting the τ axis at τ = cu . If the common tangent is not horizontal, the inference is that the specimens as tested did not have the same specific volume at failure or that the tests were not fully undrained. This would occur if the specimens had been allowed to consolidate to equilibrium at the test cell pressure before the start of shear (ie the tests carried out were consolidated-undrained, rather than unconsolidated-undrained). In this case, the test results provide an indication of the increase in cu with decreasing water content, due to increasing initial stress, perhaps corresponding to increasing depth in the field. Alternatively, the specimens may not have been fully saturated. In this case, the air is compressed and dissolved in the pore water as the cell pressure is increased, so that changes in specimen volume and void ratio occur without the passage of pore fluid (air or water) into or out of the specimen. The notion that test results with an inclined tangent may be described in terms of an ‘undrained friction angle ϕu′ and an ‘undrained cohesion intercept cu′ , is fundamentally flawed. Multi-stage tests, in which a single specimen is brought to failure at three successively greater cell pressures, are not recommended because the later stages are effectively tests on damaged material that may give erroneous results.

A4.4.3 Drained or undrained conditions As excavation in front of an embedded retaining wall is an unloading process, the stability of the wall might often be expected to be more critical in the long term as the negative excess pore water pressures induced on excavation dissipate and the clay tends to soften and swell. In the short term, the negative excess pore water pressures maintain the average effective stress s′ (=½[σ′1 + σ′3]) temporarily high, so that the Mohr circle of effective stress is initially well within the failure envelope (Figure A4.11).

Figure A4.11

Failure after dissipation of negative excess pore water pressures induced on excavation (after Powrie, 2014)

As the negative excess pore water pressures dissipate, the clay swells and the average effective stress s′ is reduced. The shear stress required to maintain the stability of the excavation remains approximately constant, and when the Mohr circle of effective stress touches the effective stress failure envelope, the soil will fail (Figure A4.11).

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Although a permanent wall must be designed based on an effective stress analysis for the anticipated long-term pore water pressure conditions, a more economical design will usually result if it can be assumed that substantially undrained conditions will prevail over the lifetime of a temporary structure. The key question is whether this assumption is reasonable. The answer will depend on factors that even in the same soil may vary from site to site. These factors, which are discussed in Chapter 5, are difficult to quantify. Softening is likely to occur first near the retained and excavated soil surfaces, which may be taken into account by using effective stress analysis or reduced undrained shear strengths in these zones. The depth L below a free surface to which the soil is affected by softening after an elapsed time t may in some circumstances be estimated approximately as:

(A4.13)

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where cv is the coefficient of consolidation (see Bolton, 1991 or Powrie, 2014). This assumes that the free surface acts as a recharge boundary, but that there is no source of recharge within the actual soil. As the horizontal permeability of a soil is often an order of magnitude or so greater than the vertical permeability, lateral recharge may often be more significant than vertical recharge in the field. This is discussed further in Chapter 5. Changes in pore water pressure indicating softening could be monitored using appropriate piezometers.

A4.5 SOIL STIFFNESS Calculations based on soil strength can be used to assess the stability of a wall, but not on their own to estimate wall and soil movements under working conditions. To do this, a stress-strain relationship for the soil is needed (Figure A4.12).

Figure A4.12

Definitions of soil stiffness (after Powrie, 2014)

The simplest approach is to describe the pre-failure behaviour of the soil as linear elastic. This is not a realistic model because the ‘stiffness’ of a clay (defined either as a tangent stiffness, dσ/dε, or as a secant stiffness Δσ/Δε where Δσ and Δε represent changes of generalised stress and strain from a defined starting point) depends on the: zz

average effective stress, p′ (=[σ′1 + σ′2 + σ′3]/3)

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zz

overconsolidation ratio

zz

stress path being followed, particularly in relation to the recent stress history.

The typical variation of soil stiffness with continued shearing is discussed in Section 5.5.5.

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The maximum shear strain increment in the soil around an embedded retaining wall with acceptably small deflections is likely to be in the order of 0.1 per cent (Atkinson and Sällfors, 1991, Mair, 1993). This can be used, in association with a stiffness-strain curve such as that described in Section 5.5.5, to estimate a suitable soil stiffness profile for use in analysis. Usually, the soil stiffness must be allowed to vary with depth to account for the effects of increasing average effective stress and decreasing overconsolidation ratio. It is generally accepted that with a judicious choice of stiffness parameters, numerical analyses (eg finite element or finite difference) using a linear elastic-plastic soil model can lead to reasonable estimates of wall movements and bending moments (eg Burland and Kalra, 1986, Powrie et al, 1999). However, the calculation of realistic ground movements requires the use of a more complex soil model that better represents the degradation of stiffness with strain indicated in Section 5.5.5 (eg Jardine et al, 1986, Simpson, 1992, Stallebrass and Taylor, 1997, Atkinson, 2000). It is important to check that the computed stresses do not take the soil beyond the strain range for which the stiffness parameters are chosen. The effect of the stress changes associated with wall installation on the stiffness of the soil during subsequent excavation in front of the wall are a further consideration (see Powrie et al, 1998).

A4.6 EFFECTS OF CONCRETE DIAPHRAGM AND BORED PILE WALL INSTALLATION A4.6.1 Introduction The process of installing an embedded retaining wall may influence the behaviour of the structure in three ways: 1

It affects the stress state in the ground adjacent to the wall. In a numerical analysis, this may have a significant effect on wall performance (movements and bending moments) during and after excavation (eg Potts and Fourie, 1984).

2

It affects the recent stress history of the soil, and so its subsequent stress-strain response (Powrie et al, 1998).

3

It may cause significant ground movements (Stroud and Sweeney, 1977).

This section summarises research into the effects of diaphragm wall and bored pile wall installation in overconsolidated clays on the stress state in the surrounding ground in relation to the first of these. This has been used to inform the general guidance given in Section 5.5.3. Ground movements, which may be significant in soft or sensitive soils, or where the water table is high, are addressed in Section 6.2.

A4.6.2 Key literature Investigations into the effects of diaphragm and bored pile wall installation on lateral earth pressures have been reported by: zz

Tedd et al (1984): field data from a secant pile retaining wall on the M25 at Bell Common, UK

zz

Powrie (1985): elastic stress analysis

zz

Gunn and Clayton (1992) and Gunn et al (1992): plane strain and axi-symmetric finite element analyses

zz

Symons and Carder (1993): field data from three sites

zz

Ng et al (1995): finite element analyses of a diaphragm wall installation at Lion Yard, Cambridge, UK

zz

Page (1995): centrifuge model tests

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zz

de Moor (1994): 2D plan view finite element analyses of the installation of a number of diaphragm wall panels in sequence

zz

Ng and Yan (1998): 3D finite difference analysis of the installation of a single diaphragm wall panel

zz

Gourvenec and Powrie (2000): 3D finite element analyses of the installation of nine diaphragm wall panels in sequence

zz

Ng and Yan (1999): 3D finite difference analyses of the installation of three diaphragm wall panels in sequence

zz

Powrie and Batten (2000b): axi-symmetric analysis of a single bored pile in stratigraphy representative of conditions at Canada Water, London, UK

zz

Richards et al (2006, 2007): field data from a contiguous bored pile retaining wall in Atherfield and Weald Clay on HS1 at Ashford, UK.

With the exception of Powrie and Batten (2000b), who considered a bored pile in a mixed sequence of soils, these studies related mainly to the installation of diaphragm wall panels in heavily overconsolidated deposits in which the in situ lateral earth pressure coefficient was generally greater than one. More recent work has focused primarily on 3D numerical analysis of diaphragm wall panel installation in normally consolidated clays and sands in which the in situ lateral earth pressure coefficient was generally less than one, for example: zz

Schafer and Triantafyllidis (2004): numerical modelling of earth and pore water pressure development during diaphragm wall construction in soft clay

zz

Conti et al (2012): numerical modelling of the installation of diaphragm wall panels of different length in a sand (K0 = 1 – sinϕ′), with stress path dependent soil stiffness

zz

Comodromos et al (2013): numerical modelling of the installation of diaphragm wall panels of different length in a clay with an in situ earth pressure coefficient of 0.54

zz

Chen et al (2014): numerical modelling of the installation of diaphragm wall panels in a soft clay (K0 = 1 – sinϕ′).

It is clear from these that while numerical models have become more complex and sophisticated, real field data remain challenging to obtain and so are rare.

A4.6.3 General considerations During installation of an in situ concrete retaining wall, the soil adjacent to the wall is likely to be unloaded laterally (eg to the hydrostatic pressure of the fluid used to support the panel or pile bore), before being reloaded when the concrete is poured. In a heavily overconsolidated deposit, the in situ lateral stresses are likely to be higher even than the hydrostatic pressure of concrete, leading to the potential for a reduction in lateral stress in the ground near the wall during wall installation. Powrie (1985) argued that diaphragm wall installation in an overconsolidated deposit would reduce the in situ lateral earth pressure coefficient, to a value still in excess of one. In a lightly or normally consolidated deposit, the reverse is the case – concreting is likely to impose lateral stresses on the soil that are greater than the in situ values, which are relatively low. Installation of even a single diaphragm wall panel or pile is a complex 3D problem, as stresses redistribute around the panel through both vertical and horizontal arching. Numerical studies from Gourvenec and Powrie (2000) have consistently demonstrated the role of the panel geometry (length × breadth × depth) in controlling the magnitude and distribution of the stress changes associated with installation, and the lateral distance of influence of installation effects. To determine the starting point for an analysis of excavation in front of an in situ wall that starts with the wall already in place, it is useful to quantify the stress change associated with wall installation in simple terms, for example, as a percentage or absolute increase or decrease in earth pressure coefficient or lateral stress. In addition to being adopted by many authors, this approach is summarised as follows, although note that it does not capture the full detail of the stress changes.

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Complicating factors, all illustrated by the data from the High Speed 1 (HS1) project at Ashford (Richards et al, 2006 and 2007), include: 1

Installation of a series of piles or panels to form a wall leads to multiple stress changes, with stress relief that occurred during the installation of one pile or panel being partly reversed during installation of an adjacent pile or panel later on. This is demonstrated by 3D finite element analyses (eg Gourvenec and Powrie, 2000, and Figure A4.13) and results in an oscillating distribution of lateral stress along the direction of the wall (eg Conti et al, 2012). However, field data from Ashford suggest that numerical analyses may tend to over-predict the amplitude of variation in reality, and as reported by Conti et al (2012) the longitudinal stiffness of the wall reduces the effect dramatically when the main excavation is made.

2

The initial lateral earth pressure coefficient is rarely uniform with depth. In an overconsolidated deposit, the lateral earth pressure coefficient will be greatest near the surface and may fall quite rapidly over the uppermost 5 m to about 10 m of soil. In general, different strata will have different strength and stiffness properties and stress histories.

3

Arching of lateral stresses, especially vertically onto the soil below the bottom of the pile or panel, means that the change in earth pressure coefficient or stress is not distributed uniformly or linearly with depth, but may be greatest over the middle of the pile or panel.

4

The change in lateral stress will reduce with distance from the face of the pile or panel. The distance of influence for stress changes perpendicular to the line of the wall is commonly reported as about 0.5 to 1.0 times the panel length.

Some of these points are illustrated quantitatively by the results of 3D finite element analyses by Gourvenec and Powrie (2000) (see Figure A4.13). Figure A4.13 shows the changes in lateral earth pressure coefficient at a distance of one metre from a panel resulting from the installation of that panel (Panel 1), the adjacent panels (Panels 2 and 3), and a further three panels on each side (giving a total of nine panels). Following an initial reduction in lateral stress during the installation of Panel 1, arching onto the completed panel then caused a slight increase in stress behind Panel 1 during the installation of the adjacent Panels 2 and 3. The installation of any panel affected only the stresses behind it (a decrease) and the panels immediately adjacent (a slight increase). Installation of more panels further away than Panels 2 and 3 had no significant effect on the lateral stresses behind Panel 1.

Figure A4.13 Earth pressure coefficient profiles one metre behind the centre of the primary panel during construction of the wall: 3D analysis with five metre panels (from Gourvenec and Powrie, 2000)

Figure A4.14 shows that in the analysis with 5 m long, 15 m deep panels, wall installation had little effect on lateral earth pressure coefficients at distances in excess of five metres (one panel length) from the wall.

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Figure A4.14 Earth pressure coefficient profiles normal to the centre of the primary panel following completion of the wall: 3D analysis with five metre panels (from Gourvenec and Powrie, 2000)

Gourvenec and Powrie (2000) used a relatively coarse finite element mesh and a relatively simple soil model, and represented the bentonite slurry and wet concrete in the diaphragm wall panel trench as imposed lateral pressures. Later analyses (eg Schafer and Triantafyllidis, 2004, Conti et al, 2012, Comodromos et al, 2013 and Chen et al, 2014) have used more refined finite element meshes, more sophisticated soil models, and/or less idealised ways of modelling the bentonite and the concrete, but have given substantially similar results.

A4.6.4 Summary of quantitative findings Diaphragm wall panels At Bell Common (Tedd et al, 1984), the earth pressure coefficient in the London Clay fell during installation from 1.7 at a depth of nine metres and 1.5 at a depth of 12 metres to about one. However, in the Claygate Beds at a depth of six metres, the initial lateral earth pressure coefficient of one was substantially unaffected. Symons and Carder (1993) describe field monitoring of the effects of wall installation at three sites (A, B and C) involving embedded walls installed in London Clay. Earth and water pressures were measured using push-in spade-shaped pressure cells each fitted with an integral high air entry pneumatic piezometer. The pressure cells were installed at locations in front of and behind the proposed retaining wall up to three months before the construction of the wall. During installation, the blade of each cell was carefully oriented to measure the lateral stress acting towards the retaining wall. Empirical corrections were necessary to the measured values of earth pressure to allow for disturbance caused by the cell. Diaphragm walls were installed at Sites B and C. T-shaped panels (consisting of a 4.0 m × 0.8 m front section and a 2.7 m × 0.8 m counterfort) were installed to a depth of 13.5 m at Site B. Planar panels (4 m in length by 0.6 m wide) were installed to a depth of 15 m at Site C. The ground conditions at site B comprised London Clay overlain by 2.4 m thickness of gravel and made ground. At Site C, 6.9 m thickness of soft alluvial deposits overlay 2.2 m of gravel above the London Clay. The spade cells were installed 1.5 m from the T-shaped panels at Site B. Spade cells were not installed at Site C, but changes in pore water pressures due to wall installation were measured in piezometers installed 1 m from the wall. The spade cells at Site B indicated a decrease in the average earth pressure coefficient of about 20 per cent in the London Clay. The piezometers at Site C indicated reductions in pore water pressure during panel excavation under bentonite with a further increase to above the pre-construction values after concreting. These pore water pressures then gradually stabilised to their pre-wall construction values.

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In the finite element analyses of Gourvenec and Powrie (2000), the installation of a diaphragm wall in panels each 5 m long and 15 m deep in an overconsolidated clay resulted in overall reductions in lateral earth pressure coefficient from ~2.5 to ~2 (~20 per cent) at the top and ~1.4 to ~1 (~28 per cent) at the toe at a distance of one metre from the line of the wall. Comodromos et al (2013) reported a 20 per cent reduction in the in situ lateral earth pressure coefficient for diaphragm wall panels 1.2 m wide × 2.8 m long × 44 m deep panels installed in a soil having K0 = 1.5.

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Later analyses of diaphragm wall panel installation in normally or lightly overconsolidated deposits indicated either no significant net change in lateral effective stress during panel or wall installation (eg Chen et al (2014), for 0.4 m wide × 4 m long × 15 m deep panels in a soil having K0 = 0.74, Comodromos et al (2013), for 1.2 m wide × 2.8 m long × 44 m deep panels in a soil having K0 = 0.54), or an increase (to an average of approximately the lateral pressure of the bentonite support slurry, although there were significant variations along the length of the wall) for 0.7 m wide × 2.5 m long × 17 m deep panels installed in a soil having K0 = 0.46 (Conti et al, 2012). Conti et al (2012) show that the maximum wall bending moments (calculated in analyses using a soil model that accounts for the increased relative stiffness of the soil behind the wall as a result of stress path reversal effects arising from wall installation) are some 25 per cent smaller than those calculated using a more conventional linear elastic/perfectly plastic soil model. Many of the reported finite element studies, together with centrifuge model tests by Powrie and Kantartzi (1996), show that the pore water pressures in the surrounding soil fall during the excavation of a diaphragm wall trench under bentonite and rise back towards their in situ values during concreting. This results in no net change during panel installation.

Bored piles In a finite element analysis of the installation of a single pile, Powrie and Batten (2000b) calculated reductions in lateral earth pressure coefficient in the range 28 to 37 per cent in strata where the initial lateral earth pressure coefficient was one or more, but an increase in the lateral earth pressure coefficient of up to 20 per cent where the initial value was only 0.5. However, this analysis did not consider the effects of installing subsequent piles. Symons and Carder’s (1993) site A was a 24 m deep contiguous bored pile wall (comprising 1500 mmdiameter bored piles at 1700 mm spacings). The spade cells installed to measure earth and pore water pressures were located 1.5 m from the wall. Ground conditions comprised up to 3 m depth of made ground overlying 3 m thickness of Claygate Beds overlying London Clay. A decrease in the average in situ earth pressure coefficient of about 10 per cent was observed at this site. Measurements at the HS1 Ashford site by Richards et al (2006) indicated an average reduction in the in situ lateral earth pressure coefficient from about 1.04 to 0.8 (~25 per cent) during installation of a contiguous bored pile wall. This average reduction is based on ‘best fit’ lines to before and after distributions of lateral effective stress. The data show considerable deviation over the depth of the wall, with the greatest reductions in lateral earth pressure coefficient occurring towards the middle of the ~20 m deep piles, just below the bottom of the support casing. Richards et al (2006) note that the average absolute reduction in lateral effective stress of about 0.25 γz at depth z (where γ is the bulk density of the soil) is similar to that noted by Symons and Carder (1993) at their site A. Pore water pressures fell during pile boring, but rose again during concreting. The result was that there was no net change during the installation process as a whole. Richards et al (2006) note that there was no change in either horizontal total stress or pore water pressure during the period of 10 months between wall installation and excavation from in front of it. Richards et al (2007) further reported that while pore pressures continued to fall over the six years following construction due to drainage through the wall, there were no further changes in horizontal total stress during that time, or indeed over the following seven years (Montalti, 2015). This means that

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even though the transverse extent of the zone of lateral stress reduction due to wall installation is limited, it is sufficient for the effect to be permanent.

A4.6.5 Summary of recommendations for simple representation of bored pile or diaphragm wall installation

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In an analysis starting with the wall already in place, it is recommended that the pore water pressures are set to the in situ values. This reflects almost all available published field data, centrifuge model test data and results of 3D numerical analyses that the pore water pressures fall and then return to their initial in situ values during wall installation, so there is no net change during the whole process. Where the soil is represented using a constitutive model that makes no special allowance for the effect of recent stress history on soil stiffness (ie does not distinguish between the stiffness of the soil behind and in front of the wall), it is recommended that for heavily overconsolidated deposits having an in situ lateral earth pressure coefficient K0 greater than one, the pre-excavation earth pressure coefficient in the analysis is set to unity. This probably overestimates the lateral stress relief that will occur in reality because of wall installation. However, this is compensated by neglecting the increased stiffness of the soil on the retained side of the wall, which will enable lateral stress to reduce more quickly with wall movement than the simple homogeneous model will allow. If K0 < 1, the pre-excavation value of lateral earth pressure coefficient should be taken to be the same as K0 (ie wall installation does not change the in situ value). If the soil is represented using a constitutive model that does allow for the effect of recent stress history on soil stiffness (ie assigns a higher stiffness to the soil behind the wall than in front), it is recommended that in a heavily overconsolidated deposit a reduction in in situ lateral earth pressure coefficient of 10 to 20 per cent is applied, or that wall installation is modelled explicitly. If 0.5 < K0 < 1, the preexcavation value of lateral earth pressure coefficient may be taken to be the same as K0. If K0 < 0.5, the pre-excavation value should be set to 0.5, which represents a possible increase during installation to approximately the bentonite support pressure.

A4.7 EARTH PRESSURE COEFFICIENTS A4.7.1 Numerical procedure for calculating earth pressure coefficients Analytical expressions for active and passive earth pressure coefficients are presented in various literature and in Annex C of EC7-1. Such expressions are useful when carrying out retaining wall calculations using computer programs, including spreadsheets. The equations given in Annex C of EC7-1 have been used to derive the charts of horizontal earth pressure coefficient presented in Figures A4.15 to A4.23.

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Figure A4.15

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Active and passive earth pressure coefficients, (β/ϕ) = -1

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Figure A4.16

Active and passive earth pressure coefficients, (β/ϕ) = -0.75

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Figure A4.17

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Active and passive earth pressure coefficients, (β/ϕ) = -0.5

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Figure A4.18

Active and passive earth pressure coefficients, (β/ϕ) = -0.25

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Figure A4.19

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Active and passive earth pressure coefficients, (β/ϕ) = 0

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Figure A4.20

Active and passive earth pressure coefficients, (β/ϕ) = 0.25

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Figure A4.21

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Active and passive earth pressure coefficients, (β/ϕ) = 0.5

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Figure A4.22

Active and passive earth pressure coefficients, (β/ϕ) = 0.75

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Figure A4.23

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A4.8 EFFECT OF METHOD OF ANALYSIS A4.8.1 Methods of analysis

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Available methods of analysis range from simple limit equilibrium calculations to complex numerical modelling of SSI using finite element or finite difference techniques. They can be categorised broadly as: zz

limit equilibrium

zz

subgrade reaction or pseudo-finite element, in which the ground is modelled as a bed of elastic springs (subgrade reaction) or as an elastic solid with the stiffness matrices calculated using a finite element program (pseudo-finite element), see Section 4.2.1 and Table 4.1

zz

finite element or finite difference.

In this section, four generic problems are solved using commercially available software packages from each of these categories to highlight, in general terms, the potential benefits of a more complex analysis. It is not the purpose of this appendix to make detailed comparisons between the software packages considered, or to illustrate in detail the application of the design recommendations made in Chapter 7, although the latter have broadly been adopted. Appendix A7 provides a step-by-step guide to the application of the design method recommended in Chapter 7.

A4.8.2 Software packages The following methods and commercially available software packages were used: zz

limit equilibrium (spreadsheet from first principles)

zz

pseudo-finite element (FREW, WALLAP)

zz

finite element (PLAXIS).

A4.8.3 Problems analysed The four generic problems considered were as follows: zz

zz

Effective stress analysis: {{

Example 1: cantilever wall (Figure A4.24)

{{

Example 2: singly-propped wall (Figure A4.25).

Total stress analysis: {{

Example 3: cantilever wall (Figure A4.26)

{{

Example 4: singly-propped wall (Figure A4.27).

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Figure A4.24

Example 1 – cantilever wall: effective stress analysis

Figure A4.25

Example 2 – propped wall: effective stress analysis

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Figure A4.26

Example 3 – cantilever wall: total stress analysis

Figure A4.27

Example 4 – propped wall: total stress analysis

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A4.8.4 Assumptions and analysis procedure

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ULS and SLS calculations were carried out for each of the four example cases in accordance with EC7 DA1, as follows: 1

A ULS calculation using DA1C2, with partial factors of 1.25 applied to tanϕ′ and 1.4 to the undrained shear strength, 1.0 to permanent actions (loads) and 1.3 to variable unfavourable actions (loads).

2

A ULS calculation using DA1C1, with partial factors of 1.0 applied to tanϕ′ and 1.0 to the undrained shear strength, 1.0 to permanent loads, 1.1 to variable loads and 1.35 to calculated stress resultants (effects of actions). EC7 does not specify whether the depth of embedment should be determined independently of the DA1C2 calculation. Current standard practice in the UK is to use the embedment depth determined from the DA1C2 calculation in the DA1C1 calculation in a SSI or finite element analysis. This is not possible in a limit equilibrium analysis – in limit equilibrium calculations, the depths determined using the DA1C1 and DA1C2 approaches will be different.

3

A SLS calculation, using the greater of the depths of embedment determined in points (1) and (2), and partial factors of 1.0 applied to soil strengths, all actions (loads) and calculated effects of actions.

The wall was assumed to be rough and the soil to have ϕ′peak = ϕ′cv , so that (δ/ϕ′)des = 1. The key assumptions and values of partial factors used are summarised in Table A4.1. Table A4.1

Key assumptions and values of partial factors used in calculations Calculation approach DA1C1

DA1C2

SLS

Factor on effective cohesion

1

1.25

1

Factor on angle of shearing resistance ϕ′

1

1.25

1

Effective stress soil/wall adhesion

0

0

0

Maximum angle of soil/wall friction δ

ϕ′

ϕ′

ϕ′

Factor on undrained shear strength cu

1

1.4

1

cu/2

cu/2

cu/2

1.1 (V)

1.3 (V)

1.0 (P)

1.0 (P)

1.35

1

1

Expected action (surcharge) applied at ground surface level on active (retained) side (assumed variable)

10 kPa

10 kPa

10 kPa

Allowance for unexpected surcharge

0 kPa

0 kPa

0 kPa

Maximum soil wall adhesion cw Factor on imposed actions (loads) Factor on effects of actions

1 (all)

Unplanned excavation (assuming a well-controlled site)

0.1 m

0.1 m

None

Maximum depth of soil softening applied to total stress analysis

0.5 m below excavation level

0.5 m below excavation level

0.5 m below excavation level

Total stress analysis: propped wall Total stress analysis: cantilever wall

MEFP of 5z

MEFP of 5z

MEFP of 5z

Flooded tension crack (γwz)

Flooded tension crack (γwz)

Flooded tension crack (γwz)

Notes In the pseudo-finite element and finite element analyses, the width of the excavation was taken as 32 m for all four examples. In all analyses and all examples, the unit weight of the soil was taken as 20 kN/m3 and the unit weight of water as 10 kN/m3.

Limit equilibrium calculations ULS limit equilibrium calculations were carried from first principles using a spreadsheet. The following principles or assumptions were adopted, as appropriate, in each case: 1

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In the effective stress analyses of both walls (Examples 1 and 2), pore water pressures around the wall were calculated according to the linear seepage approximation.

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2

In the effective stress analyses following DA1C1 (in which the partial factor applied to the soil strength is unity), the minimum lateral stress behind the wall near the top was set at zero for both the propped and the unpropped walls (Examples 1 and 2) otherwise, the cohesive component of the strength results in a slightly negative lateral effective stress).

3

In analyses of unpropped walls (Examples 1 and 3), free earth support conditions were assumed with the pivot depth calculated as one of the unknowns in the analysis in all three sets of calculations. No change in the values of the earth pressure coefficients below the pivot to account for a possible reversal in the relative direction of wall friction was applied. To do so would lead to unrealistically large depths of embedment compared with previous back analysis of model tests and case studies, and compared with the fixed-earth support simplification.

4

Spreadsheet calculations for propped walls (Examples 2 and 4) were carried out with passive pressures behind the wall above the level of the prop, consistent with the implied direction of wall rotation about a rigid pivot. The value of the passive earth pressure coefficient used was set to account for the soil tending to move downward relative to the wall, rather than upward as is normally the case in a passive zone. The assumption of active limiting lateral stresses behind the wall above the prop, perhaps more representative of a yielding prop, would result in a slight increase in the calculated wall embedment, but a potentially significant reduction (up to ~30 per cent) in prop load.

5

A minimum equivalent fluid pressure (MEFP) of 5z kPa/m was applied below the pivot in front of the wall in the undrained analysis of the unpropped wall (Example 3). The effect of this is insignificant – it increases the calculated embedded depth by a maximum of 2 cm compared with assuming zero lateral stresses in this zone.

Pseudo-finite element calculations The pseudo-finite element calculations carried out using FREW and WALLAP required further assumptions to be made about the: zz

construction sequence

zz

in situ stress conditions

zz

soil, wall and prop stiffness.

These additional assumptions were as follows.

Cantilever wall (Examples 1 and 3) zz

Stage 0 – install wall: {{ {{

{{

zz

Lateral stress coefficient, K0 = 1.0 Young’s modulus of soil = 48 000 kPa (effective stress, Example 1) = 60 000 kPa (total stress, Example 3) (equivalent to a uniform shear modulus of 20 000 kPa) wall EI = 469 000 kNm²/m. This corresponds to a hard/hard secant bored pile wall (750 mm pile diameters at 650 mm spacing). The value of EI was taken as 0.7E0I (Section 4.2.3).

Stage 1 – excavation to final formation level: {{

excavation to 4.1 m depth in ULS analysis and 4.0 m depth in SLS analysis

{{

soil and wall stiffness as Stage 0.

Singly-propped wall (Examples 2 and 4) zz

Stage 0 – install wall: {{ {{

{{

Lateral stress coefficient, K0 = 1.0 Young’s modulus of soil = 48 000 kN/m² (effective stress, Example 2) = 60 000 kN/m² (total stress, Example 4) (equivalent to a uniform shear modulus of 20 000 kPa) wall EI = 469 000 kNm2/m. This corresponds to a hard/hard secant pile wall (750 mm pile diameters at 650 mm spacing). The value of EI was taken as 0.7E0I (Section 4.2.3).

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zz

zz

Stage 1 – excavate to 1 m below prop level: {{

excavation to 3.1 m depth in ULS analysis and 3.0 m depth in SLS analysis

{{

soil and wall stiffness as Stage 0

Stage 2 – install prop and excavate to final formation level {{

install prop. Prop stiffness, k = 100 000 kN/m/m

{{

excavate to 8.1 m depth in ULS analysis and 8.0 m depth in SLS analysis

{{

soil and wall stiffness as Stage 0.

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In all analyses, the wall depth was taken as the minimum (determined iteratively) at which the DA1C2 analysis converged. Steady-state pore water pressures in the effective stress analyses were calculated using the linear seepage approximation. FREW will only calculate structural stress resultants (effects of actions – bending moments and shear forces) at user-specified nodes. If the nodes are too far apart, the true maxima or a sharp change in stress may be missed. However, too close a node spacing may cause numerical instability. It is possible for the minimum node spacing for numerical stability to be too large to ensure calculation of the true maximum bending moment and/or shear force, resulting in some (generally small) inaccuracy.

A4.8.5 Finite element calculations The finite element analyses modelled one half of a 32 m wide excavation, using a mesh 46 m wide and 50 m deep. An initial sensitivity analysis showed that this resulted in boundaries far enough away from the area of interest around the embedded retaining wall that would not influence the results. Standard fixities were assumed, ie nodes along the vertical boundaries were on rollers (restrained in the horizontal direction only), and nodes along the lower horizontal boundary were pinned (restrained both vertically and horizontally). The retaining walls were modelled by elastic plate elements to allow easy extraction of the bending moments and shear forces. Interface elements were introduced between the wall and the surrounding soil to allow control of the interface friction and relative soil/wall movement. In the effective stress analyses (Examples 1 and 2), steady-state groundwater seepage pressures were computed directly by the finite element program assuming constant head boundaries at the retained and excavated soil surfaces (ie freely available recharge from above and continual removal of water from within the excavation), an impermeable wall and zero flow through the vertical line of symmetry. The introduction of a water-filled tension crack for Example 3 and a minimum equivalent fluid pressure (MEFP) of 5z for Example 4 required a staged approach as follows: Step 1 An initial analysis with no measures to implement a water-filled tension crack or MEFP. Step 2 Determination of the difference between the calculated total stress on the back of the wall from the analysis in Step 1 and the water-filled tension crack or the MEFP. Step 3 A second analysis in which the difference calculated in Step 2 was applied to the back of the wall to the analysis in Step 1 (note that an equal and opposite pressure must then be applied to the ground). The process was repeated until the lateral stresses at the bottom of the flooded tension crack and in the soil immediately below it were the same.

A4.8.6 Results Results for each example and analysis scenario (DA1C1, DA1C2 and SLS) are summarised in Figures A4.28 to A4.31. Each of these figures comprises: zz

Tabulated data of wall depth and maximum wall bending moment, shear force, deflection and maximum prop load as applicable, with the maximum bending moment and wall depth also given in graphical form.

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zz

Profiles of total horizontal pressure (either calculated directly in the undrained analyses, or as the summation of the horizontal effective stress and the pore water pressure in the case of the effective stress analyses) and wall bending moment with depth.

For the avoidance of doubt, the indicated prop loads and stress resultants (effects of actions) associated with DA1C1 are, in all cases, the raw calculated values multiplied by the required partial factor of 1.35.

A4.8.7 Observations from Figures A4.28 to A4.31 Example 1: cantilever wall, effective stress analysis

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Wall embedment depth is governed by DA1C2, as are the maximum wall bending moments and shear forces. DA1C2 maximum bending moments are generally consistent across all methods of analysis. The maximum SLS bending moments calculated using the SSI analysis are generally about half those calculated for DA1C2, and the maximum SLS shear forces are about 35 per cent of the corresponding DA1C2 values.

Example 2: propped wall, effective stress analysis Embedment depth is again governed by DA1C2, as generally are the maximum wall bending moments, shear forces and prop loads. Depths of wall embedment calculated using SSI analysis (finite element and pseudo-finite element programs) are slightly less than with the limit equilibrium approach. Wall bending moments are also generally smaller while shear forces and prop loads computed by SSI analysis are larger than the corresponding values obtained from limit equilibrium calculations. The maximum SLS wall bending moments and shear forces calculated using SSI analysis are generally about 60 to 75 per cent of the corresponding DA1C2 values.

Example 3: cantilever wall, total stress analysis Wall embedment is governed by DA1C2 – SSI analysis and limit equilibrium calculations give broadly similar wall embedment depths. DA1C1 governs the computed ULS wall bending moments. Here, there is no great benefit in terms of reduced bending moments or wall embedment from the use of SSI analysis. This is because both the bending moments and the embedment depth are governed by the hydrostatic pressure of water in a flooded tension crack, for which no stress redistribution due to SSI analysis is possible.

Example 4: propped wall, total stress analysis Wall embedment is governed by DA1C2 – SSI analysis and limit equilibrium calculations give broadly similar wall embedment depths. The prop load and to a lesser extent the maximum wall bending moment calculated in a limit equilibrium analysis is hugely sensitive to the assumption made regarding the lateral stresses behind the wall above the prop. The limit equilibrium results shown in Figure A4.31 are for passive pressures behind the wall above the prop. The prop load decreases by a factor of more than four and the maximum positive bending moment increases by 55 per cent when the limiting stresses in this zone are set to active. DA1C1 governs the ULS design in terms of the effects of actions whichever calculation method is adopted, because the total stresses are determined in all cases by the application of the MEFP of 5z kPa/m, which is then factored up in DA1C1. Here, the analytical results are governed by the lateral stresses in the soil behind the wall above the prop, rather than stress redistribution due to SSI.

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Example 1: cantilever wall, effective stress analysis

Figure A4.28

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Example 2: propped wall, effective stress analysis

Figure A4.29

Results for Example 2 – propped wall: effective stress analysis

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Example 3: cantilever wall, total stress analysis

Figure A4.30

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Example 4: propped wall, total stress analysis

Figure A4.31

Results for Example 3 – propped wall: total stress analysis

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A4.8.8 Conclusions

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For the simple wall geometries and soil conditions considered in this appendix: 1

In the DA1C2 analysis of unpropped cantilever walls (effective stress and total stress) and singlypropped walls (total stress), SSI analysis gave broadly similar wall embedment depths to a limit equilibrium calculation. Patterns of wall bending moment and shear force were also broadly similar. In the total stress analysis, bending moments and embedment depth were governed by the hydrostatic pressure of water in a flooded tension crack, giving no opportunity for stress redistribution due to SSI analysis.

2

The effects of SSI in reducing wall embedment and the calculated effects of actions (bending moments and shear forces) were most significant in the effective stress analysis of the propped wall. In this case, the wall embedment calculated from a DA1C2 limit equilibrium calculation was slightly greater than the corresponding values calculated from pseudo-finite element and finite element methods, and the SLS effects of actions calculated using SSI analysis were generally less than those calculated by limit equilibrium methods. So, use of SSI analysis should lead to savings in wall material costs.

3

In the total stress analysis of the propped wall, prop loads and bending moments were governed in all cases (DA1C1, DA1C2 and SLS) by the lateral stresses behind (ie on the retained side of) the wall above the prop. The results of the SSI calculations were bracketed by limit equilibrium calculations assuming active and passive conditions.

4

Effects of actions (prop loads, shear forces and bending moments) calculated from SSI analysis using the DA1C2 partial factors were generally greater than those calculated using the DA1C1 partial factors for the effective stress analyses. In the total stress analysis of the wall propped near its crest, the reverse was the case (ie effects of actions calculated using the DA1C1 partial factors were generally greater than those using DA1C2).

5

In the DA1C2 effective stress analysis of the propped wall, prop loads calculated using limit equilibrium methods were smaller than those obtained from SSI analysis, even when passive pressures were assumed behind the wall above the prop. Prop loads obtained from limit equilibrium calculations in which active conditions are assumed behind the wall above as well as below the prop are further reduced, and should be treated with caution in design (see Section 8.1.3).

Limit equilibrium analyses can be used with confidence to calculate depths of wall embedment and the ULS effects of actions where stress redistribution due to SSI is not significant, and to provide an approximate check on the results of more sophisticated SSI analysis. For walls where the effects of stress redistribution are significant (eg where the wall is heavily loaded or forms part of a larger structural system, or where interactions with nearby structures or more accurate estimates of ground movements are required), and for wall types where simple limit equilibrium earth pressure distributions are less certain (eg walls that are singly-propped at low level or multi-propped), an appropriate SSI analysis should be carried out.

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A5 Derivation of rock strength parameters for use in design calculations The shear strength of a weak rock is typically defined by a curved failure envelope in effective strength space. The magnitude and shape of the envelope can be determined by carrying out a number of triaxial tests on rock samples at a range of stress levels. If triaxial tests are not available, which is common, then the failure envelope can be derived based on the methodology described by Hoek et al (2002) using unconfined compressive strength data and rock quality information (which are typically available) and borehole descriptions. The Hoek-Brown failure criterion is defined as follows:

(A5.1)

where σ′1 and σ′3 are the major and minor principle effective stresses at failure σ′ci is the unconfined compressive strength of the intact rock mb is the Hoek-Brown constant s and a are constants which depend upon the rock mass characterisation:

GSI is the geological strength index D is the disturbance factor due to blasting and stress relaxation, and

The Hoek-Brown coefficient mi needs to be corrected to take account of the rock mass and possible blast damage as follows:

(A5.2)

The following calculation is an example of how equivalent Mohr-Coulomb parameters can be derived for a weak rock based on some typical parameters. For this example, a GSI value of 60 and a characteristic unconfined compressive strength of 1.6 MPa have been assumed as typical values for a weak claystone. Table 3 from Hoek et al (2002) gives the mi value for claystone to be 4 ± 2, an average value of 5 can be assumed. For a cast in situ embedded retaining wall, a D value of 0 can be assumed as excavation does not involve any blasting. For the example mudstone, this results in the following parameters for the Hoek-Brown failure criterion: σ′ci = 1.6 MPa mb = 1.198 s = 0.01174 a = 0.5028

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(A5.3)

The Hoek-Brown failure criterion in principal stress space is given by:

(A5.4)

The Mohr-Coulomb equivalent in principal stress space is given by the following relationship:

(A5.5)

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The equivalent Mohr-Coulomb parameters should be chosen to give a best fit linear failure criterion over the relevant normal stress range. This can be done by eye or by using the following equation that balances the areas above and below the Hoek-Brown failure criterion.

(A5.6)



(A5.7)

where σ3n = σ′3max /σ′ci and σ′3max is the upper limit of confining stress over which the relationship between the Hoek-Brown and Mohr-Coulomb failure criterion is to be considered. For example, for a 20 m deep retaining wall supporting 10 m of rock with a unit weight of 20 kN/m3 with no water table, the limiting stress to consider on the passive side of the wall is 400 kPa and 200 kPa on the active side. Using Equations A5.6 and A5.7 the following parameters are derived for the rock on the active side and on the passive side of the wall (Table A5.1). Table A5.1

Equivalent Mohr-Coulomb parameters for both sides of the wall

Side of wall

ϕ′

c’

Passive

32.91°

56 kPa

Active

27.51°

80 kPa

The equivalent Mohr-Coulomb failure criteria for the active and passive side of the wall is compared to the Hoek-Brown failure criteria shown in Figure A5.1.

Figure A5.1

Comparison of failure criteria for the both sides of the wall

So, for σ3, values between 0 and 0.2 MPa the area under the black line (the Hoek-Brown envelope) is equivalent to the area under the blue line (the Mohr-Coulomb criteria for the passive side). For σ3 values between 0 and 0.2 MPa the area under the black line is equivalent to the area under the red line (the Mohr-Coulomb criteria for the active side). 344

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A6 Ground movements and case study data

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A6.1 GROUND MOVEMENTS DUE TO WALL INSTALLATION A6.1.1 Overview Ground movements arising from bored pile and diaphragm installation in stiff clays are summarised in a range of sources, as listed in Section 6.1.1. In particular, Figures 6.6 to 6.9 show the combined data from Clough and O’Rourke (1990), Thompson (1991), Carder (1995), Puller (1996) and Carder et al (1997). Some data fall outside the envelope of settlements arising from the construction of secant bored pile walls shown on Figure 6.8b. These data relate to walls constructed at the British Library, Euston, London, Vintners Place north east wall, London and at Blackfriars, London. Thompson (1991) states that the retaining wall at the British Library is very complicated and in many areas involved the construction of sheet piles around the wall before the construction of the secant bored piles. The settlements measured at the British Library site also included consolidation settlement due to local dewatering at this location. The sites at Blackfriars and Vintners Place were underlain by some 5 m to 7 m of sand and gravel deposits overlying stiff London Clay. Thompson (1991) reports that at these locations, the deposits comprised mainly sand with only a small proportion of gravel. The higher local settlement was probably due to ground disturbance arising from hydraulic imbalance in these deposits during pile boring. These examples illustrate the importance of good workmanship and effective control of construction operations to minimise ground movements. They also highlight the localised nature of ground movements arising from construction problems. Clough and O’Rourke’s (1990) upper bound movement limit is likely to over-estimate ground movements arising from bored pile and diaphragm walls installed in stiff clay under conditions of good workmanship. The Clough and O’Rourke upper bound limit includes ground movements arising from diaphragm walls installed in soft alluvial deposits overlying completely decomposed granite in Hong Kong and walls installed in soft clay at Studenterlunden, in addition to walls installed in stiff clays.

A6.1.2 Case study data Case studies relevant to wall installation effects are summarised in Tables A6.1 to A6.5.

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Location

UK

Site

New Palace Yard

11

Construction sequence/ support system

Diaphragm wall (1 m thick) Top-down, multi-propped

Wall type/dimensions

Wall deflection effects – case study data (from Clough and O’Rourke, 1990)

Site no

Table A6.1

10 m of made ground/ sand and gravel over London Clay

Retained soil stratigraphy Restraining soil

5.5

Depth to water table (m)

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30

Wall depth (m)

Simpson et al (1979)

St John (1975)

Burland and Hancock (1977)

Reference

7.5

3.4 m of fill over 0.25 m of sand and gravel over London Clay 4 m of fill over 4 m of sand and gravel over London Clay

Secant pile wall H/S 0.75 m CFA

Contiguous pile wall (0.75 m piles at 0.9 m)

1 Ludgate Place UK

UK

UK

UK

UK

UK

UK

63 Lincolns Inn Field

Holborn Bars

Peterbrough Court

Leith House

Vintners Place (N.Wall)

Blackfriars 1

Blackfriars 2

Vintners Place (NE.Wall)

Minster Court

Aldersgate

18

19

20

21

22

23

24

25

26

27

28

UK

UK

UK

4.65

0.9 m of fill over 0.8 m of sand and gravel over London Clay

Secant pile wall H/S 0.75 m CFA

Linsey House

17

7.3 m of fill over 1.5 m of alluvium over 5 m of sand and gravel over London Clay

Secant pile wall H/S (0.75 m)

Diaphragm wall (1 m)

Diaphragm wall (0.8 m)

Secant pile wall H/C (1.08 m)

4

7.3 m of fill over 1.5 m of alluvium over 5m of sand and gravel over London Clay

Secant pile wall (1.2 m)

4.8 9.3 10

6 m of fill over 6 m of sand and gravel over London Clay 5.6 m of fill over 4.7 m of sand and gravel over London Clay 8 m of fill over 4 m of sand and gravel over London Clay

4

6.8

3

1.4

1.4

5.5

7 m of fill over 4 m of sand and gravel over London Clay

Secant pile wall H/C (1.08 m)

4.3 m of fill over 2.5 m alluvium over 2.1 m of sand and gravel over London Clay

Contiguous pile wall (1.2 m)

0.9 m of fill over 0.8 m of sand and gravel over London Clay

10 m of made ground/sand and gravel over London Clay

2.8 m of fill over 0.5 m of sand and gravel over London Clay

Top down, multipropped

Secant pile wall

Secant pile wall

Diaphragm wall (1 m thick)

3

UK

7 m older head and Claygate beds over London Clay

New Palace Yard

Top propped

11

Secant pile wall (1.18 m piles at 1.08 m centres)

UK

Depth to water table (m)

Bell Common (M25)

Restraining soil

4

Construction sequence/support Retained soil stratigraphy system

Location

Site

Wall type/dimensions

Wall deflection effects – case study data (from Thompson, 1991)

Site no

Table A6.2

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33

21.45

26

11

22.5

30

14.5

22

17

21.3

21

21

30

20

Fernie et al (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Thompson (1991)

Simpson et al (1979)

St John (1975)

Burland and Hancock (1977)

Symons and Tedd (1989)

Tedd et al (1984)

Wall depth Reference (m)

UK

UK

A406/A10 Junction UK

UK

UK

UK

UK

Walthamstow (CPW)

Rayleigh Weir

East of Falloden Way (A406) (CPW)

New Palace Yard

Waterloo International Terminal

5

6

7

9

10

11

15

Top down, multi-propped

Supported by 5 m berm during excavation of Diaphragm wall (0.8 m thick) central area, temporary props, then permanent prop cast

Diaphragm wall (1 m thick)

Cantilever

8.5 m of made ground/alluvial clay/gravel over London Clay

10 m of made ground/sand and gravel over London Clay

23 m of glacial till over London Clay

3 m of made ground over London Clay

Contiguous pile wall (1.5 m piles at 1.7 m centres)

Contiguous pile wall (1.5 m piles at 2 m centres)

2.4 m of made ground/sand and gravel over London Clay

Diaphragm wall, counterfort, (4 m × 0.8 m front, 2.7 × Low propped 0.8 m counterfort) Low propped

1.5 m of made ground over London Clay

5 m of made ground and terrace gravels over London Clay

Top propped

Contiguous pile wall (1.5 m piles at 1.7 m centres)

Secant pile wall (1.2 m piles Cantilever at 1.03 m centres)

Hackney Wick (A406)

7 m older head and Claygate beds over London Clay

Top propped

Secant pile wall (1.18 m piles at 1.08 m centres)

Bell Common (M25) UK

4

London Clay to surface

UK Low propped

Walthamstow (CF)

3

Retained soil stratigraphy

Diaphragm wall, counterfort, (4 m × 0.8 m front, 3.2 m × 1.5 m counterfort)

Construction sequence/ support system 21 m of glacial till over London Clay

UK

East of Falloden Way (A406) (DW)

2

Wall type/dimensions

Diaphragm wall (5 m × 1 m) Cut-and-cover

Location

Site

Wall deflection effects – case study data (from Carder, 1995)

Site no

Table A6.3

London Clay

Glacial till

Restraining soil

3

Depth to water table (m)

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30

20

Li et al (1992)

Simpson et al (1979)

St John (1975)

Burland and Hancock (1977)

Brookes and Carder (1996)

Darley et al (1994)

Carder et al (1991)

Watson and Carder (1994)

Carswell et al (1993)

Bennett et al (1996)

Symons and Tedd (1989)

Tedd et al (1984)

Carder et al (1994)

Brookes and Carder (1996)

Wall depth Reference (m)

London

Location

Construction sequence/ support system

Diaphragm wall (6.1 m Unknown × 0.8 m)

Wall type/dimensions

Location

UK

Site

Aldershot Road Underpass

Site no

347

Construction sequence/ support system

Counterfort diaphragm Temporary props with wall permanent low prop

Wall type/dimensions

Wall deflection effects – case study data (from Carder et al, 1997)

London (unknown)

45

Table A6.5

Site

Wall deflection effects – case study data (from Puller, 2003)

Site no

Table A6.4

Restraining soil

Restraining soil

2 m made ground over 2 m sand London Clay and gravel over London Clay

Retained soil stratigraphy

London clay: overconsolidated fissured stiff silty clay

Retained soil stratigraphy

Depth to water table (m)

Depth to water table (m)

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14.6

Wall depth (m)

15

Wall Depth (m)

Carder et al (1997)

Reference

Farmer and Attewell (1973)

Reference

A6.2 GROUND HEAVE A6.2.1 Estimation of ground heave movements Beneath an excavation in clay, heave occurs immediately due to undrained deformation and subsequently as the clay draws in water and swells. Generally, undrained deformation implies shearing at constant volume. However, close to the excavated surface there may be insufficient suction to ensure this, leading to undrained expansion as the clay becomes unsaturated. The simplest model of soil behaviour to use for estimating deformations is that of a linearly elastic isotropic material with elastic parameters E and ν (or G and K). Movements may be estimated by treating the excavation as a negative load as for an embankment with vertical sides (Poulos and Davis, 1980).

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(A6.1)

where E′ and Eu are the Young’s modulus of elasticity of the ground under long-term drained and shortterm undrained conditions respectively ν′ and νu are the values of Poisson’s ratio under drained and undrained conditions respectively G is the shear modulus of the ground. For undrained conditions, νu = 0.5 and for drained conditions, ν′ = 0.1 to 0.3, typically 0.2 for stiff clays. For the reasons discussed in Chapters 4 and 5, this is not a realistic model. However, such a simple model when used in conjunction with appropriate values of stiffness obtained from its use in the back analysis of well monitored excavations in similar conditions can provide a quick and convenient means of estimating the order of magnitude of likely movements. Ho (1991) carried out back analysis of four wellinstrumented and monitored excavations in London Clay using such a model (PDISP, formerly VDISP, see Websites) and estimated:

(A6.2)

where cu is the undrained shear strength of the London Clay below excavation level. This ratio of (Eu /cu) can be used in elastic calculations to provide an estimate of short-term ground heave arising from the effects of excavation stress relief in London Clay. Additional heave may occur near the excavated surface owing to undrained expansion, as previously described. It is well established that the Young’s modulus of a stiff, overconsolidated clay is highly non-linear and strain dependent. Simpson et al (1979), Atkinson and Sällfors (1991) and Mair (1993) show that the range of shear strains that can be expected around a strutted retaining wall in its working condition is typically 0.01 to 0.1 per cent. Similarly, soil behaviour is also highly non-linear (Simpson 1992, Atkinson, 2000). Predictions of ground movements can be made using numerical analyses (eg finite element, finite difference methods) in conjunction with an appropriate soil constitutive model and one that incorporates the likely variation of the soil’s stiffness with shear strain and appropriately models the initial in situ stress conditions in the ground. However, care should be adopted in the use and application of the results of such analyses. It is important that the numerical model and the parameters adopted are first calibrated through the back analysis of similar excavations in comparable ground conditions. Websites PDISP: www.oasys-software.com/products/engineering/pdisp.html

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A6.2.2 Case study data Table A6.6 lists relevant ground heave case studies.

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Table A6.6

Relevant ground heave case studies

Site no

Site

Location

Soil stratigraphy

Stress relief (kPa)

Reference

1

QEII conference centre UK

10 m of London Clay over 145 Woolwich and Reading beds

Burland and Kalra (1986)

2

Cantley sugar silos

UK

6 m gravel over London Clay

130

Ho (1991)

3

St Pauls, London

UK

London Clay

147

Ho (1991)

4

New Palace Yard

UK

London Clay

370

Ho (1991)

5

Stock Exchange

UK

London Clay

177

Ho (1991)

A6.3 GROUND MOVEMENTS DUE TO WALL DEFLECTION A6.3.1 Overview Many researchers have published measurements and patterns of wall deflections and ground movements arising from excavation in front of retaining walls, most notably Peck (1969), Clough et al (1989), Clough and O’Rourke (1990), St John et al (1992), Carder (1995), Fernie and Suckling (1996), Carder et al (1997), Long (2001), Finno et al (2002), Moorman (2004), Lam (2010) and Wang et al (2010). Appendix A6.3.2 provides a list of case studies considered by these researchers where available. Peck (1969) summarised data on settlements behind walls. Three different zones were identified, depending upon ground conditions. Generally, where workmanship is average or above average and the soil conditions are not difficult, settlements are indicated not to exceed one per cent of the excavation depth. From available case study data of walls embedded in sands and stiff clays, Clough and O’Rourke (1990) showed that the maximum horizontal and vertical ground movements behind these walls were typically less than 0.5 per cent of the excavation depth. These data indicate that there is no significant difference between the maximum movements observed from a range of different wall types, suggesting that the stiff ground conditions may have had a significant effect on wall behaviour. Clough and O’Rourke (1990) present envelopes of maximum horizontal movements and settlements at ground surface behind different wall types embedded in sand, stiff to hard clay and soft to medium clay. The surface displacements and distance from the wall are expressed as ratios of the maximum excavation depth (H) and the distribution of settlement is shown as a proportion of the maximum settlement behind the wall. Carder (1995) defined three categories of wall support stiffness to categorise the performance of the retaining walls contained in his database. These categories are defined in Table 6.2. Carder considered the measured horizontal and vertical movements at ground surface associated with bored pile and diaphragm walls falling into each of the support stiffness categories. Little difference was found in the wall performance between bored pile walls and diaphragm walls in each of the support stiffness categories. Observed maximum wall deflections (δhmax) relating to excavations in London Clay are presented by St John et al (1992), see Figure A6.1.

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Figure A6.1

Observed maximum lateral wall deflections for excavations in London Clay (after St John et al, 1992)

For top down construction (high support stiffness), St John et al (1992) indicate maximum (δhmax) values of < 0.2%H (typically 0.15%H, where H is the maximum excavation depth) and typical values of about 0.4%H for cantilever (low support stiffness) walls. Fernie and Suckling (1996) considered the horizontal deflections of bored pile walls, diaphragm walls and sheet pile walls in terms of system stiffness (Equation 6.3). They found that for a FoS against base heave of greater than three, the lateral deflection of walls wholly embedded in stiff soils was less than 0.3%H and that the typical value of δhmax for walls wholly embedded in London Clay in top-down conditions was about 0.15%H. Carder et al (1997) present wall deflections and ground movements relating to bored pile and diaphragm walls caused by wall installation in stiff clay and further excavation in front of walls that were permanently propped at final formation level only. These correspond to the moderate stiffness category defined by Carder (1995). Figure 6.15 shows the combined data collated from Clough and O’Rourke (1990), Carder (1995) and Carder et al (1997) for surface movements arising from excavation in front of bored pile, diaphragm and sheet pile walls embedded in stiff clays. The data are presented in terms of the high and low support stiffness categories, as defined by Carder (1995). Figure 6.15a shows that data from two sites fall outside the envelope of movements relating to lowsupport stiffness walls. These data relate to walls constructed at Bell Common and Neasden. At both of these sites unusual site-specific circumstances dictated the measured ground movements. At Bell Common, temporary sheet piles supporting an adjacent 3.5 m deep excavation were propped against the permanent secant pile wall. At Neasden, block movement of London Clay resulted in horizontal displacement of the wall and its tie back anchor supports. Figure 6.16 shows monitored ground surface settlements arising from excavation in front of walls embedded in sand. The data have been taken from Clough and O’Rourke (1990). Settlements due to wall installation have been excluded. Figure 6.16 indicates maximum surface settlement of less than 0.3 per cent of maximum excavation depth immediately adjacent to the wall decreasing to zero settlement at a lateral distance of some two times the maximum excavation depth.

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Long (2001) extended the database to around 300 case studies (Table A6.11) and considered the horizontal deflections of walls embedded in a stiff stratum, but retaining varying amounts of soft ground, against system stiffness. Long (2001) found that, for a FoS against base heave of about 3 or more, the measured maximum wall (δvmax) deflections varied depending upon the proportion of soft ground retained by the wall (see Figure A6.2). Significant scatter was observed in the data, probably reflecting, in part, local construction problems and variable quality of workmanship.

a Walls with s0.6H – stiff soil at formation level

c

Walls with s>0.6H – soft soil at formation level

d Cantilever walls Figure A6.2

Normalised maximum wall deflection versus system stiffness (after Long, 2001)

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Long (2001) reports that where large wall deflections (greater than 0.3%H) were observed for walls wholly embedded in stiff soils, they were principally due to: zz

movements associated with an initial cantilever stage at the beginning of the construction sequence

zz

an overly flexible retaining system

zz

creep of anchorages

zz

structural yielding.

Table A6.7 lists the average values of δhmax as a percentage of maximum excavation depth (H) for cases where the observed values of δhmax were less than 0.3%H for walls wholly embedded in stiff soils. Table A6.7 Average δhmax values due to excavation in front of walls embedded in stiff soil for data where δhmax < 0.3%H (after Long, 2001)

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δhmax (%H) Wall support

s0.6H

stiff soil at dig level

stiff soil at dig level

0.16 %

No data

Top down Notes H = maximum excavation depth

s = thickness of soft ground overlying stiff ground

The average δhmax value of 0.16%H in Table A2.7 is consistent with the findings of St John et al (1992) and Fernie and Suckling (1996). Long (2001) reports average δhmax values of about 0.4%H for cantilever walls wholly embedded in stiff soils (Figure A6.2). This is consistent with observations reported by St John et al (1992) and Carder (1995) for low support stiffness category walls (Figure 6.15a and Table 6.3). Long (2001) also considered maximum ground surface settlements δvmax behind walls wholly embedded in stiff soils. Average values of δhmax were found to be < 0.2%H for top-down (high support stiffness) walls. This is greater than the value of 0.1%H reported by other observers (Table 6.3). It is possible that Long’s data represent total settlements that include those due to wall installation. Long shows that for walls embedded in a stiff stratum with a large FoS against base heave that retain a significant thickness of soft soil (>0.6H) and have soft soil at formation level, wall deflections and associated ground settlements may increase significantly compared to the case where stiff ground exists at formation level (Figure A6.2c). This indicates that, for walls wholly embedded in stiff soil with a FoS of three or more against base heave, wall deflections are relatively insensitive to variation in wall thickness and stiffness provided the overall system stiffness is not significantly reduced. This means that economies in wall type and size can be achieved through the adoption of flexible walls (eg sheet pile walls) in stiff soils, without significant increase in ground movements. Moorman (2004) studied a database of over five hundred international case studies, predominantly in soft soils. The case studies were classified by shear strength and throughout the report, comparison is made between soft clays characterised by shear strengths of 75 kPa. Both horizontal and vertical movements behind the wall are considered, for a range of different wall types and support mechanisms. In particular, horizontal movements in soft clays showed a wide scatter but typically, movements (δhmax) were less than 1.0%H. The walls that showed movements of greater than 1.0%H were, with the exception of two case studies, all sheet pile walls. For stiff clays, horizontal movements showed less scatter and typical movements of 0.4% < δhmax /H. Vertical settlements in soft clays again show wide scatter and movements of up to 1.0% < δhmax /H except for sheet pile walls, which show greater movement (typically up to 1.8%H). Moorman also reports on the location of maximum movement in relation to the excavation depth, influence of support system and compares his database to those presented in previous studies.

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Finno et al (2007) report the effects of excavation geometry on 3D ground movements caused by excavation in clay. The effect of wall system stiffness and FoS against basal heave is also considered. Results are presented in terms of the PSR. Lam (2010) collected a database of 150 published international case studies, particularly focusing on walls that are embedded in a stiff material or rock, but retain soft to firm clays with a cu of less than 75 kPa.

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Wang et al (2010) present the results from a database of around 300 case studies of ground movements from deep excavations in soft soils in Shanghai. Mean values of lateral displacement as a percentage of excavation depth are discussed for several types of wall. It is concluded that for all types of wall, lateral movements increase with increasing excavation depth. A mean value of maximum ground surface settlement of 0.42%H is proposed (typically from a range of movements between 0.1H and 0.8H). The settlement zone was found to range between 1.5H and to 3.5H from the excavation, for this particular set of data. The paper assesses the influence of other factors such as system stiffness and wall thickness on movement. Comparisons are made between the Shanghai database and other published data.

A6.3.2 Case study data Case studies detailing the ground movements due to wall deflection are listed in Tables A6.8 to A6.12. In addition, discussions on various databases of global case studies, predominantly for ground movements in soft soil, include Finno et al (2002), Moorman (2004), Lam (2010), and Wang et al (2010). For these, details of specific sites are unknown.

Guidance on embedded retaining wall design

355

Kingpost

Diaphragm wall

Washington, OCC Building USA

Chater Station

33

34

Hong Kong

Kingpost, no lagging

Seattle, Washington, USA

1st National Bank

Multi-propped top down

Multi-propped

Multi-propped

22

37

2 m of fill over 22 m of Pleistocene terrace deposits of clay and sand over Cretaceous Potomac formation 9 m of fill over 8 m of marine deposits over 27 m of decomposed granite over granite rock

2

25.2

19.2

32

Interbedded stiff clay and fine sandy silt

3.04 m of fill, over 4.87 m of very stiff silty clay over 11 m of silty sand over 6.4 m of sand over 9.7 m of silty clay over 4.6 m of glacial till

10 m stiff clay over interbedded stiff clay and fine sandy silt

Kingpost

Houston, USA

Houston Buildings

31

Anchored

37

36

11 m of very stiff clay over 3 m of sand and gravel over 5 m glacial till over interbedded sands, silts, and clays

Kingpost, 4 m intervals, wood Multi-propped lagging 10 cm to 15 cm thick

Seattle, Washington, USA

Columbia Center

30

28

29

4.5 m of fill and Lake Edmonton Clay over 13 m of glacial till over 5.5 m of Saskatchewan sands over Cretaceous bedrock

Contiguous pile wall (0.91 m to Top-down, multi-propped 1.07 m diameter)

Edmonton, Canada

Churchill Square

Diaphragm wall (1 m thick) 30

13

27

18.3

23.7

16.7

31

15

16

18

8

Wall depth Excavation (m) depth (m)

18

5.5

Depth to water table (m)

7.5 m of made ground and gravel

London Clay

Restraining soil

Diaphragm walls One anchor, propped by (0.6 m thick) floors top-down

UK

YMCA

12

0.5 m of made ground over 30 m London Clay

Retained soil stratigraphy

10 m of made ground/ sand and gravel over London Clay

UK

New Palace Yard

11

Construction sequence/ support system

Diaphragm wall Anchored (4.57 m × 0.6 m)

Wall type/ dimensions

10

3.7

1.52

2.5

6

13

2

12

5

Embedment depth (m)

6

3.9

2.7

3.1

1.7

7

3.1

1.15

Support spacing (m)

Davies and Henkel (1980)

Ware et al

Shannon and Strazer (1970)

Ulrich (1989)

Grant

Eisenstein and Medeiros (1983)

St John (1975) Burland et al (1979)

Burland and Hancock (1977) St John (1975) Simpson et al (1979)

Sills et al (1977) Simpson et al (1979) St John (1975) Carswell et al (1991)

Reference

Please note that the references cited in these tables are derived from other source materials, and so are incomplete.

Top down, multi-propped

UK

1

Location

Wall deflection effects – case study data (from Clough and O’Rourke, 1990)

Neasden Lane Underpass

Site no Site

Table A6.8

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356

CIRIA, C760

Diaphragm wall Cantilever (3.66 m × 1.22 m)

Reading (A329(M))

16

UK

3 m of made ground and terrace gravel over London Clay

Supported by 5 m berm 8.5 m of made ground/ during excavation of central alluvial clay/gravel over area, temporary props, then London Clay permanent prop cast

Diaphragm wall (0.8 m thick)

Waterloo International London, UK Terminal

15

USA

1.83

Multi-propped

Kingpost, lagging

8th and G st

39

7.3 m of medium to dense sand and gravel over 4.6 m of stiff clay over 6.4 m of dense sand and interbedded stiff clay over 5.2 m of dense sand and gravel over 4 m of hard clay over 10.7 m of dense clayey sand over schistose Gneiss

2.5

1.83

Multi-propped

0

3.7

Support spacing (m)

7.3 m of sand and gravel over 4.6 m of stiff clay over 6.4 m of dense sand and interbedded stiff clay over 5.2 m of dense sand and gravel over 4 m of hard clay over 10.7 m of dense clayey sand over schistose Gneiss

Kingpost, lagging

G St test site USA

38

USA

10.4

9.3

Embedment depth (m)

1.83

Multi-propped

Kingpost, lagging

7th and G St

10.4

13

Wall depth Excavation (m) depth (m)

7.3 m of sand and gravel over 4.6 m of stiff clay over 6.4 m of dense sand and interbedded stiff clay over 5.2 m of dense sand and gravel over 4 m of hard clay over 10.7 m of dense clayey sand over schistose Gneiss

1

1 m of fill over 0.6 m of clay over 7.8 m of moraine over bedrock

Anchored, using tie rods and rockbolts

King post, wooden lagging

37

Stockholm

Bergshamra

36

8

Sheet pile wall

UK

Hatfield

35

3 m of fill over 2 m of gravel over 2.1 m of sandy gravel over 5.7 m of sand over 2 m of till over 8 m of gravel/sand over chalk

Depth to water table (m)

Anchored, one high anchor

Restraining soil

Retained soil stratigraphy

Location

Site no Site

Construction sequence/ support system

Wall type/ dimensions

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357

Burland et al (1979) St John (1975) Carder and Symons (1989)

Li et al (1992)

O’Rourke et al

O’Rourke et al

O’Rourke et al

Broms and Stille (1976)

Symons et al (1987)

Reference

UK

UK

UK

UK

UK

UK

UK

Bell Common (M25)

Hackney Wick (A406)

Walthamstow (CPW)

A406/A10 Junction

Limehouse Link

Rayleigh Weir

East of Falloden Way (A406) (CPW)

3

4

5

6

7

8

9

10

UK

UK

Walthamstow (CF)

New Palace Yard

UK

East of Falloden Way (A406) (DW)

2

11

UK

1

Top propped

Secant pile wall (1.18 m piles at 1.08 m centres)

Diaphragm wall (1 m thick) Top-down, multi-propped

Cantilever

Low propped

Contiguous pile wall (1.5 m piles at 1.7m centres)

Contiguous pile wall (1.5 m piles at 2 m centres)

Tunnel portal, cantilever

Low propped

Diaphragm wall, counterfort, (4 m × 0.8 m front, 2.7 m × 0.8 m counterfort)

Diaphragm wall (4.2 m × 1 m)

Top propped

Contiguous pile wall (1.5 m piles at 1.7 m centres)

Cantilever

Low propped

Diaphragm wall, counterfort, (4 m × 0.8 m front, 3.2 m × 1.5 m counterfort)

Secant pile wall (1.2 m piles at 1.03 m centres)

Cut and cover

Anchored

Construction sequence/ support system

Diaphragm wall (5 m × 1 m)

Diaphragm wall (4.57 m × 0.6 m)

Location Wall type/dimensions

Wall deflection effects – case study data (from Carder, 1995)

Neasden Lane Underpass

Site no Site

Table A6.9

18

12

1.15

Tedd et al (1984) Symons and Tedd (1989)

Carder et al (1994)

Brookes and Carder (1996)

Sills et al (1977) Simpson et al (1979) St John (1975) Carswell et al (1991)

Reference

Brookes and Carder (1996) Burland and Hancock (1977) St John (1975) Simpson et al (1979)

23 m of glacial till over London Clay 10 m of made ground/ sand and gravel over London Clay

Darley et al (1997)

Moran and Laimbeer (1994)

6 m of made ground and terrace gravel over London Clay 3 m of made ground over London Clay

Carder et al (1991)

30

5

Support spacing (m)

2.4 m of made ground/ sand and gravel over London Clay

5.5

8

Embedment depth (m)

Carswell et al (1993) Watson and Carder (1994)

20

13

Wall depth Excavation (m) depth (m)

1.5 m of made ground over London Clay

3

Depth to water table (m)

Bennett et al (1996)

London Clay

Glacial till

London Clay

Restraining soil

5 m of made ground and terrace gravels over London Clay

7 m older head and Claygate beds over London Clay

London Clay to surface

21 m of glacial till over London Clay

0.5 m of made ground over 30 m London Clay

Retained soil stratigraphy

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358

CIRIA, C760

UK

16

Diaphragm wall (3.66 m × 1.22 m)

Reading (A329(M)) Cantilever

3 m of made ground and terrace gravel over London Clay

3.1

Burland et al (1979) St John (1975) Carder and Symons (1989)

Lings et al (1994)

St John (1975) Burland et al (1979)

Li et al (1992)

2

Supported by 5 m berm 8.5 m of made ground/ during excavation of central alluvial clay/gravel over area, temporary props, then London Clay permanent prop cast

Diaphragm wall (0.8 m thick)

UK

Waterloo International Terminal

15

16

Cole and Burland (1972) Burland et al (1979)

Diaphragm wall (0.8 m thick)

Britanic House UK

14

Gault Clay overlain by 3 m of made ground and gravel

18

Supported by berm during 2.5 m of sand and excavation of central area, temporary struts, gravel over London Clay then floor cast

Reference

Top-down, multi-propped

Support spacing (m)

Diaphragm wall (8.5 m × 0.6 m)

Lion Yard

13

UK

Embedment depth (m)

7.5 m of made ground and gravel

Wall depth Excavation (m) depth (m)

One anchor, propped by floors top-down

Depth to water table (m)

Diaphragm walls (0.6 m thick)

YMCA

UK

Restraining soil

Retained soil stratigraphy

Construction sequence/ support system

Location Wall type/dimensions

12

Site no Site

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Guidance on embedded retaining wall design

359

UK

Norwich

Japan

Singapore

Croydon

Holborn

Waterloo

Barnsley

Japan

Detroit

Lyon

Italy

Buffalo

Ontario

Zurich

Oakland

Seattle

Chicago

London

Chelsea

Guildhall

Vauxhall

YMCA

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

UK

UK

UK

UK

UK

USA

USA

USA

Switzerland

USA

USA

France

USA

UK

UK

UK

UK

Location

Wall type/ dimensions

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Multi-propped

Multi-propped

Multi-propped

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Multi-propped

Single prop

Single prop

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Construction sequence/ support system

Retained soil stratigraphy

Wall deflection effects – case study data (from Fernie and Suckling, 1996)

Site no Site

Table A6.10

London Clay

London Clay

London Clay

London Clay

London Clay

Soft clay

Stiff soil

Stiff soil

Silt/sand

Very dense sand

Dense sand and gravel

Soft clay

Stiff clay

Soft clay

Soft clay

Coal measures

Medium gravel

London Clay

London Clay

Soft clay

Stiff clay

Upper chalk

Restraining soil

Depth to water table (m)

Wall depth (m)

16

14.5

6.5

13

8

13.4

19

15.5

15.2

11

5.5

8

7

17.1

10

5.78

11

11.4

13

13.8

18

Excavation depth (m)

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CIRIA, C760

Embedment depth (m)

10

3.63

3

4.33

5.2

4.46

7.64

3.8

13.7

3.04

3.67

7.8

3.2

11.2

4.3

7.5

5.8

8.65

5

3.25

6.88

2.57

Support spacing (m)

St John

Littlejohn

Littlejohn

Corbett

Hodgson

Gnaedinger

Peck

Peck

Gysi

Bauer (1984)

Peck

Rampello

Kastner

Abedi

Sato

Curtis

Li

Ward

Brooks

Wallace

Maruoka

Grose

Reference

UK

Charing Cross

New Palace Yard UK

A329

Humber Bridge

Oslo

New York

Houston

Brittanic House

Neasden

Aldersgate

Chingford

Swindon

Mark Lane

Mark Lane

Benwell Rd

Manchester

Manchester

Manchester

Edinburgh

JLE 111

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

USA

USA

Norway

UK

UK

Location

Site no Site

Wall type/ dimensions

Single prop

Cantilever

Single prop

Cantilever

Cantilever

Cantilever

Single prop

Multi-propped

Cantilever

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Cantilever

Multi-propped

Multi-propped

Construction sequence/ support system

Retained soil stratigraphy

London Clay

Coal measures

Sandstone

Sandstone

Sandstone

London Clay

London Clay

London Clay

Kimmeridge Clay

London Clay

London Clay

London Clay

London Clay

Stiff clay

Rock

Soft clay

Kimmeridge Clay

8

6

7

5

5

4.5

5.5

7

4.5

8

23

8

14

18.3

18.5

10

29.5

6.9

11

Excavation depth (m)

Woolwich and Redding Beds

Wall depth (m)

18.5

Depth to water table (m)

London Clay

London Clay

Restraining soil

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361

Embedment depth (m)

8.2

8.4

8

7

7

6.3

6.3

3.5

6.3

9.2

3.3

2

4.67

6.1

3.7

1.67

4.92

9.7

3.08

2.75

Support spacing (m)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Carswell

Fernie

Sills

Cole

Peck

Saxena

Peck

Busbridge

Carder

Burland

Wood

Reference

USA

UK

Eastbourne

Bell Common

A1(m)

Cambridge

Singapore

Switzerland

Chicago

Chicago

Washington

Washington

Chicago

Chicago

Boston

San Francisco

San Francisco

San Francisco

Victoria embankment

San Francisco

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

USA

USA

USA

USA

USA

USA

USA

USA

USA

USA

UK

UK

UK

Wall type/ dimensions

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Cantilever

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Single prop

Single prop

Single prop

Construction sequence/ support system

For details of source references, see Fernie and Suckling (1996)

Note

UK

Eastbourne

328

UK

Location

Site no Site

Retained soil stratigraphy

Soft clay

London Clay

Soft clay

Soft clay

Soft clay

Stiff clay

Soft/medium clay

Soft/medium clay

Hard clay

Hard clay

Soft/medium clay

Soft/medium clay

Moraine

Stiff clay

Gault Clay

Medium sand

London Clay

Alluvial clay

Alluvial clay

Restraining soil

Depth to water table (m)

Wall depth (m)

13.7

18

22

11

10

19

8.5

8

25

17

9

8

17.2

12.9

10

9.3

9

14

11

Excavation depth (m)

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362

CIRIA, C760

Embedment depth (m)

3.43

6

2.75

10

23.5

3.8

2.83

2.67

6.25

11

3

2

4.29

2.58

3.3

10.6

8.9

22.4

17.6

Support spacing (m)

O’Rourke

St John

Clough

Clough

Clough

Clough

Clough

O’Rourke

O’Rourke

O’Rourke

O’Rourke

O’Rourke

Huder

Tan

Ng

Symons

Tedd

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Reference

UK

UK

Holborn

Minster Court

Britannic Hse

Chelsea

Walthamstow

Barbican

Charing Cross

John Lewis KUT

Victoria Embankment

London

Guildhall

Vauxhall

Chingford

41

27

43

44

45

46

47

48

49

50

51

52

53

Mark Lane

UK

Croydon

40

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

UK

Location

Site no Site

54

Counterfort diaphragm wall

Wall type/ dimensions Temporary props with permanent low prop

Construction sequence/ support system

Contiguous

Secant

Diaphragm

Diaphragm

Diaphragm

Secant

Diaphragm

Diaphragm

Diaphragm

Secant

Diaphragm

Diaphragm

Diaphragm

Secant

Secant

Wall type/ dimensions

Multi-propped

Single prop

Multi-propped

Single prop

Single prop

Props and berm

Props and berm

Multi-propped

Multi-propped

Multi-propped

Props and berm

Props and berm

Props and berm

Single prop

Multi-propped

Construction sequence/ support system

London Clay London Clay London Clay London Clay London Clay London Clay London Clay

5 m of soft soil above hard soil 1 m of soft soil above hard soil 4 m of soft soil above hard soil 1.4 m of soft soil above hard soil 2 m of soft soil above hard soil 5 m of soft soil above hard soil 2.5 m of soft soil above hard soil

London Clay

London Clay

London Clay

London Clay

London Clay

London Clay

London Clay

3.5 m of soft soil above hard soil

6 m of soft soil above hard soil

London Clay

2 m of soft soil above hard soil

14.6

10

7

8

14.5

6.5

8

18

12

11

16

7.9

13

14

9

11

11.4

4.6

4

3.5

9.2

3.63

3

5.2

6

3.8

2.75

4

7.4

4.33

4.67

7.3

8.65

5

Carder et al (1997)

Reference

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

St John et al (1992)

Long (1989)

Wood and Perrin (1984)

Stevens et al (1977)

Watson and Carder (1994)

Corbett et al (1975)

Cole and Burland (1972)

Tse and Nicholson (1993)

Ward (1992)

Brooks and Spence (1992)

Reference

Support spacing (m)

Support spacing (m)

Embedment depth (m)

Embedment depth (m)

Wall depth Excavation (m) depth (m)

Wall depth Excavation (m) depth (m)

Depth to water table (m)

Depth to water table (m)

London Clay

Restraining soil

Restraining soil

2 m made ground over 2 m sand and gravel over London Clay

Retained soil stratigraphy

Retained soil stratigraphy

Wall deflection effects – case study data (from Long, 2001)

UK

Location

Wall deflection effects – case study data (from Carder et al, 1997)

Aldershot Road Underpass

Table A6.12

347

Site no Site

Table A6.11

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Guidance on embedded retaining wall design

363

UK

USA

USA

USA

Mark Lane

JLE HI

MaIden Way

Bermondsey

Canada Water

Humber Bridge

Cambridge

Channel Tunnel

Lyon

Switzerland

Dublin-Jervis

Dublin-Clarend

Dublin-MandS

MBTA. Boston

Oakland

Houston

Seattle

West. Station, Seattle

Pion. Square, Seattle

54

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

USA

USA

USA

Ireland

Ireland

Ireland

Switzerland

France

UK/France

UK

UK

UK

UK

UK

UK

Location

Site no Site

Secant

Contiguous

Sheet piles

Unknown

Sheet piles

Diaphragm

Sheet piles

Soldier pile

Secant

Diaphragm

Diaphragm

Sheet piles

Diaphragm

Diaphragm

Secant

Diaphragm

Contiguous

Diaphragm

Contiguous

Wall type/ dimensions

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Single prop

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Single prop

Single prop

Construction sequence/ support system

Glacial till Glacial till

1 m of soft soil above hard soil 3 m of soft soil above hard soil

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Glacial till

Moraine

Stiff clay

Gault Clay

Gault Clay

Kimmeridge Clay

Woolwich and Reading beds

21.9

15.2

23.8

18.3

19

15.2

7.2

6.2

9.7

17.2

8

6.5

10

24.5

17

19.5

Woolwich and Reading beds

8

5.5

Wall depth Excavation (m) depth (m)

7.5

Depth to water table (m)

London Clay

London Clay

London Clay

Restraining soil

3 m of soft soil above hard soil

2 m of soft soil above hard soil

7 m of soft soil above hard soil

Retained soil stratigraphy

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

364

CIRIA, C760

Embedment depth (m)

5.5

3.8

2.64

6.1

3.36

3.36

6

5

8.5

4.29

3.2

4

3.3

4.92

5.7

6

5.5

8.2

6.3

Support spacing (m)

Borst et al (1990)

Borst et al (1990)

Peck (1969)

Peck (1969)

Peck (1969)

Becker and Haley (1990)

Long (1997)

Long (1997)

Long (1997)

Huder (1972)

Kastner and Ferrand (1992)

Young and Ho (1994)

Ng and Lings (1995)

Busbridge (1974)

Powrie and Batten (1997)

Dawson et al (1996)

Symons and Carder (1991)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Reference

USA

Washington

Washington

Washington

Houston-Exxon

Houston-1Shell

Houston-Cokex

Tiong Bahru

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE U

Singapore CE II

Singapore CE U

Singapore CE U

Singapore CE U

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

74

74

74

77

78

79

80

81

81

81

81

81

86

81

86

86

86

81

81

81

81

81

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

USA

USA

USA

USA

USA

Location

Site no Site

Sheet and H

Sheet and H

Sheet and H

Sheet and H

Sheet and H

Sheet and H

Sheet and H

Sheet and H

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Sheet piles

Contiguous

Soldier piles

Contiguous

Sheet piles

Piled

Wall type/ dimensions

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Construction sequence/ support system

1.5 m of soft soil above hard soil

3 m of soft soil above hard soil

4.5 m of soft soil above hard soil

Retained soil stratigraphy

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Hard clay

Hard clay

Restraining soil

Depth to water table (m)

12

11.5

11.5

11

10

10

10

10

11.5

11.5

10.5

10.2

9

9

9

15.3

17.1

18

16.2

15

25

17

Wall depth Excavation (m) depth (m)

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

Guidance on embedded retaining wall design

365

Embedment depth (m)

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4.6

3.5

6

6.5

7.5

6.25

11

Support spacing (m)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al(1997)

Wong et al (1997)

Wong et al (1997)

Leonard et al(1987)

Ulrich (1989a)

Ulrich (1989a)

Ulrich (1989a)

Eisenstein and Medeiroz (1983)

O’Rourke (1992)

O’Rourke (1992)

Reference

Singapore

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE II

Singapore CE U

Singapore CE U

Singapore CE II

Singapore CE U

Singapore CE U

Singapore CE U

Singapore CE U

Waterloo

Eastbourne

Buffalo

Ontario

Zurich

Lyon-P Kieb

81

81

81

81

81

81

81

81

86

86

81

86

86

86

86

111

112

113

114

115

116

France

Switzerland

Canada

USA

UK

UK

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Location

Site no Site

Sheet piles

Diaphragm

Sheet piles

Sheet piles

Diaphragm

Diaphragm

Diaphragm

Diaphragm

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Sheet and H

Sheet and H

Wall type/ dimensions

Single prop

Single prop

Multi-propped

Multi-propped

Multi-propped

Props and berm

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Construction sequence/ support system

3.5 m of soft soil above hard soil

2 m of soft soil above hard soil

Retained soil stratigraphy

Sandy gravel

Silt/sand

Very dense sand

6.75

15.5

15.2

11

Dense sand and gravel

5.78

14.5

13.5

21.5

20

13.5

13.5

13.5

12.5

12

12

11.5

11

11

17

17

Wall depth Excavation (m) depth (m)

11

Depth to water table (m)

Gravel/sand

Medium gravel

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Restraining soil

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

366

CIRIA, C760

Embedment depth (m)

6.75

13.7

3.04

3.67

7

5.8

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

Support spacing (m)

Kastner (1982)

GYSI (7)

Bauer (1984) (7)

Peck (1969)

Fernie et al (1996)

Li et al (1992)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Reference

France

Lyon-R Ney

Lyon-S. Gam.

117

118

Cairo Metro

Lisbon-Carlos

Hannover

Hannover

Duisburg

Offenbach

Lubeck

Salzburg

Bruckmuhl

Sao Paulo, ES1

Sao Paulo, ES2

Argyle Station

Manchester

New York

Han River, Seoul

YMCA, London

120

121

122

123

123

125

126

127

128

129

130

131

132

133

134

135

136

UK

China

USA

UK

Hong Kong

Brazil

Brazil

Germany

Germany

Germany

Germany

Germany

Germany

Portugal

Egypt

Germany

The Netherlands

Karlshrue

Maas, Rotterdam

119

France

Location

Site no Site

Diaphragm

Secant/H

Piled

Contiguous

Diaphragm

Soldier pile

Soldier pile

Soldier piles

Soldier piles

Secant

Secant

Soldier pile

Secant

Secant

Diaphragm

Precast

Sheet and tubes

Sheet piles

Diaphragm

Diaphragm

Wall type/ dimensions

Multi-anchored

Multi-propped

Multi-propped

Single prop

Multi-propped

Multi-propped

Multi-propped

Single prop

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Single prop

Single prop

Multi-propped

Construction sequence/ support system Sandy gravel Sandy gravel

6.5 m of soft soil above hard soil 3 m of soft soil above hard soil

Weathered rock London Clay

1 m of soft soil above hard soil

Rock

Sandstone

Residual soil

Residual soil

Residual soil

Sand, marl

Gravel, clay

Silt

Clay

Gravel, sand, silt

Sand, gravel, clay

13.5 m of soft soil above hard soil

7 m of soft soil above hard soil

Clay/sands

5.5 m of soft soil above hard soil Sand, marl,

Clay/sands

6 m of soft soil above hard soil

Silts/sands

Sands

Restraining soil

Retained soil stratigraphy

Depth to water table (m)

11.5

13.8

10

21

5

10.7

9.95

Wall depth Excavation (m) depth (m)

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

Guidance on embedded retaining wall design

367

16

25

18.5

7

18.7

19

9

14.1

10

9.6

19.3

23

21.9

Embedment depth (m)

10

2:78

3.7

8

6.2

3.8

3.6

7

5

4.25

6.4

4.8

6.6

2.6

2.76

3.3

17

3.75

5.2

3.1

Support spacing (m)

St John (1975)

Choi and Lee (1998)

Saxena (?)

Fernie and Suckling (1996)

Morton et al (1980)

Massad (1985)

Massad (1985)

Viendenz (1984)

Breymann (1992)

Rodatz et al (1996)

Krajewski et al (1997)

Hettler et al (1997)

Blümel and Wemheuer (1980)

Rizkallah and Vogel (1992)

Mattos-Fernandes (1985)

El-Nahhas and Eisenstein (1989)

Bakker and Brinkgrieve (1991)

Josseaume et al (1997)

Kastner (1982)

Kastner (1982)

Reference

Lisbon-ivens

Colomb., Seattle USA

University Street Seattle, USA Station

Seattle

Houston-Herm

Houston-Bank

Houston-FCB

Houston-Smith

Houston-Texas

Houston-Cullen

Houston-321

Washington

State Trans

142

143

144

71

147

148

149

150

151

152

153

74

155

Davis Square

Lisbon-Colom

141

157

Lisbon-DD Ave.

140

60 State St

Contiguous

Copenhagen

139

156

Piled

DenmarkSweden

Oresund-Sydh

138

Boston, USA

Boston, USA

Boston, USA

USA

USA

USA

USA

USA

USA

USA

USA

USA

Portugal

Portugal

Portugal

Diaphragm

Diaphragm

Diaphragm

Soldier

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Contiguous

Soldier pile

Soldier pile

Soldier pile

Soldier pile

Tubes/H

Soldier pile

UK

Neasden

137

Denmark

Diaphragm

Location

Site no Site

Wall type/ dimensions

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Single anchor

Single anchor

Multi-anchored

Construction sequence/ support system

Stiff clay Stiff clay

2.6 m of soft soil above hard soil 4 m of soft soil above hard soil

3 m of soft soil above hard soil

3 m of soft soil above hard soil

3m of soft soil above hard soil

m of soft soil above hard soil

Stiff clay

1 m of soft soil above hard soil

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Boulder clay

London Clay

1 m of soft soil above hard soil 5 m of soft soil above hard soil

Restraining soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

368

CIRIA, C760

17.1

9.1

9.1

15.2

9.1

8.2

16

15.5

9.1

16.8

9

23

18.3

37

9

14

17

11

10

8

Embedment depth (m)

3.36

3.36

3.36

3

3

7

3.2

3

8

3.35

3

1.5

2.6

1.75

3.25

3.25

3.25

5.5

7

2

Support spacing (m)

Becker and Haley (1990)

Becker and Haley (1990)

Becker and Haley (1990)

Ware et al (1973)

Ulrich (1989b)

Ulrich (1989b)

Ulrich (1989b)

Ulrich (1989b)

Ulrich (1989b)

Ulrich (1989b)

Ulrich (1989b)

Winter (1990)

Borst et al (1990)

Grant (1985)

Correia and da Costa Guerra (1997)

Correia and da Costa Guerra (1997)

Correia and da Costa Guerra (1997)

Duc Long and Bredenberg (2000)

Hess et al (1997)

Sills et al (1977)

Reference

Boston, USA

1 Memorial

Harvard Square

Boston

Harvard Square

Boston

Boston

Salt Lake City

CE II

CE II

CE II

CE U

CE II

CE II

CE II

CE II

A1(M)

Hatfield

Lisbon

Paris-R Gau

Paris-13e

158

159

160

159

160

163

164

81

81

81

86

81

81

81

81

173

35

175

176

177

France

France

Portugal

UK

UK

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

USA

USA

USA

USA

USA

USA

Location

Site no Site

Diaphragm

Sheet

Diaphragm

Sheet piles

Sheet piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier piles

Soldier pile

Soldier pile

Diaphragm

Diaphragm

Diaphragm

Wall type/ dimensions

Multi-anchored

Single anchor

Multi-anchored

Single anchor

Single anchor

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Construction sequence/ support system

Stiff clay

3 m of soft soil above hard soil

Sand Sand/gravel

5.5 m of soft soil above hard soil 4 m of soft soil above hard soil

Sands

Gravels

3 m of soft soil above hard soil

Sand

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Till

Till

Stiff clay

Restraining soil

2 m of soft soil above hard soil

2 m of soft soil above hard soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

Guidance on embedded retaining wall design

369

17.4

12.3

13.8

9.3

9.3

19

18.5

17

15

16

12.5

12.5

12.2

12.5

18.9

15.7

15.7

8.2

Embedment depth (m)

4.35

8.3

2.75

6.8

10.6

4

4

4

4

4

4

4

4

3.2

2.7

6.2

5.2

3.36

Support spacing (m)

Josseaume and Stenne (1979)

Gignan (1984)

Fernandes (1985)

Symons et al (1988)

Symons et al (1988)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Cliendo et al (1990)

Houghton and Dietz (1990)

Hansmire et al (1989)

Hansmire et al. (1989)

Becker and Haley (1990)

Reference

Diaphragm

Berlin, Germany

Berlin, Germany

P Platz DB

Hofgarten

Berlin

Berlin

SONY

Salzburg

Salzburg

Urreiting

Fr’hafen

Vienna

Dusseldorf

Dusiburg

Grauholz

Johannsburg

Milwaukee

Norwich

181

182

183

183

185

186

186

188

189

190

191

192

193

194

196

197

Dartford

Diaphragm

Geneva, Switzerland

Le Mail

180

198

Diaphragm

The Netherlands

Le Havre

179

UK

UK

USA

South Africa

Switzerland

Dusiburg

Germany

Austria

Austria

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Multi-anchored

Single anchor

Multi-anchored

Single anchor

Multi-anchored

Construction sequence/ support system

Barrettes

Contiguous Multi-anchored

Multi-anchored

Deep soil mix Multi-anchored

Soldier piles

Contiguous

Soldier piles

Soldier piles

Soldier piles

Secant

Soldier piles

Soldier piles

Soldier piles

Diaphragm

Berlin, Germany

Austria

Secant

Germany

Soldier piles

Diaphragm

France

Calais

178

Germany

Diaphragm

Location

Site no Site

Wall type/ dimensions

1.2 m of soft soil above hard soil

5 m of soft soil above hard soil

3 m of soft soil above hard soil

3 m of soft soil above hard soil

4 m of soft soil above hard soil

Chalk

Chalk

Sands

Firm silt

Sands, silts

Sand, gravel

Sand, gravel

Sand, marl

Gravel, clay

Gravel, sand

Gravel, clay

Gravel, clay

Sand/gravel

Sands

Sands

Sands

Sands

Sand/gravel

Sand/gravel

Sands

4.5 m of soft soil above hard soil 9 m of soft soil above hard soil

Restraining soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

370

CIRIA, C760

9

18

5

18.3

17

20

19.8

18.5

12.5

18.3

11.5

11

14.3

12.3

18.5

17

18

14.8

16.5

24

Embedment depth (m)

2

2.57

1.67

3.05

2.4

2.9

3.5

2.9

2.9

5.5

4.9

4

3.58

3.6

0.75

2.83

15

2.5

10.5

8

Support spacing (m)

Wood et al (1989)

Grose and Toone (1992)

Anderson (1998)

Day (1990)

Steiner and Werder (1991)

Ulrichs (1982)

Ulrichs (1980)

Pötscher et al (1984)

Ostermayer (1983)

Naderer (1988)

Breymann (1992)

Breymann (1992)

Kudella and Mayer (1998)

Weibenbach and Gollub (1995)

Triantafyllidis (1998)

Nussbaumer (1998)

Triantafyllidis et al (1997)

Monnet et al (1994)

Maquet (1981)

Delattre et al (1995)

Reference

UK

New Palace Yard UK

UK

UK

Bell Common

British Library

National Gallery Extension

Aldersgate

Limehouse

P0 Square

HKandS Bank

Chater Station

Hong Kong

Rowes Whr

75 State St

125 Sum. St

Salzburg

Wien

Cairo Metro

River

Havelock

199

11

201

202

28

204

205

206

207

208

209

210

211

186

190

121

215

216

Singapore

Singapore

Egypt

Austia

Austia

Boston

Boston

Boston

Hong Kong

Hong Kong

Hong Kong

Boston

UK

UK

Location

Site no Site

Diaphragm

Diaphragm

Diaphragm

Soldier piles

Soldier piles

Diaphragm

Diaphragm

Diaphragm

Soldier piles

Diaphragm

Diaphragm

Diaphragm

Diaphragm

Diaphragm

Secant

Secant

Diaphragm

Secant

Wall type/ dimensions

Multi-propped

Multi-propped

Top down

Top down

Top down

Top down

Top down

Top down

Top down

Top down

Top down

Top down

Top down

Top-down

Top-down

Top-down

Top-down

Top-down

Construction sequence/ support system

Decomposed granite

Clay/sands Stiff clay Stiff clay

3.5 m of soft soil above hard soil 20 m of soft soil above hard soil 16.5 m of soft soil above hard soil

Sand, silt

Gravel, clay

Stiff clay

Stiff clay

Decomposed granite

14 m of soft soil above hard soil

3 m of soft soil above hard soil

Decomposed granite

5 m of soft soil above hard soil

Stiff clay

28

Till

4 m of soft soil above hard soil

5 m of soft soil above hard soil

26

Woolwich and Reading beds

4 m of soft soil above hard soil

16.5

29

10

24

11.5

18.3

19.8

16.8

16

23.4

16

23

10

24.4

London Clay

London Clay

3 m of soft soil above hard soil

18.5

9

Embedment depth (m)

8 m of soft soil above hard soil

London Clay

2 m of soft soil above hard soil

Wall depth Excavation (m) depth (m)

London Clay

London Clay

4 m of soft soil above hard soil

Depth to water table (m)

4.2 m of soft soil above hard soil

Restraining soil

Retained soil stratigraphy

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

Guidance on embedded retaining wall design

371

4

3.3

5

6.3

3.8

3.36

3.36

3.36

4.7

4

4

3.3

4

3.3

7

5

3.08

8.9

Support spacing (m)

Gronin et al (1991)

Gronin et al (1991)

EI-Nahhas and Eisenstein (1989)

Fross and Klenovec (1992)

Breymann (1992)

Becker and Haley (1990)

Becker and Haley (1990)

Becker and Haley (1990)

Triantafyllidis (1996)

Davies and Henkel (1980)

Humpheson et al (1986)

Whittie et al (1993)

Stevenson and De Moor (1994)/De Moor and Stevenson (1996)

Fernie et al (1991)

Long (1989)

Simpson (1992)

Burland and Hancock (1977)

Tedd et al (1984)

Reference

Singapore

Singapore

Singapore

Norway

Norway

CBD

CE II

CE U

CE II

CE U

CE U

CE II

CE U

CE U

CE II

CE U

Multistorey car park

Interchange

Canal

B

D

Oslo Gronland 1

Oslo Tech School

Oslo Telephone

217

81

86

81

86

86

81

86

86

81

86

228

229

230

231

232

233

234

235

Norway

Bangkok

Bangkok

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Location

Site no Site

Diaphragm

Sheet

Sheet

Diaphragm

Diaphragm

Sheet

Contiguous

Diaphragm

Arbed

Contiguous

Arbed

Sheet and H

Contiguous

Arbed

Contiguous

Sheet and H

Sheet and H

Diaphragm

Sheet piles

Wall type/ dimensions

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Construction sequence/ support system Stiff clay

14 m of soft soil above hard soil

Stiff clay Rock

14 m of soft soil above hard soil 11.5 m of soft soil above hard soil

Rock

Stiff clay

15 m of soft soil above hard soil

18.5 m of soft soil above hard soil

Stiff clay

6 m of soft soil above hard soil

Rock

Stiff clay

15 m of soft soil above hard soil

6 m of soft soil above hard soil

Clayey sand

17.3 m of soft soil above hard soil

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Stiff clay

Restraining soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

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18.5

6

11.5

16

15.5

6.5

20

17.3

19

18.5

17.5

17

17

15

14

11

10

10

14.7

Embedment depth (m)

6.1

2

4.5

3.2

5.1

5

4

2.88

4

4

4

4

4

4

4

4

4

4

2.1

Support spacing (m)

DiBiagio and Roti (1972)

NGI (1962b)

NGI (1962a)

Balasubramaniam et al (1991)

Balasubramaniam et al (1991)

Wong and Chua (1999)

Vuillemin and Wong (1991)

Lee et al (1998)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Wong et al (1997)

Broms et al (1986)

Reference

Norway

France

USA

Oslo Olav Kyrres

Oslo City

Nieuw Maas

Japan 1

Sheung Wan

CE U

Quai Gloria

Hartford, Conneticut

UOB

H’Fok A

CTC

Somerset

MOE 12

MOE 19

Bugis

CBD

Parking

236

237

238

239

240

86

242

243

244

245

246

247

248

249

250

217

252

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Singapore

Hong Kong

Japan

Norway

Norway

Location

Site no Site

Diaphragm

Sheet piles

Diaphragm

Sheet piles

Sheet piles

Diaphragm

Sheet piles

Sheet piles

Diaphragm

Soldier pile

Diaphragm

Diaphragm

Diaphragm

Sheet

Diaphragm

Sheet

Sheet

Wall type/ dimensions

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Single anchor

Single anchor

Multi-anchored

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Construction sequence/ support system

Rock Sand Stiff clay CDG

19 m of soft soil above hard soil 19.5 m of soft soil above hard soil 12 m of soft soil above hard soil 19 m of soft soil above hard soil

Dense gravels Soft clay Soft clay Soft clay Peats/silts Soft clay Soft clay Soft clay Soft clay Soft clay

30 m of soft soil above hard soil 19 m of soft soil above hard soil 37 m of soft soil above hard soil 10 m of soft soil above hard soil 24 m of soft soil above hard soil 12 m of soft soil above hard soil 30 m of soft soil above hard soil 17 m of soft soil above hard soil 12 m of soft soil above hard soil

Sands

13.5 m of soft soil above hard soil

9 m of soft soil above hard soil

Rock

18.5 m of soft soil above hard soil

Stiff clay

Restraining soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

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9.5

15

18.3

6.4

6.8

15.2

12

7.3

13

7

12

12

30

13.75

19.5

19

18.5

Embedment depth (m)

5.9

2.5

2.29

1.6

1.7

3.8

2

1.83

2.6

5

10

4

2.73

6.88

6

2.5

2

Support spacing (m)

Vuillemin and Wong (1991)

Broms et al (1986)

Hulme et al (1989)

Tan et al (1985)

Tan et al (1985)

Leonard et al (1987)

Lee et al (1985)

Davies and Walsh (1983)

Wallace et al (1992)

Murphy et al (1975)

Vezinhet et al (1989)

Wong et al (1997)

Fraser (1992)

Fernie and Suckling (1996)

Van Tol and Brassinga (1991)

Bruskeland (1991)

Karlsrud and Myrvall (1976a,b) Karlsrud (1986)

Reference

Quen M

Tax

Formosa

Cathay

A

C

B

Vaterland 1

Vaterland 2

Studenterlu

Jerbanetorget

Bank of Norway

Eastbourne 1

Eastbourne 2

255

256

257

258

259

260

231

262

263

264

265

266

267

268

Pietrafitta

Power

254

269

Taiwan

Airline

253

Diaphragm

Diaphragm

Sheet

Bangkok, Thailand

Bangkok, Thailand

Bangkok, Thailand

Italy

UK

UK

Sheet

Diaphragm

Diaphragm

Oslo, Norway Diaphragm

Oslo, Norway Diaphragm

Oslo, Norway Diaphragm

Oslo, Norway Sheet

Oslo, Norway Sheet

Diaphragm

Diaphragm

Sheet

Diaphragm

Diaphragm

Contiguous

Wall type/ dimensions

Taiwan

Taiwan

Taiwan

Taiwan

Taiwan

Location

Site no Site

Multi-propped

Single prop

Single prop

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Construction sequence/ support system

Soft clay Soft clay Soft clay Soft clay Soft clay

37 m of soft soil above hard soil 35 m of soft soil above hard soil 18 m of soft soil above hard soil 15 m of soft soil above hard soil 15 m of soft soil above hard soil

Soft to firm clay

Soft clay

16 m of soft soil above hard soil

20 m of soft soil above hard soil

Soft clay

16 m of soft soil above hard soil

Firm clay

12 m of soft soil above hard soil

Soft clay

Soft clay

20 m of soft soil above hard soil

12 m of soft soil above hard soil

Soft clay

8 m of soft soil above hard soil

Soft clay

Firm clay

8 m of soft soil above hard soil

12 m of soft soil above hard soil

Firm clay

15 m of soft soil above hard soil

Soft clay

Firm clay

9 m of soft soil above hard soil

15 m of soft soil above hard soil

Restraining soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

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5.5

14

11

16

10

16

11

11

7.2

18.5

9.8

21

18.45

7.65

10.7

16.2

9.6

Embedment depth (m)

7.8

13

10

3.2

5

5.3

2

2

1.8

4.6

3.1

2.63

2.64

1.91

2.68

3.24

2.4

Support spacing (m)

Rampello et al (1992)

Fernie and Suckling (1996)

Fernie and Suckling (1996)

Roti and Friis (1985)

Karlsrud (1981, 1983)

Karlsrud (1981, 1983, 1986)

NGI (1962g)

NGI (1962f)

Balasubramaniam et al (1991)

Balasubramaniam et al (1991)

Balasubramaniam et al (1991)

Ou et al (1993)

Ou et al (1993)

Ou et al (1993)

Ou et al (1993)

Ou et al (1993)

Ou et al (1993)

Reference

Multi-propped

Sheet

Steel pipe pile

Chicago, USA

Sewage Tr. Tokyo Japan

Japan

Mexico

China

China

China

UK

Inland steel

Osaka A

Japan 2

Lake zone, Mexico

Shanghai-Jin Mao

Shanghai-Heng Long

Shanghai

River wall, Middlesborough

Detroit

TP, Bogota

Newton

Chi Ching

Far East

Christiana

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

Diaphragm

Diaphragm

Diaphragm

Diaphragm

Sheet

Sheet

Diaphragm

Diaphragm

Diaphragm

Top down

Top down

Top down

Top down

Multi-anchored

Single anchor

Single anchor

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Multi-propped

Steel pipe pile

Diaphragm

Multi-propped

Diaphragm

Oslo, Norway Sheet

Taiwan

Taiwan

Singapore

Colombia

USA

Japan

Multi-propped

Sheet

Chicago, USA

Chicago

270

Multi-propped

Wall type/ dimensions

Location

Site no Site

Construction sequence/ support system

Soft clay Soft clay Soft clay/silt Soft clays Soft clays Firm clay Soft clay

12 m of soft soil above hard soil 10 m of soft soil above hard soil 34 m of soft soil above hard soil 12 m of soft soil above hard soil 15 m of soft soil above hard soil 24 m of soft soil above hard soil 23 m of soft soil above hard soil

Soft/firm clay

24 m of soft soil above hard

Soft/firm clay

36 m of soft soil above hard soil Soft/firm clay

Soft clay

20 m of soft soil above hard soil

29 m of soft soil above hard soil

Soft clay

Soft clay

25 m of soft soil above hard soil 20 m of soft soil above hard soil

Soft clay

Soft clay

Soft clay

Restraining soil

30 m of soft soil above hard soil

19 m of soft soil above hard soil

15 m of soft soil above hard soil

Retained soil stratigraphy

Depth to water table (m)

Wall depth Excavation (m) depth (m)

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9.6

20

13.9

14.5

16

?

9.5

17.85

18.2

19.65

15.7

17.1

20.6

26

11

13.4

Embedment depth (m)

3

3.33

2.78

3.63

3.75

11.2

4.25

3.57

3.64

3.93

2.62

4.3

3

4.3

2

4.46

Support spacing (m)

Finstad (1991)

Ou et al (1993)

Ou et al (1993)

Nicholson (1987)

Maldonado (1998)

Fernie and Suckling (1996)

Baggett and Butting (1977)

Onishi and Sugawara (1999)

Zhao et al (1999)

Zhao et al (1999)

Auvinet and Organista (1998)

Fernie and Suckling (1996)

Tamano et al (1996)

Tominaga et al (1985)

Flaate (1966)

Fernie and Suckling (1996)

Reference

A6.4 GROUND MOVEMENT TRENDS IN RELATION TO EXCAVATION DEPTH Where available, data used for Figures 6.11, 6.15 and 6.16 have been plotted without normalisation by excavation depth. This gives an estimation of movement in relation to excavation depth. These graphs have been plotted for both horizontal and vertical movement, for all material types (Figures A6.3 and A6.5) and for stiff clays only (Figures A6.4 and A6.6). For horizontal movements, deflections are the maximum deflection at the wall. For vertical movements, the maximum settlement may not occur directly behind the wall, but has been generally shown to occur within 0.75H (as demonstrated in Figure 6.15c), the settlements shown are the maximum within 75 per cent of the excavation depth behind the wall.

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The datamarker fill on each of the plots is representative of the wall stiffness, where a hollow symbol is a cantilever or singly-propped wall (assumed low system stiffness) and a filled symbol is a multi-propped or top-down constructed wall (assumed high system stiffness).

Figure A6.3

Horizontal movements with excavation depth (all materials, all system stiffness)

Figure A6.4

Horizontal movements with excavation depth (stiff clay only, all system stiffness)

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Figure A6.5 Vertical movements with excavation depth taken from within 0.75 excavation depths from the wall (all materials, all system stiffnesses)

Vertical movements with excavation depth taken from within 0.75 excavation depths from Figure A6.6 the wall (stiff clay only, all system stiffnesses)

A6.5 CORNER EFFECTS The shape of an excavation will affect the magnitude and distribution of ground movements around it. This is illustrated by contours of ground surface settlements measured for excavations in stiff London Clay at New Palace Yard (Burland and Hancock, 1977) and in coarse grained soils at Yen Chow Street, Hong Kong (Lui and Yau 1995), see Figure A6.7.

Figure A6.7 Ground surface settlement contours at New Palace Yard, London (after Burland and Hancock, 1977)

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377

The corners of the excavation often restrict movement. A number of researchers have considered this effect, notably St John (1975), Ou et al (1996), Ou and Shiau (1998), Simic and French (1998) and Lee et al (1998). 2D and 3D numerical analyses have been carried out to study details of behaviour. Due to the specific nature of such studies, their extrapolation for general application is limited. However, some useful findings have been made. These are discussed as follows.

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St John (1975) compared the results of finite element analyses assuming plane strain and axi-symmetric conditions for an unsupported excavation, which was square in plan with depth equal to one-third of the length of one side. Uniform elastic soil was assumed with initial hydrostatic groundwater pressures. The results of the analysis, which are also reported by Burland et al (1979), indicated a significant reduction in the horizontal ground movements towards the corners of the excavation. The plane strain and axisymmetric analyses gave similar vertical movements. The horizontal movements from the axi-symmetric model were some 50 per cent of those computed in the plane strain model. Ou et al (1996) report a study related to the excavation for the Hai-Hua building, Taipei, shown in Figure A6.8. They carried out a 2D finite element analysis of the main section and a 3D analysis of the corner section of the building, and developed correlations for wall deflections between the 2D and 3D results. The ground conditions were principally firm clays and the parameters were developed for a Duncan and Chang (1970) model (elastic-plastic material described by a hyperbolic relationship). At inclinometer positions 11, 12, 13, on the long sides of the excavation, Figure A6.8 Plan of the Hai-Hua building, Taipei (after Ou et al, 1996) it was found that the 2D plane strain analysis gave good agreement with field measurements, but that 3D effects were important at inclinometers 14 and 15. The measured deflections at 14 and 15 were some 50 per cent and 40 per cent of the movements computed by the 2D plane strain analysis. Ou et al developed correlations between the 3D and 2D analyses to enable predictions to be made of wall deflections near corners. These correlations worked well for the project they studied. Simic and French (1998) used a 3D analysis of an underground station box, formed in diaphragm walls, to seek savings in reinforcement when comparing results with plane strain analysis. They concluded that steel quantities could be reduced by about 25 per cent for the project they studied, mainly because the walls near the corners of the excavation were calculated as being less heavily loaded.

A6.6 MOVEMENT AT DIFFERENT EXCAVATION STAGES The maximum horizontal movement observed will be affected by the movement occurring during the initial stages of excavation, before any supports are installed. Table A6.9 summarises data from six different case studies. Typically the cantilever movement has been shown to be around 70 per cent of the total horizontal movement. The following plots expand on the data shown in Figure 6.24 and show the relationship between cantilever height and cantilever movement for the same set of case studies.

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Figure A6.9 Table A6.13

Relationship between cantilever height and initial cantilever movements for the case studies presented in

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379

Project title

Diaphragm wall (5 m × 1 m)

Secant pile wall (1.18 m piles at 1.08 m centres)

East of Falloden Stiff clay Way (A406) (DW)

Stiff clay

Stiff clay

Stiff clay

Bell Common (SPW)

Rayleigh Weir (CPW)

Rayleigh Weir (CPW)

2

2

4

9

9

Diaphragm wall (5 m × 1 m)

East of Falloden Stiff clay Way (A406) (DW)

2

Contiguous pile wall (1.5 m piles at 1.7 m centres)

Contiguous pile wall (1.5 m piles at 1.7 m centres)

Diaphragm wall (5 m × 1 m)

Contiguous bored pile wall (Nant y Bwch west) NW P33 1080 mm diameter at 1450 mm centres

Wall type

East of Falloden Stiff clay Way (A406) (DW)

Weak rock

General ground condition

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

6.3

6.3

6.3

5.6

excavation to 5 m temporary props installed excavation to formation level 8 m permanent prop installed temporary prop removed. 8.7

8.7

wall installation excavation and construction of cill beam excavation to 5 m (3b) 10 construction of roof excavation to 8 m excavation to formation level after backfilling.

excavation to 3.5 m depth roof installation and excavation to formation.

install wall and install ground anchors tension anchors excavate to formation level.

Construction stage summary

Final excavation depth (h) (m)

5

5

5

3.5

3.5

3.5

2

Height of cantilever (hc) (m)

0.57

0.57

0.50

0.56

0.56

0.56

0.36

hc/h

Case studies with available displacement data for cantilever stages and final excavation stages

A465 Section DL_1 3 Brynmawr to Tredegar (CPW)

Ref

Table A6.13

I1 (11.5 m away from the wall)

N155

None

I3

I2

I1 (same section as I2)

Inclinometer ID

0.0

Just behind wall

0.0

0.0

1.9

Inclinometer location (distance from wall in m)

12.8

12.0

42.5

3.9

3.1

1.5

1.0

Horizontal movement at cantilever stage (mm)

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15.5

59.9

4.5

3.2

2.5

15.0

Horizontal movement at full excavation with prop installed (mm)

0.8

0.7

0.9

0.9

0.6

0.1

Cantilever movement/ total movement

Cantilever movement at least 85 per cent of total.

Movement also recorded when temporary prop is removed and load transferred to permanent prop at base of excavation

Cantilever movements here are assumed to be just before roof was constructed, ie assuming construction of the roof does not cause significant movement further into the excavation.

Some excavation to install and stress anchors, but anchors stressed before bulk excavation started.

Comments

General ground condition

Stiff clay

Stiff clay

Stiff clay

Project title

A406/A10 Junction (DW)

Walthamstow (CF)

Aldershot Road underpass (CF)

Ref

7

3

347

Counterfort diaphragm wall

Diaphragm wall, counterfort (4 × 0.8 m front, 3.2 m × 1.5 m counterfort)

Diaphragm wall, counterfort (4 m × 0.8 m front, 2.7 × 0.8 m counterfort)

Wall type

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

zz

12.5

excavation to 3 mbgl installation of capping beam installation of upper prop 10 installation of lower prop bulk excavation removal of props.

excavation to 4 m temporary prop installed excavation to formation level permanent prop installed temporary prop removed.

Construction stage summary

Final excavation depth (h) (m)

3

4

Height of cantilever (hc) (m)

0.30

0.32

hc/h

Geosensor (accurate of absolute wall movements assuming East and West wall movements identical at the top)

I5

Inclinometer ID

0.0

Inclinometer location (distance from wall in m)

4.7

6.4

Horizontal movement at cantilever stage (mm)

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6.7

11.5

Horizontal movement at full excavation with prop installed (mm)

0.7

0.6

Cantilever movement/ total movement

Exact details of cantilever movement unknown (somewhere between 6.4 mm and 8.7 mm) – this movement is estimated from a few months after the excavation to 3 m, but before the props were installed.

No data on cantilever movement

Comments

A7 Design example A7.1 WORKED EXAMPLE

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A7.1.1 Introduction This appendix presents a complete worked example for a typical three level basement construction in an urban environment. For the purpose of consistency and to allow direct comparison, the same worked example presented in Appendix A11 in Gaba et al (2003) has been used, although the parameter selection and design approach will follow EC7 rather than the Gaba et al (2003) design method. To demonstrate the advantages in economic design that can be realised by undertaking a thorough site investigation and implementing a project under good site management and control, the same example is analysed twice – once using information from a minimal site investigation, and a second time with a welldesigned appropriate site investigation and good on-site management during construction. It should be noted that this example is not intended to give recommendations for site investigation scope or the interpretation of parameters, as these are project specific. The intention is merely to demonstrate, using real site investigation data, that significant economic advantage can be achieved in design and construction by appointing skilled designers and investing in appropriate site investigation and good onsite management during construction.

A7.1.2 Proposed basement The geometry of the basement and the stratigraphy is the same as that presented in Appendix A11 of Gaba et al (2003). The total depth of the basement excavation is 16 m and the construction sequence is fully top-down with support provided to the wall by three levels of permanent slab (denoted P1, P2 and P3). For each excavation stage, formation level will be 0.5 m below the soffit of the permanent slab. In the permanent case, a 1.0 m thick basement slab will also support the wall. The basement is 80 m long and 40 m wide. The stratigraphy at the site comprises 4.0 m of made ground, overlying 7.0 m of gravel which in turn overlies a clay stratum with a thickness of 50 m. Chalk underlies the clay. The following is a summary of the main requirements and assumptions: zz

The retaining wall comprises a 900 mm hard/hard secant pile wall (the external casing diameter is 880 mm although the resultant pile diameter is 900 mm due to the presence of the outside driving teeth, the auger diameter is 750 mm).

zz

The retaining wall is a load bearing element in the short and long term with a permanent line load of 150 kN/m and a variable line load of 50 kN/m at all construction stages.

zz

Permanent drainage will be provided beneath the base slab to prevent long term build-up of water pressure.

zz

An access road is to be located behind the wall offset at more than one metre. The road will be subject to normal traffic loading, which will be modelled as a uniformly distributed load of 10 kPa across the width of the model.

To resist long-term swelling pressures from the underlying clay, a grid of tension piles are installed before the basement is excavated. These piles also have plunge columns installed to support the permanent slabs as the excavation proceeds.

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The permanent structure is shown in Figure A7.1.

Figure A7.1

Geometry of proposed basement

An outline of the proposed construction sequence is shown in Figure A7.2.

Figure A7.2

Proposed construction sequence

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A7.1.3 Structural parameters Retaining wall The basement wall is required to provide a barrier to groundwater flow in the permanent condition. To achieve a water-retaining structure, interlock between the primary and secondary piles is required to a depth of at least 1.0 m below formation level, in this example 17.5 m below ground level including an allowance of 0.5 m for unplanned excavation. The interlock is achieved by specifying installation tolerances on position and verticality and by choosing the spacing between the piles in accordance with the principles described in Chapter 3.

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For the example presented here, the best achievable tolerances discussed in Chapter 3 have been adopted, ie 25 mm on position and 1 in 200 on verticality. To determine the spacing between secondary piles, the nomogram shown in Figure A7.3 has been prepared using the principles described in Appendix A3. The proposed spacings between secondary piles as shown in Figure A7.3 is 1350 mm, which results in a spacing between primary and secondary piles of 675 mm (1350/2). The resultant cut into the primary pile (225 mm) leaves half of the primary pile intact and so is acceptable. The flexural stiffness of a line of secondary circular piles is calculated from the pile diameters (Dsec) and pile spacing (s) as follows: Isec = πDsec4/64.s (A7.1)

Figure A7.3

Nomogram for interlock of segmental cased piled walls (verticality 1:200, position tolerance ±25 mm)

Figure A7.4

900 mm diameter secondary pile through casing

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The stiffness of the cased section of the secondary piles (Isec) where D = 900 mm and s = 1350 mm is: Isec = 0.024 m4/m

(A7.2)

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The cutting of the primary piles of diameter Dprim by the secondary piles, results in a complex shape that is not easily calculated. However, its contribution to the wall stiffness (Iprim) can be estimated by treating the primary piles as an equivalent rectangle of width w and depth d given by the following equations:



(A7.3)

Iprim = 0.013 m4/m

(A7.4)

So, the total stiffness per metre run of the cased section of the retaining wall is given by: Iwall = Isec + Iprim = 0.037 m4/m

(A7.5)

Below the cased section of the wall (at a depth of 17.5 m below ground level), the wall stiffness is derived from the secondary piles only with a reduced diameter assumed to be equal to the auger diameter (750 mm) at 1350 mm spacings (see Figure A7.6).

Figure A7.5

900 mm primary pile

Figure A7.6 casing

750 mm diameter secondary pile below

Wall Iwall = πD4/64.s Iwall = 0.0115 m4/m Young’s modulus of concrete, E0 = 30 MPa (C32/40 concrete, see Table 3.1 of EC2-1). Wall EI during construction = 0.7 E0I Wall EI in the long term = 0.5 E0I A summary of the derived wall stiffness parameters is given in Table A7.1. Table A7.1

Summary of flexural stiffness values for retaining wall

Wall section

I (m4/m)

Short-term EI (kNm2/m)

Long-term EI (kNm2/m)

Cased section (ground level to 17.5 m)

0.037

7.77 × 105

5.55 × 105

Uncased section (17.5 m to toe level)

0.0115

2.42 × 105

1.73 × 105

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As the beam elements used in the finite element mesh do not have any width, the weight of the element should be equal to the difference between the weight of the soil that would be removed during construction and the concrete used to construct the wall. Assuming a unit weight of 20 kN/m3 for soil and 24 kN/m3 for concrete gives an effective weight of 4 kN/m3 for the beam elements. A typical Poisson’s ratio of 0.15 for concrete is adopted.

Props The props (slabs) are assumed to act axially across the width of the basement and have a stiffness (k) given by: Prop stiffness, k = EA/leff s where

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leff is the effective length of the prop, which is defined as the point of zero axial movement. For the symmetric basement presented here the effective length of the prop is half the basement width (20 m) s is the prop (slab) spacing. The slabs are continuous across the basement and so the prop spacing is unity Young’s modulus of the concrete E0 is taken to be 30 MPa (C32/40). During construction, E is taken to be 0.7E0 and in the long term E is taken to be 0.5E0 . The values of k used in the retaining wall analysis are summarised in Table A7.2. Table A7.2

Summary of prop stiffness for design example Slab thickness

Stiffness k during construction

Stiffness k in long term

500 mm (P1, P2 and P3)

525 000 kN/m/m

375 000 kN/m/m

The base slab is modelled with solid elements, which represent the actual depth of the structure and so requires elastic material properties that are typical for concrete: Young’s modulus of concrete, E0 = 30 MPa (C32/40) During construction, E = 0.7E0 = 21 MPa In the long term, E = 0.5E0 = 15 MPa Poisson’s ratio, v = 0.15 Unit weight, γ = 24 kN/m3

Tension piles The tension piles are assumed to be installed on a regular 10 m grid and each has a diameter of 1.5 m and a length of 24 m below the soffit of the base slab. The piles are modelled as beam elements in the numerical model and so their structural properties are smeared in the out-of-plane dimension to mimic the spacing of the piles. The properties are summarised in Table A7.3. Table A7.3

Tension pile structural properties for analysis

Diameter

Spacing

Toe level

1.5 m

10.0 m

-40 m

Axial stiffness

Bending stiffness

EA

EI

4.95 × 106 kN/m

7.0 × 105 kNm2/m

A7.1.4 Analysis assumptions General As discussed in Section 4.2.3, using the finite element method for checking the ULS of a geotechnical structure is not straightforward as there is no definitive, agreed technique for applying partial factors to soil properties.

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Simpson (2012) discussed the following two options: zz

Strategy 1 – applying partial factors to soil parameters at the start of the analysis to model the whole construction sequence with design soil parameters.

zz

Strategy 2 – starting the analysis with characteristic parameters and reducing the strength to the design values at critical stages to check no ULS occurs.

For the reasons discussed in Section 4.2.3, Strategy 2 has been adopted together with the commercially available geotechnical finite element software PLAXIS (see Websites). Websites

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Geometry

PLAXIS: www.plaxis.nl

For the purposes of undertaking the design of the embedded retaining wall, it is reasonable to assume that the basement retaining wall at the centre of the excavation will behave approximately in a plane strain manner with no significant movement (or strain) in the out-of-plane direction. So, this situation is well suited to 2D plane strain analysis using a finite element program. The chalk stratum is significantly stronger and stiffer than the overlying strata and can be reasonably assumed to represent a rigid boundary (where movement is prevented in the vertical and horizontal direction) for the finite element mesh. The lateral boundaries of the mesh should be placed at a sufficient distance from the retaining wall so as to not affect its behaviour, but not so far away as to substantially increase the computational time and memory requirements. The appropriate position for the lateral boundaries is a function of the excavation depth and width. For the example presented here the lateral boundary is set at a distance equal to 2.5 times the excavation width from the retaining wall. On the horizontal boundaries, horizontal movements are restrained, but the mesh is free to move vertically.

Figure A7.7

Finite element mesh

To increase the accuracy of the analysis, a zone of mesh refinement close to the retaining wall has been included. The result is a mesh with 2328 elements with an average size of 1.773 m. The elements in the area of interest are less than 0.5 m.

Material behaviour The clay is homogeneous with a coefficient of permeability, k < 10 -8 m/s. Based on experience and the guidance provided in Chapter 5, it is assumed to behave in an undrained manner during construction (excavation) and in a drained manner in the long term. As previously noted, the wall will be impermeable and will provide a groundwater cut-off in the clay. There is no source of groundwater recharge at or below excavation level, ie there are no water-bearing permeable horizons that could

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provide drainage paths within the clay. This means there will only be excavation disturbance to a depth of 0.5 m below formation level (see Chapter 5). No further softening of the clay is assumed during excavation on the restraining side. The behaviour of all strata will be represented by a linear elastic perfectly plastic soil model. The short-term behaviour of the clay layer will be represented by a Tresca model. The short- and long-term behaviour of the made ground and gravel layers, and the long-term behaviour of the clay stratum are represented by a Mohr-Coulomb model. All structural elements are modelled as linear elastic.

Interface friction

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The reduced interface friction between the wall and the ground has been modelled by introducing zero thickness interface elements into the mesh along these boundaries. To avoid stress discontinuities and numerical problems, the interface elements are extended one metre beyond the toe of the wall. The frictional strength of the interface elements (δ) are as follows: zz

made ground and gravel layers and clay stratum (long term only): δ = ϕ

zz

clay stratum (short term only): cw = 0.5cu .

It should be noted that while the wall is load bearing in the short and long term, there is no need to modify the value or direction of the interface friction as this will automatically be dealt with by the finite element analysis (see Chapters 4 and 7).

A7.2 SCENARIO 1: MINIMAL SITE INVESTIGATION A7.2.1 Description of site investigation In an attempt to save initial expenditure on the proposed basement, the client for the proposed development tendered the site investigation to several contractors without a minimum scope. As the contractors were in a competitive situation, the winning contractor had chosen to undertake a single cable percussion borehole to the base of the clay stratum. Classification testing was undertaken on disturbed samples from all layers. No installations were included in the borehole for determining the groundwater level.

A7.2.2 Determination of characteristic parameters Made ground A few SPTs were undertaken in the made ground layer, although due to the highly variable nature of this material, a conservative view has to be taken on its engineering properties. The borehole logs described the material as predominately granular in nature (comprising reworked sands and gravels as well as varying quantities of crushed concrete and bricks), so the following characteristic parameters were ascribed based on experience and typical parameters suggested in Chapter 5:

Stratum Made ground

Bulk unit weight

Young’s modulus

Angle of shearing resistance

Effective cohesion

Coefficient of permeability

Poisson’s ratio

(gb)

(E′)

(ϕcv ′/ϕpk ′)

(c′)

(k)

(v′)

18

5 MPa

25°/25°

0 kPa

1 × 10 -6 m/s

0.3

The in situ stress was calculated based on the Jaky formula: K0 = 1 – sin ϕ = 0.58

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Gravel deposit

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In order to estimate the properties of the gravel layer, the SPT N60 values recorded in the single borehole have been used, using the correlations presented in Chapter 5. The recorded SPT N60 value for the gravel layer are shown in Figure A7.8. The particles are described in the borehole log as being sub-angular to sub-rounded and the particle size distribution curves result in a uniformity coefficient of about 3.0. Due to the limited data, a characteristic N60 value of 10 is taken for the gravel layer, resulting in a density index of about 40 per cent according to Table 5.4. So from Section 5.5.4 and Table 5.14: ϕ′cv,k = 30 + ϕ′ang,k + ϕ′PSD,k ϕ′pk,k = 0 + ϕ′ang,k + ϕ′PSD,k + ϕ′dil,k where ϕ′cv,k is the characteristic constant volume angle of shearing resistance ϕ′pk,k is the characteristic peak angle of shearing resistance

Figure A7.8

SPT N60 values in gravel layer

ϕ′ang,k is the contribution from the angularity of the particles = 2° ϕ′PSD,k is the contribution from the soil’s particle size distribution = 2° ϕ′dil,k is the contribution from soil dilation = 2° Based on this approximate method, the characteristic constant volume and peak angles of shearing resistance are 34° and 36° respectively. To estimate the drained Young’s modulus of the gravel layer, the relationship proposed by Stroud (1989) as reported in Section 5.5.5 is used: E′ = 2.0 N60 (MPa) This gives a characteristic E′ value of 20 MPa for the gravel layer. Other design parameters for the gravel layer, such as bulk unit weight, permeability and Poisson’s ratio were ascribed based on experience and typical parameters suggested in Chapter 5:

Stratum

Bulk unit weight

Young’s modulus

Angle of shearing resistance

Effective cohesion

Coefficient of permeability

Poisson’s ratio

(gb)

(E′)

(ϕcv ′/ϕpk ′)

(c′)

(k)

(v′)

19

20 MPa

34°/36°

0 kPa

1 × 10 -4 m/s

0.25

Gravel

The in situ stress was calculated based on the Jaky formula: K0 = 1 – sin ϕ = 0.41

Clay stratum To characterise the clay stratum and estimate its strength and stiffness, the single borehole has provided index testing and SPT N values. The plasticity index for the material was determined to be 45 to 50 per cent. The plot of SPT N against elevation is shown in Figure A7.9.

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The characteristic design line has been fitted through the data points with the following equation: N = 10 + 1.45z where z is the depth below the top of the clay stratum.

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Due to the limited data, the characteristic design line is necessarily close to the lower bound. Using the relationship proposed by Stroud (1989) as reported in Section 5.5.4, the undrained shear strength can be estimated using the SPT N value by the following relationship: cu = f1 N60 The value of f1 is dependent on the plasticity index. With no triaxial testing to support the correlation, a conservative value of f1 = 4.5 is chosen for a plasticity index of around 50 per cent (see Figure 5.9). This gives the following characteristic undrained shear strength profile: cu = 45 + 6.53 z

Figure A7.9

SPT N values in clay stratum

where z is the depth below the top of the clay stratum. Based on experience relating to the back analysis of case studies in clay, the following relationship is proposed between undrained shear strength and the Young’s modulus: Eu = 1000cu = 45 + 6.53 MPa where z is the depth below the top of the clay stratum E′ = 750cu = 33.75 + 4.9 MPa where z is the depth below the top of the clay layer Other design parameters for the clay stratum, such as bulk unit weight, permeability and Poisson’s ratio were ascribed based on experience and typical parameters suggested in Chapter 5:

Stratum

Bulk unit weight

Angle of shearing resistance

Effective cohesion

Coefficient of permeability

Poisson’s ratio

(gb)

(ϕcv′)

(c′)

(k)

(v′/vu)

20

25°

5 kPa

1 × 10 -9 m/s

0.25/0.5

Clay

Groundwater level No groundwater monitoring installations were included in the single borehole and so no reliable measurement of groundwater level was available. The driller’s logs indicated a water strike in the gravel at a depth of 7.5 m below ground level, rising to 7.1 m below ground level over a duration of 20 minutes. No further groundwater strikes were noted during the drilling of the borehole. Due to the limited information relating to possible spatial and temporal variations in the groundwater level, an appropriately conservative assumption has to be made with regard to the design value. Due to the degree of uncertainty, and following the guidance provided in Section 7.3, a design level of two metres below ground level has been judiciously chosen.

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A7.2.3 Site control No particular requirements are specified in the project-specific technical documentation or employer’s requirements for special controls during construction and so the designer is required to assume unplanned excavation at each excavation stage amounting to that stated in Section 5.8, ie the lesser of: zz

0.5 m

zz

10 per cent of the total height retained for cantilever walls, or the height retained below the lowest support level for propped or anchored walls.

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A7.3 SCENARIO 2: APPROPRIATE WELL CONSIDERED SITE INVESTIGATION A7.3.1 Description of site investigation Constructive dialogue between the client and the designer at the start of the project led to a reasonable budget being allocated for an appropriate site investigation. The designer involved the client in discussions regarding the scope of the investigation and how this early expenditure should be seen as part of the project risk management as well as providing a better understanding of the ground conditions and engineering behaviour of the ground. The site investigation scope and specification was defined by the designer following the guidelines of EC7 as set out in Chapter 5. The site investigation contract was competitively tendered based on the site investigation specification and contract documents produced by the designer on behalf of the client. The site investigation comprised: zz

four boreholes, including two that proved the depth of the clay stratum

zz

in situ SPT testing in all boreholes

zz

triaxial testing of thin-walled U100 clay samples

zz

large shear box testing of gravel samples to directly assess angle of friction

zz

classification and index testing of all strata

zz

installation of standpipe piezometers in the gravel layer in three of the boreholes, including pressure transducers and a data-logger to get continuous readings for an adequate period of time.

A representative from the designer’s organisation was on site for the duration of the site investigation to ensure the technical objectives were achieved and the site investigation contractor followed the requirements of the specification. By having a representative on site from the designer’s organisation, it was possible to respond to events immediately, which minimised delays and the risk of missing important information as well as getting a ‘hands on’ feel for the ground conditions as they were encountered for a full understanding of how these should be accommodated in design.

A7.3.2 Determination of characteristic parameters Made ground Due to the inherent variability of the made ground layer, and the difficulty in undertaking representative laboratory testing in this stratum, no enhancement to the engineering properties assumed in Section A7.2.2 was possible. The engineering parameters required for the embedded retaining wall analysis is given here:

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Stratum Made ground

Bulk unit weight

Young’s modulus

Angle of shearing resistance

Effective cohesion

Coefficient of permeability

Poisson’s ratio

(gb)

(E′)

(ϕcv ′/ϕpk ′)

(c′)

(k)

(v′)

18

5 MPa

25°/25°

0 kPa

1 × 10 -6 m/s

0.3

The in situ stress was calculated based on the Jaky formula: K0 = 1 – sin ϕ = 0.58

Gravel deposit

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The angle of shearing resistance for the gravel was assessed directly by undertaking a total of six large shear box tests on representative samples of the deposit in accordance with the guidance provided in Section 5.5.4 for coarse grained soils. The measured peak angle of shearing resistance was as follows: Sample

1

2

3

4

5

6

Average

Angle of shearing resistance ϕ′pk

43.7°

38.4°

42.9°

41.0°

40.6°

38.1°

40.8°

To augment the directly assessed angle of shearing resistance presented above, an estimate based on the correlation proposed in Section 5.5.4 and Table 5.14, which relies on the SPT N60 value was undertaken. The recorded SPT N60 value for the gravel layer are shown below in Figure A7.10. It should be noted that N60 values are obtained from all four boreholes. The particles are described in the borehole logs as being sub-angular to sub-rounded and the particle size distribution curves result in a uniformity coefficient of about 3.0. Due to the extra SPT data from the additional three boreholes, a characteristic N60 value of 20 is taken for the gravel layer, resulting in a density index of about 55 per cent according to Table 5.4. From Section 5.5.4 and Table 5.14: ϕ′cv,k = 30 + ϕ′ang,k + ϕ′PSD,k ϕ′pk = 30 + ϕ′ang,k + ϕ′PSD,k + ϕ′dil,k where ϕ′cv,k is the characteristic constant volume angle of shearing resistance ϕ′pk,k is the characteristic peak angle of shearing resistance ϕ′ang,k is the contribution from the angularity of the particles = 2° ϕ′PSD,k is the contribution from the soils particle size distribution = 2° ϕ′dil,k is the contribution from soil dilation = 2° Figure A7.10

SPT N60 values in gravel layer

Based on this approximate method, the characteristic constant volume and peak angles of shearing resistance are 36° and 38° respectively. Using the measured average value of peak angle of shearing resistance (40.8°) and the estimate based on the approach presented in Section 5.5.4, a characteristic design angle of ϕpk of 39° was chosen.

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To estimate the drained Young’s modulus of the gravel layer, the relationship proposed by Stroud (1989) as reported in Section 5.5.5 is used: E′ = 2.0 N60 (MPa) This gives a characteristic E′ value of 40 MPa for the gravel layer. Other design parameters for the gravel layer, such as bulk unit weight, permeability and Poisson’s ratio were ascribed based on experience and typical parameters suggested in Chapter 5: Stratum

Bulk unit weight

Young’s modulus

Angle of shearing resistance

Effective cohesion

Coefficient of permeability

Poisson’s ratio

(gb)

(E′)

(ϕcv ′/ϕpk ′)

(c′)

(k)

(v′)

19

40 MPa

36°/39°

0 kPa

1 × 10 m/s

0.25

Gravel

-4

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The in situ stress was calculated based on the Jaky formula: K0 = 1 – sin ϕ = 0.37

Clay stratum The undrained shear strength of the clay layer has been determined by undertaking triaxial tests on thin-walled U100 samples. The resultant undrained shear strength profile with depth from the four boreholes is shown in Figure A7.11. There is considerable scatter in the undrained shear strength data. To augment the undrained strength data the SPT data can be used with the correlation proposed by Stroud (1989) as reported in Section 5.5.4. The SPT N60 data from the four boreholes is shown in Figure A7.12.

Figure A7.11 Undrained shear strength profile with depth Figure A7.12 from triaxial testing

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The characteristic design line has been fitted through the data points with the following equation: N = 15 + 1.715z where z is the depth below the top of the clay stratum. Due to the additional data, the characteristic design line can be representative of a cautious estimate of the mean, rather than a lower bound as in the previous example shown in Figure A7.9. Using the relationship proposed by Stroud (1989) as reported in Section 5.5.4, the undrained shear strength can be estimated using the SPT N60 value by the following relationship: cu = f1 N60

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The value of f1 is dependent on the plasticity index. A value of f1 = 5.0 is chosen for the plasticity index determined from the results of index testing on clay samples recovered from the site investigation. This gives the following characteristic undrained shear strength profile: cu = 75 + 8.575z where z is the depth below the top of the clay stratum. The proposed design line is compared to the undrained shear strength from triaxial tests and the factored SPT N60 values (using an f1 value of 5.0) in Figure A7.13. It can be seen that the proposed design line represents a cautious estimate of the mean through the SPT N60 data and the undrained shear strength data from triaxial testing. Based on experience relating to the back analysis of case studies in clay, the following relationship is proposed between undrained shear strength and the Young’s modulus: Eu = 1000cu = 75 + 8.575 MPa where z is the depth below top of clay stratum E′ = 750cu = 56.25 + 6.43 MPa where z is the depth below top of clay stratum. Other design parameters for the clay stratum, such as bulk unit weight, permeability and Poisson’s ratio were ascribed based on experience and typical parameters suggested in Chapter 5.

Figure A7.13 Undrained shear strengths from triaxial testing and factored SPT

Stratum Clay

394

Bulk unit weight

Angle of shearing resistance

Effective cohesion

Coefficient of permeability

Poisson’s ratio

(gb)

(ϕcv ′/ϕpk ′)

(c′)

(k)

(v′/vu)

20

25°

5 kPa

1 × 10 m/s

0.25/0.5

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Groundwater level

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Standpipe piezometers were installed in three of the four boreholes. The response zones were located close to the bottom of the gravel deposit where the groundwater level was expected, based on the designer’s hydrogeological understanding of the site. An automatic pressure transducer was installed in each of the boreholes and connected to a data-logger, which took readings every 15 minutes. The pressure transducers were left in the boreholes for 14 days. The recorded groundwater level data are shown in Figure A7.14.

Figure A7.14

Water level recorded in gravel layer over 14 day period

Over the 14 day period the recorded water level varied between 7.6 m and 8.05 m below ground level with small fluctuations at each location. There is no clear diurnal cycle and no source of water identified in the area of the wall that could artificially raise the groundwater level in the short term. So for the construction stages of the analysis a design water level of seven metres below ground level has been adopted. This is based on the highest measurements shown in Figure A7.14 plus an allowance of 0.5 m (in accordance with guidance provided in Section 7.3). In the long-term condition, once the permanent structure has been completed, the design with a water level of two metres below ground surface is checked to account for uncertainty regarding the future use of the area surrounding the basement during its design life.

A7.3.3 Site control The earthworks and piling specification produced by the wall designer on behalf of the client stipulate that extra caution shall be exercised and appropriately tight on-site management and demonstrable control measures should be put in place to ensure that unplanned excavation does not occur. So a practical minimum tolerance for over-excavation of 100 mm at each excavation stage has been considered and allowed for in the analysis.

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A7.4

DETERMINATION OF DESIGN PARAMETERS

A7.4.1 Soil parameters The characteristic parameters for each of the strata have been derived in Sections A7.2.2 and A7.3.2 for the minimal and appropriate well-considered site investigation conditions respectively. To undertake the design of the embedded retaining wall in accordance with EC7 requirements and in-line with the guidance provided in Chapter 7, the characteristic values (X k) are converted to design parameters (Xd) by applying the appropriate partial factors (γM) for DA1C1 and DA1C2 calculations, as stated in Chapter 5, such that:

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(A7.7)

The strength of the coarse-grained layers (made ground and gravel deposit) and the clay stratum in the long term have been assumed to be given in terms of effective strength parameters as follows: Effective stress: tanϕ′d = tanϕ′k /γϕ′ c′d = ck′/γc′ The short-term behaviour of the fine-grained clay stratum has been assumed to be given in terms of total stress strength parameters as follows. Total stress: cu,d = cu,k /γcu According to the UK NA to EC7-1, the partial factors given in Table 7.1 are adopted. For convenience, these are restated in Table A7.4. Table A7.4

Material partial factors

Partial factor

DA1C1

DA1C2

γϕ′

1.0

1.25

γc′

1.0

1.25

γcu

1.0

1.4

For Scenario 1 (minimal site investigation) and Scenario 2 (appropriate well-considered site investigation), the design parameters for the DA1C1 and DA1C2 calculations are given in Table A7.5.

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Table A7.5

Summary of design parameters adopted in calculations Scenario 1

Parameter

DA1C1

Made ground

gb

Gravel

DA1C2

18 kN/m3

18 kN/m3

25°

20.5°

25°

20.5°

c′

0 kPa

0 kPa

0 kPa

0 kPa

E′

5 MPa

5 MPa

5 MPa

5 MPa

v′

0.3

0.3

k

1 × 10 -6 m/s

1 × 10 -4 m/s

0.58

0.65

gb

0.58

0.65

19 kN/m3

19 kN/m3

ϕ′

36°

30.2°

39°

32.9°

c′

0 kPa

0 kPa

0 kPa

0 kPa

E′

20 MPa

20 MPa

40 MPa

40 MPa

v′

0.25

0.25

k

1 × 10 -4 m/s

1 × 10 -4 m/s

K0

0.41

0.50

gb

Clay

DA1C1

ϕ′

K0

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Scenario 2 DA1C2

20 kN/m

0.37

0.46 20 kN/m

3

3

cu

45 + 6.53z kPa

32 + 4.66z kPa

75 + 8.575z kPa

53.6 + 6.13z kPa

Eu

45 + 6.53z MPa

45 + 6.53z MPa

75 + 8.575z MPa

75 + 8.575z MPa

vu

0.5

0.5

0.5

0.5

cw

22.5 + 3.27z kPa

16 + 2.34z kPa

37.5 + 4.29z kPa

26.8 + 3.06z kPa

ϕ′

25°

20.5°

25°

20.5°

c′

5 kPa

4 kPa

5 kPa

4 kPa

E′

33.75 + 4.9z MPa

33.75 + 4.9z MPa

56.25 + 6.43z MPa

56.25 + 6.43z MPa

v′

0.25

0.25

k

1 × 10 -9 m/s

1 × 10 -9 m/s

K0

1.0

1.0

1.0

1.0

The SLS calculation was undertaken using the DA1C1 soil design parameters as set out in Table A7.5. No partial factors applied to unfavourable variable actions (loadings) and no allowance for unplanned excavation and for softening was given, in-line with the guidance in Section 7.3.

A7.4.2 DESIGN WATER PRESSURES As described in Sections A7.2.2 and A7.3.2, the design groundwater levels for Scenarios 1 and 2 are: zz

Scenario 1 – two metres below ground level due to the unreliable and insufficient available data, both during construction and in the long term.

zz

Scenario 2 – seven metres below ground level during construction and two metres below ground in the long term.

For both cases, water pressures are assumed to be hydrostatic from the groundwater level.

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A7.4.3 Design actions (surcharges and loads) In accordance with the guidance in Section 7.3, the partial factors shown in Table A7.6 should be applied to unfavourable variable actions (loading) acting on or behind the wall: Table A7.6

Partial factors applied to unfavourable actions (surcharges)

Combination

Variable

Permanent

DA1C1

1.11*

1.0*

DA1C2

1.3

1.0

Note

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* These values are modified to take account of the 1.35 partial factor that is applied to the effect of actions for DA1C1 as discussed in Section 7.3.

The basement retaining wall is subject to a variable surcharge of 10 kPa at ground level on its retained side. The partial factors summarised in Table A7.6 result in design actions (surcharge values) of: zz

11 kPa for the DA1C1 calculation

zz

13 kPa for the DA1C2 calculation

zz

10 kPa for the SLS calculation.

The wall is also subject to 150 kN/m of permanent load and 50 kN/m of variable load applied vertically at the top of the wall throughout the construction sequence. Using the partial factors given in Table A7.6, the following design actions (vertical loads) are applied to the wall at its top: zz

205 kN/m for the DA1C1 calculation

zz

215 kN/m for the DA1C2 calculation

zz

200 kN/m for the SLS calculation.

A7.5

SET UP OF FINITE ELEMENT MODEL

A7.5.1 DA1C2 As discussed in Chapter 7, the DA1C2 analysis is undertaken first in order to determine the required embedment depth for the wall. By a separate calculation, a minimum wall embedment depth of five metres has been determined for the vertical load bearing requirements of the wall. So, the analysis will be set up with five metre embedment and a check on lateral stability undertaken. The stages of the PLAXIS DA1C2 analysis are summarised in Table A7.7.

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Table A7.7 Stage

Stages for DA1C2 PLAXIS analysis Description

Behaviour of clay layer

1

Initialise stress with characteristic soil parameters

Drained

2

Install wall and tension piles

Undrained

3

Apply 13 kPa surcharge behind wall and 215 kN/m Undrained line load to top of wall

4

Excavate to 2.5 m depth plus allowance for over-excavation

4a

Apply partial factors to soil parameters

Comment

Unplanned excavation: Undrained

zz

0.25 m for Scenario 1 (2.75 m)

zz

0.1 m for Scenario 2 (2.6 m).

Undrained

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P1 slab modelled as fixed end anchor. 5

5a

Install P1 slab at depth of 1.75 m and excavate to 7.0 m plus allowance for over-excavation

Apply partial factors to soil parameters

Undrained

Unplanned excavation: zz

0.5 m for Scenario 1 (7.5 m)

zz

0.1 m for Scenario 2 (7.1 m).

Undrained P2 slab modelled as fixed end anchor.

6

Install P2 slab at depth of 6.25 m and excavate to 11.5 m plus allowance for over-excavation

Unplanned excavation: Undrained

zz

0.5 m for Scenario 1 (12.0 m)

zz

0.1 m for Scenario 2 (11.6 m).

Apply softening in clay to 0.5 m depth. 6a

Apply partial factors to soil parameters

Undrained P3 slab modelled as fixed end anchor.

7

Install P3 slab at depth of 10.75 m and excavate to 16.0 m plus allowance for over-excavation

Unplanned excavation: Undrained

zz

0.5 m for Scenario 1 (16.5 m)

zz

0.1 m for Scenario 2 (16.1 m).

Apply softening in clay to 0.5 m depth. 7a

Apply partial factors to soil parameters

Undrained

8

Replace over-excavated material and construct base slab

Undrained

9

Switch clay parameters to drained

Drained

10

Consolidate for 100 years to dissipate excess pore pressures in clay layer and reach steady state seepage under the wall

Drained

10a

Apply partial factors to soil parameters

Drained

11

Switch clay parameters to undrained

Undrained

12

Check for water pressure 2 m below ground level

Undrained

12a

Apply partial factors to soil parameters

Undrained

Unplanned excavated material replaced with clay. Base slab modelled with solid elements with properties of concrete (linear elastic).

Fix pore water pressure on boundary of mesh to represent source. Set zero flow boundary on base of mesh and along the line of symmetry.

Raise groundwater level to check for future rise in groundwater level.

Key Highlighted stages are ULS checks at key stages

A7.5.2 DA1C1 As discussed in Chapter 7, the DA1C1 analysis is undertaken after the required wall embedment depth is confirmed by the DA1C2 analysis described in Section A7.5.1.

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The steps in the DA1C1 analysis are described in Table A7.8. Table A7.8 Stage

Stages for DA1C1 PLAXIS analysis Description

Behaviour of clay layer

Initialise stress with characteristic soil parameters

Drained

2

Install wall and tension piles

Undrained

3

Apply 11 kPa surcharge behind wall and 205 kN/m Undrained line load to top of wall

4

Excavate to 2.5m depth plus allowance for over-excavation

1

Comment

Unplanned excavation: Undrained

zz

0.25 m for Scenario 1 (2.75 m)

zz

0.1 m for Scenario 2 (2.6 m).

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P1 slab modelled as fixed end anchor. 5

Install P1 slab at depth of 1.75 m and excavate to 7.0 m plus allowance for over-excavation

Undrained

Unplanned excavation: zz

0.5 m for Scenario 1 (7.5 m)

zz

0.1 m for Scenario 2 (7.1 m).

P2 slab modelled as fixed end anchor. 6

Install P2 slab at depth of 6.25 m and excavate to 11.5 m plus allowance for over-excavation

Unplanned excavation: Undrained

zz

0.5 m for Scenario 1 (12.0 m)

zz

0.1 m for Scenario 2 (11.6 m).

Apply softening in clay to 0.5 m depth. P3 slab modelled as fixed end anchor. 7

Install P3 slab at depth of 10.75 m and excavate to 16.0 m plus allowance for overexcavation

Unplanned excavation: Undrained

zz

0.5 m for Scenario 1 (16.5 m)

zz

0.1 m for Scenario 2 (16.1 m).

Apply softening in clay to 0.5 m depth. Unplanned excavated material replaced with clay.

8

Replace over-excavated material and construct base slab

Undrained

9

Switch clay parameters to drained

Drained

10

Consolidate for 100 years to dissipate excess pore pressures in clay layer and reach steady state seepage under the wall

Drained

11

Switch clay parameters to undrained

Undrained

12

Check for water pressure 2 m below ground level Undrained

Base slab modelled with solid elements with properties of concrete (linear elastic).

Fix pore water pressure on boundary of mesh to represent source. Set zero flow boundary on base of mesh and along the line of symmetry. Raise groundwater level to check for future rise in groundwater level.

The resulting effects of actions (wall bending moments, shear forces and prop forces) from the DA1C1 analysis are then multiplied by 1.35 to obtain the design effects of actions, as discussed in Chapter 7.

A7.5.3 SLS analysis For the basement considered in this design example, the close proximity of sensitive buildings requires the direct assessment of the serviceability state for the retaining wall as described in Section 7.3.2. The steps in the serviceability analysis are described in Table A7.9.

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Table A7.9

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Stage

Stages for serviceability PLAXIS analysis Description

Behaviour of clay layer

1

Initialise stress with characteristic soil parameters

Drained

2

Install wall and tension piles

Undrained

3

Apply 10 kPa surcharge behind wall and 200 kN/m Undrained line load to top of wall

4

Excavate to 2.5 m depth

Undrained

5

Install P1 slab at depth of 1.75 m and excavate to 7.0 m

Undrained

6

Install P2 slab at depth of 6.25 m and excavate to 11.5 m

Undrained

7

Install P3 slab at depth of 10.75 m and excavate to 16.0 m

Undrained

8

Construct base slab

Undrained

9

Switch clay parameters to drained

Drained

10

Consolidate for 100 years to dissipate excess pore pressures in clay layer and reach steady state seepage under the wall

Drained

11

Switch clay parameters to undrained

Undrained

Comment

No unplanned excavation. P1 slab modelled as fixed end anchor. No unplanned excavation. P2 slab modelled as fixed end anchor. No unplanned excavation. P3 slab modelled as fixed end anchor. No unplanned excavation or allowance for softening. Base slab modelled with solid elements with properties of concrete (linear elastic). Fix pore water pressure on boundary of mesh to represent source. Set zero flow boundary on base of mesh and along the line of symmetry.

The pertinent results from the SLS analysis are: zz

the bending moments and shear forces in the wall that are used in the assessment of crack widths (if required, see Section 7.5.2)

zz

serviceability prop forces for the design of props (see Section 8.1.6)

zz

structural deflections (principally the wall, props and base slab)

zz

movement of the ground behind the wall.

A7.6 RESULTS A7.6.1 Scenario 1: minimal site investigation The results from the Scenario 1 sets of analyses are summarised in this section. The computed effects of actions (bending moments and shear forces) in the retaining wall for each construction stage for the DA1C1 and DA1C2 analyses are shown in Figures A7.15 and A7.16 respectively. The wall bending moment and shear force envelopes for the DA1C1 and DA1C2 analyses together with the results of the SLS analyses are shown in Figure A7.17. As discussed in Chapter 7, the maximum values shown by these analyses represent the values to be adopted in the structural design of the wall. The maximum computed axial forces in each of the floor slabs are presented in Table A7.10.

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Figure A7.15

Scenario 1 – DA1C1 effects of actions (wall bending moments and shear forces)

Figure A7.16

Scenario 1 – DA1C2 effects of actions (wall bending moments and shear forces)

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Figure A7.17

Scenario 1 – Design effects of actions (wall bending moments and shear forces) from ULS and SLS analyses

Table A7.10

Scenario 1 – Summary of effects of actions (prop/slab forces)

Prop

DA1C1

DA1C2

SLS

P1

295 kN/m

263 kN/m

202 kN/m

P2

737 kN/m

832 kN/m

506 kN/m

P3

1150 kN/m

1616 kN/m

831 kN/m

The computed horizontal deflection of the retaining wall at each construction stage in the SLS analysis, and the associated settlement at ground level behind the retaining wall for the Scenario 1 SLS analysis, are shown in Figure A7.18. For comparison, the upper bound movement line for high stiffness support systems shown in Figure 6.15b is also shown.

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Figure A7.18

Scenario 1 – SLS wall deflections and ground surface settlement behind retaining wall

A7.6.2 Scenario 2: appropriate well considered site investigation The results from the Scenario 2 sets of analyses are summarised in this section. The computed effects of actions (bending moments and shear forces) in the retaining wall for each construction stage for the DA1C1 and DA1C2 analyses are shown in Figures A7.19 to A7.20 respectively. The wall bending moment and shear force envelopes for the DA1C1 and DA1C2 analyses together with the SLS analyses are shown in Figure A7.21. As discussed in Chapter 7, the maximum values shown by these analyses represents the design values to be adopted in the structural design of the wall. The maximum axial forces in each the floor slabs are presented in Table A7.11.

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Figure A7.19

Scenario 2 – DA1C1 effects of actions (wall bending moments and shear forces)

Figure A7.20

Scenario 2 – DA1C2 effects of actions (wall bending moments and shear forces)

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Figure A7.21

Scenario 2 – Design effects of actions (wall bending moments and shear forces) from ULS and SLS analyses

Table A7.11

Scenario 2 – Summary of effects of actions (prop/slab forces)

Prop

DA1C1

DA1C2

SLS

P1

160 kN/m

136 kN/m

115 kN/m

P2

513 kN/m

441 kN/m

376 kN/m

P3

950 kN/m

794 kN/m

672 kN/m

Figure A7.22 shows the computed horizontal deflection of the retaining wall at each construction stage in the SLS analysis, and the associated ground surface settlement behind the retaining wall for the Scenario 2 SLS analysis. For comparison, the upper bound movement line for high stiffness support systems in Figure 6.15b is also shown.

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Figure A7.22

Scenario 2 – SLS wall deflections and ground surface settlement behind retaining wall

A7.6.3 Comparison of Scenario 1 and 2 The input parameters for Scenario 1 and Scenario 2 differ due to the quality and scope of the site investigation undertaken in preparation for the project, and also due to the quality of site control exercised during construction. The maximum design bending moments and shear forces predicted in the retaining wall and the maximum prop/slab forces are compared for Scenario 1 and Scenario 2 in Table A7.12. Table A7.12

Comparison of results from Scenario 1 and 2 analyses

Scenario

Maximum wall bending moment (kNm/m)

Maximum Maximum prop/ wall shear slab force (kN/m) force (kN/m)

1

-1200/1340

1050

2

-770/350

673

Maximum computed wall deflection (mm)

Maximum computed ground surface settlement behind wall (mm)

1616

33

10

950

20

5

There are significant potential benefits of undertaking an appropriate and well-considered site investigation together with good site management and control during construction. This is evident when shown in the comparison of structural effects of actions, computed wall deflections and associated ground surface settlements presented in Table A7.12.

A7.6.4 Discussion of predicted movements The different magnitude of wall and ground movement predicted by the analyses for Scenarios 1 and 2 can be attributed to the significantly improved soil parameters adopted in Scenario 2. However, it is worth noting the comparisons with the normalised case study data discussed in Chapter 6.

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Wall displacements As discussed in Section 6.1.2, a first estimate of the wall movement due to excavation can be made using the normalised plots proposed by Clough et al (1989). This method requires the assessment of a system stiffness, which is related to the flexural stiffness of the wall and the prop spacing: System stiffness = EI/δwh4 where EI is the Young’s modulus of the wall multiplied by its second moment of area (7.77 × 105 kNm2/m for the cased section of the hard/hard secant wall) γw is the unit weight of water (generally taken to be 10 kN/m3)

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h is the average spacing between props (4.5 m for the proposed basement). For the basement considered here, the resultant system stiffness is 189. The simplified FoS against basal failure (Fb) given by Bjerrum and Eide (1956) is given by:

(A7.8)

where cu,1 is the average undrained shear strength above formation level cu,2 is the average undrained shear strength below formation level, within the postulated failure mechanism (assumed to extend to a depth of (√2/2)B below formation, see Figure 6.2) Nc is the bearing capacity factor, is a function of the ratio of depth to breadth (taken to be 6.5 for the current example) γ is the bulk unit weight of soil H is the height of the excavation B is the width of the excavation. Using the undrained shear strength profile derived for Scenarios 1 and 2, the resultant FoS against basal failure are 2.7 and 4.2 respectively. Using these values and the derived system stiffness, Figure 6.4 predicts maximum wall movements of 40 mm (0.25 per cent of excavation depth) and 33.6 mm (0.21 per cent of the excavation depth) for Scenarios 1 and 2 respectively. These values compare to between 24 mm (0.15 per cent of excavation depth) and 32 mm (0.2 per cent of excavation depth) for walls with stiff support systems presented by St John et al (1992) for excavations in London Clay.

Ground movement profile As noted in Section 7.3.2, to make reliable predictions of ground movement with any degree of confidence is extremely complex and typically requires sophisticated laboratory testing data, an analysis that adopts an advanced non-linear soil model and quality reliable case study data. The design example in this appendix has used finite element analysis with a simplified linear elastic soil model with a Mohr-Coulomb or Tresca failure criteria. While this type of model may be suitable for undertaking ULS design of an embedded retaining walls, the absence of a strain-dependant stiffness model means it is unlikely to reliably predict the ground movements. Linear elastic soil models with a single stiffness value can be calibrated using case study data to give a reasonable match to the maximum ground movement (Grammatikopoulou et al, 2008). However, the shape of the settlement trough and the influence zone will not be accurately represented. This can be seen in Figures A7.15 and A7.19 where the ground movement profile from the PLAXIS analyses are compared to the data from Chapter 6. The maximum settlement predicted for Scenario 1 and 2 by the PLAXIS analysis is 10 mm and 5 mm respectively. The normalised case study data suggests that for a 16 metre deep excavation predominately in stiff clay would be around 12.5 mm. The data does not take account of the subtle differences in the Scenario 1 and Scenario 2 examples, so it can be concluded that

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the magnitude of the maximum movement is predicted with a reasonable degree of accuracy. The shape and extent however of the settlement trough is not well represented, with the trough being too shallow and too wide, extending beyond the influence zone of 3.5 times the excavation depth shown by the case study data (56 m influence zone based on a 16 m deep excavation). It should be noted that if the predicted ground movement profile from a finite element analysis, which assumes linear elasticity is to be used to estimate building damage, the incorrect settlement profile may have significant consequences as the slope of the ground and the induced distortion of any nearby building is likely to be underestimated. Conversely, the assumption that any nearby building will distort with the ground is likely to be a conservative assumption. If the ability of the building stiffness to modify the ground settlement profile can be taken into account, significant reductions in the predicted settlement and distortion can be made as discussed in Section 6.4.

Parameters Although the basement considered in the design example in this publication is the same as that used in Gaba et al (2003), direct comparison with the results presented here cannot be made. This is because the soil parameters derived to illustrate the difference between Scenario 1 and Scenario 2 are not the same as those used in Gaba et al (2003). A comparison of the critical soil parameters is given in Table A7.13. Soil parameters that are significantly different between the two examples are highlighted. Table A7.13 Comparison of characteristic parameters used in current study and parameters used in Gaba et al (2003) design example Gaba et al (2003)

EC7 characteristic

Gravel

Made ground

Parameter

Clay

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A7.6.5 Comparison with results from Gaba et al (2003) study

Scenario 1

Scenario 2

Moderately conservative

Worst credible

Serviceability

γb

18 kN/m3

18 kN/m3

18 kN/m3

18 kN/m3

18 kN/m3

ϕ′

25°

25°

25°

23°

25°

c′

0 kPa

0 kPa

0 kPa

0 kPa

0 kPa

E′

5 MPa

5 MPa

5 MPa

5 MPa

10 MPa

v′

0.3

0.3

0.3

0.3

0.3

k

1 × 10 m/s

1 × 10 m/s

1 × 10 m/s

1 × 10 m/s

1 × 10 -6 m/s

K0

0.58

0.58

0.6

0.6

0.6

γb

19 kN/3

19 kN/m3

19 kN/m3

19 kN/m3

19 kN/m3

ϕ′

36°

39°

35°

32°

35°

c′

0 kPa

0 kPa

0 kPa

0 kPa

0 kPa

E′

20 MPa

40 MPa

25 MPa

25 MPa

50 MPa

v′

0.25

0.25

0.25

0.25

0.25

k

1 × 10 -4 m/s

1 × 10 -4 m/s

1 × 10 -4 m/s

1 × 10 -4 m/s

1 × 10 -4 m/s

K0

0.41

0.37

0.4

0.4

0.4

γb

20 kN/m3

20 kN/m3

20 kN/m3

20 kN/m3

20 kN/m3

cu

45 + 6.53z kPa

75 + 8.575z kPa

75 + 5z kPa

N/A

75 + 5z kPa

Eu

45 + 6.53z MPa

75 + 8.575z MPa

37.5 + 2.5z MPa

N/A

75 + 5z MPa

vu

0.5

0.5

0.5

N/A

0.5

cw

22.5 + 3.27z kPa

37.5 + 4.29z kPa

37.5 + 2.5z kPa

N/A

37.5 + 2.5z kPa

ϕ′

25°

25°

25°

21°

25°

-6

-6

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Table A7.13 Comparison of characteristic parameters used in current study and parameters used in Gaba et al (2003) design example (contd) Gaba et al (2003)

EC7 characteristic

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Clay

Parameter

Scenario 1

Scenario 2

Moderately conservative

Worst credible

Serviceability

c′

5 kPa

5 kPa

5 kPa

0 kPa

5 kPa

E′

33.75 + 4.9z MPa

56.25 + 6.43z MPa

30 + 2z MPa

30 + 2z MPa

60 + 4z MPa

v′

0.25

0.25

0.25

0.25

0.25

k

1 × 10 -9 m/s

1 × 10 -9 m/s

1 × 10 -9 m/s

1 × 10 -9 m/s

1 × 10 -9 m/s

K0

1.0

1.0

1.0

1.0

1.0

In the design example by Gaba et al (2003), the groundwater was assumed to be able to rise to a level of two metres below groundwater levels, which is the same conservative assumption taken in Scenario 1. However, overall, the soil parameters assumed in Gaba et al (2003) are closer to the Scenario 2 assumptions.

Structural design The maximum design values for prop forces, bending moments and shear forces for Scenarios 1 and 2 (which follow the principles of EC7) and the Gaba et al (2003) design example are given in Table A7.14. Table A7.14

Comparison of structural design values with those reported in Gaba et al

Parameter

Scenario 1

Scenario 2

Gaba et al (2003)

Maximum prop force in P1

295 kN/m

160 kN/m

222 kN/m

Maximum prop force in P2

832 kN/m

513 kN/m

686 kN/m

Maximum prop force in P3

1616 kN/m

950 kN/m

1860 kN/m

Maximum wall shear force

1050 kN/m

673 kN/m

978 kN/m

Maximum positive wall bending moment

1340 kNm/m

350 kNm/m

543 kNm/m

Maximum negative wall bending moment

-1200 kNm/m

-770 kNm/m

-2048 kNm/m

The maximum values presented in Table A7.14 show that for the prop forces and shear force in the wall, the values predicted by Scenario 1 are similar to those presented for the Gaba et al (2003) case. The predicted values for Scenario 2 are significantly less, mostly due to the lower water level assumed for this case. There is however a significant difference between the maximum predicted bending moments between the current study and the Gaba et al (2003) example. Examination of the original FREW files has shown that the assumption of zero friction on the back of the wall in the made ground and the gravel layer (due to the application of the vertical load on the wall as shown in Box 7.1) was having a significant effect on the predicted bending moment. By using a FE analysis for the current example, this assumption was no longer required with the resultant reduction in maximum bending moment.

Serviceability In the Gaba et al (2003) design example, the wall movement was assessed directly by the FREW analysis and the ground settlement behind the wall was estimated using the semi-empirical method described in Figure 6.17. The maximum wall movement and the settlement behind the wall from the analyses presented in the current publication are compared to the results of the serviceability analyses presented in Gaba et al (2003) and in Figures A7.23 and A7.24. The wall movements shown in Figure A7.23 illustrated that the wall movements predicted by the serviceability analyses carried out as part of the work by Gaba et al (2003) and this publication produce very

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similar deflected shapes with only the maximum value being affected by the stiffness assumptions. It is interesting to note the similarity in the deflected wall shape predicted by the two independent analytical methods (ie FREW and PLAXIS). Figure A7.24 shows the maximum settlement trough from the Gaba et al (2003) analysis, which used a semi-empirical method based on wall movement (Figure 6.15) and the Scenarios 1 and 2 analyses based on the serviceability PLAXIS analyses. For comparison, the empirical data from Figure 6.15b for a 16 m deep excavation is also shown. It is important to note that as discussed previously, the linear elastic model used in the PLAXIS analysis is significantly over estimating the influence zone of the basement wall and so under estimating the possible distortion to any structure (when compared to the semi-empirical and case study data). The comparison of ground settlement troughs presented in Figure A7.24 highlights the importance of using well-documented case studies to validate movement predictions that come from more sophisticated numerical analyses. If a designer were to rely on the settlement trough predicted by the linear-elastic perfectly-plastic PLAXIS analysis, in Figure A7.23 Comparison of wall movements from isolation without reference to case study data, the serviceability analyses from Gaba et al (2003) and distortion experienced by neighbouring structures Scenarios 1 and 2 could be significantly underestimated, while the zone influence could be significantly over estimated.

Figure A7.24 Comparison of settlement behind wall from serviceability analyses from Gaba et al (2003) and Scenarios 1 and 2 and case study data

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A8 Distributed prop load (DPL) method

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A8.1 GENERAL Twine and Roscoe (1999) gives a detailed description of the principles and application of the distributed prop load (DPL) method for the determination of temporary prop loads. They provide design guidance based on extensive field measurements of prop loads for flexible and stiff walls and for the range of ground conditions commonly encountered in the UK. The DPL method should only be applied to excavations of similar depth and plan geometry and in ground conditions that are comparable to the case study data considered in Twine and Roscoe (1999). The designer should establish this before carrying out calculations of prop load using the DPL method. Temporary prop loads calculated using the DPL method provide a conservative estimate of the prop loads to be expected in the field in normal circumstances. Temporary prop loads calculated in this way: zz

require the designer to ensure that wall embedment is adequate for the application of the DPL

zz

do not consider the possibility of groundwater pressures rising to most unfavourable levels over the duration of the temporary works

zz

do not explicitly check for the possibility of progressive failure – the designer should carry out a risk assessment and/or separate calculations to show that progressive failure will not occur (Section 7.3.3).

Temporary prop loads calculated using the DPL method should be compared with SLS prop loads calculated as previously described in accordance with the general principles applicable to temporary works design (Section 5.11). Where the SLS prop loads calculated in this way differ significantly from those derived from the DPL method, the designer should carefully investigate and understand the reasons for the difference and adopt appropriate values in the design of the temporary props. Reliability of, and control over, groundwater pressures is often particularly significant in this regard. The design of the temporary works should be demonstrably robust (Section 5.11). If, as a consequence of this evaluation, DPL-derived prop loads are adopted in preference to those corresponding to the calculated values, the designer should ensure the following: 1

The wall will satisfy its design and performance criteria (Section 2.4) and will remain stable at all times over the duration of the temporary works under the application of the adopted prop loads.

2

The maximum wall bending moments and shear forces calculated in point (1) lie within the envelope of ULS effect of actions adopted in the structural design of the wall.

3

The maximum wall bending moments and shear forces calculated under the application of DPL derived prop loads, assuming un-factored soil design parameters, lie within the envelope of SLS effect of actions adopted in checking compliance with crack width criteria for reinforced concrete walls and allowable stress criteria for steel sheet pile walls, if applicable.

The essential features of the DPL method are described in Section A8.2, adapted from Twine and Roscoe (1999) where the treatment of additional loading from temperature effects has been adapted from that presented to allow for the degree of restraint of the prop.

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A8.2 DPL DESIGN METHOD Details are provided in Twine and Roscoe (1999). The DPL method for calculating prop loads for propped temporary excavations is based on the back analysis of field measurements of prop loads relating to 81 case studies, of which 60 are for flexible walls (steel sheet pile, king post walls) and 21 are for stiff walls (contiguous, secant, diaphragm walls). The case study data relate to excavations ranging in depth from 4 m to 27 m, typically 5 m to 15 m in soft and firm clays (soil class A, Table A8.1), 10 m to 15 m in stiff and very stiff clays (soil class B, Table A8.1) and 10 m to 20 m in coarse-grained soils (soil class C, Table A8.1).

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The method for determining prop loads is shown in Figure A8.1.

Figure A8.1

Method for calculating the DPL

The data are classified based on the type of ground retained by the excavation, see Table A8.1. Table A8.1

Classification of ground types

Soil class

Description

A

Normally and slightly overconsolidated clay soils (soft to firm clays).

B

Heavily overconsolidated clay soils (stiff and very stiff clays).

C

Coarse-grained soils.

D

Mixed soils (walls retaining both fine-grained and coarse-grained soils).

The classes in Table A8.1 are subdivided according to wall stiffness, ie flexible (F) walls and stiff (S) walls. Flexible walls retaining soft clay soil (Class AF) have been further subdivided according to base stability conditions. Class C soils have been subdivided into dry and submerged. Figure A8.2 shows the characteristic prop load diagrams for Class A, B and C soils.

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Figure A8.2

414

Characteristic DPL diagrams for Class A, Class B and Class C soils

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A8.3 ALTERNATIVE METHODS

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Alternative methods for the assessment of prop loads for multi-propped walls based on modified earth pressure distributions are given in DDGT (2014). Katsigiannis et al (2015) carried out parametric studies for multi-propped walls in stiff clays, comparing the maximum prop forces predicted between Twine and Roscoe (1999), DDGT (2014) recommendations and finite element method (FEM) predictions for various earth pressure at-rest coefficients K0 . The results of these comparisons show that the German recommendations give prop loads in good agreement with the numerical analysis results (see Figures A8.3 and A8.4). The difference between the DPL and FEM predictions is particularly evident in the upper prop level where the DPL method uses a uniform distribution of earth pressure with depth resulting in higher prop loads.

Figure A8.3

Geometry of example modelled in stiff clay (deep excavation with five levels of propping)

Figure A8.4

Comparison of maximum prop forces for the geometry shown in Figure A8.3 (after Katsigiannis et al, 2015)

Guidance on embedded retaining wall design

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Index Ab initio observational method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204–211, 216–218, 230 access aspects, inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 access roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 accidental design situations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 24 calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 128, 143 loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44, 192, 234–235 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232, 234–235 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185, 192, 197–202, 206–209, 212–215, 217–219, 229 actions see loadings active lateral earth pressure coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 59, 61–65, 67–77, 79–82, 90, 321–330 Acts/Statutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38, 283–285 adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61–64, 73, 89, 193–197, 200–202 adjacent site-specific constraints berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46–47 calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99, 129, 132–133, 141, 143 cost aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 design requirements/performance criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 22, 32–33 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55, 65–67, 69, 73, 78 friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59–60 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146–147, 153, 156–158, 162–164, 173, 182, 352 interacting walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67–68 loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65–67, 132–133 serviceability limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400–401 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199, 203, 222–223, 300–301, 316–319 aesthetics, wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225, 228, 230 analysis see also finite element analysis/methods; limit equilibrium analysis; stress analysis bored pile and diaphragm walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316–321 earth pressures/lateral stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–90, 316–348 effect of method of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87–90, 331–339 elastic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 method of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69–90, 331–339 quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17–18, 95, 316–321 shear stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 60–63, 67–68, 87–89, 303–306, 308–314 soil behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–90 statistical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135–136 tension cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63–65, 73, 89, 334, 336–337, 342 weak rock behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–90 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47–48, 53, 231–232, 244–253 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59, 65, 73, 77, 79, 81–82, 87–90 forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187–191, 198–202, 208–209, 212, 217–218, 229–230 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151, 153, 164, 172–173, 176–177 maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247, 252, 261 angularity of particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122–123 applied loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 147, 187, 191, 203, 313 approving/checking bodies, wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .105, 108, 127, 139–140, 154 arching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81–84, 147, 162, 234, 317–318 atmospheric zones, cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 autogeneous healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 axial forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400–407, 410–411 back analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203–206, 212–214, 219, 236–237, 350, 413–414 basements anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29, 39–40, 43, 48, 382–411 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 382–411

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propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 382, 385–386, 391, 399–408, 410–411 secant walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294, 382, 408 software packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202, 386–388, 398–411 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 wall deflections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194, 224 beam on springs analysis techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 bearing capacity construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43, 76–78 design parameter selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187, 194–196, 201–202, 228 wall movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 bedded rocks, classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97–98 bending moments design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400–407, 410–411 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56, 66, 70–78, 81–90 steel sheet piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187–191, 198, 200–202, 208–209, 212, 217–218, 229–230 wall movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152, 171, 178 bending strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86–87 bent-out bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48, 50 berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46–47, 231, 239–243, 252–253 BIM see Building Information Modelling bond-type grouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245–246, 248 bored pile walls see also contiguous piles concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 geomechanics/quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316–321 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146–147, 154–163, 169, 345, 351–352, 380 boreholes anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245–246, 261 design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103–104, 107–108, 111–113, 124 ground conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 103–104 rock strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 site investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388–394 boring anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56–57 ground investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104–105, 107, 115–117, 126–127, 142, 146 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 rig tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 bottom-up construction sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41, 43–45, 53, 153, 194–196, 231 British Standards (BS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–5, 283–285 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232, 244–248, 252 codes of practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–4, 37, 60–61, 69, 220, 230, 265 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29, 36–37, 40, 223, 247 EN 1990 to 1999 see Euronorm design standards Geotechnical investigation/testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 93–96, 98–99, 102, 110–111, 123 ground movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144–146 high modulus and combi walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 limit equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 parameter determination/selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93–105, 110–112, 121–123, 132, 135 reinforced structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265–266 rock classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96–99 secant walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 sheet pile walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290–291 soil classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93–96 soil-structure interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86–87 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232, 244–248 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184, 187, 193, 220–226 water quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 brittle failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 25, 138, 206 BS see British Standards buckling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 87, 206, 221, 234–235

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building damage assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 154–155, 176–183, 202, 409 Building Information Modelling (BIM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 building loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132–133 bulk unit weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 110, 388–390, 392–394 cages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29–33, 35, 39, 43, 48, 202–203, 222–224, 299–302 calculations see Design Approach 1 calculations; design calculations calibration aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213–216, 219, 261 cantilever heights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 378–381 cantilever moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164, 176, 378–381 cantilever walls berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239, 243 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72–75, 78–79, 87–88, 90 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152, 164–165, 167, 169, 176, 378–381 groundwater pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 limit equilibrium analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72–75, 78–79, 87–88, 90, 187–188, 331–339 pseudo-finite element methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335–338, 340, 342 soil support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41, 44–45, 47, 52–53 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331–339 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239, 243 Young’s modulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 carbon dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 carbon emissions/footprints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 28–29, 34, 37–39, 285 cased methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30–33, 36, 43 casting slabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 cast in situ systems construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 36–37, 43, 48 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193, 220, 222–224 wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56, 124, 220, 265, 293–296, 300, 343 Category 1 walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 15–16, 92, 103, 126–127 Category 2 walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 16–20, 92, 103–105, 126–127, 170 Category 3 walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 16–17, 92, 104–105, 127 CDM see Construction (Design and Management) Regulations CEN TC288 execution standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–4 centrifuge tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 CFA see continuous flight auger piling characteristic value determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–144, 343–344, 388–398, 409–411 checking and approving bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 circular shafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44–45, 300, 302 classification aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–99, 109–111, 123 clays distributed prop loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413–415 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146–150, 153–172, 176, 345, 349–357, 360–377, 380–381 parameter determination and selection 93–94, 97, 100–102, 108–121, 126–129, 133, 138–144, 389–390, 393–394, 410 Climate Change Act 2008 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38, 283 close proximity see adjacent site-specific constraints coarse grained soils design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93–96, 107–113, 122–124, 126, 141–142 ground movements/wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146, 149, 154–155, 162, 169–170 codes of practice (CoP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–4, 37, 60–61, 69, 220, 230, 265 coefficients lateral earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–57, 59–65, 67–77, 79–82, 87–90, 317, 320–330 of permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108, 111–112, 141, 306, 387–394 cohesion anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388–394 design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 120–121, 146 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61–62, 65, 67–68, 80 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306–314, 334–335 soil shear strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120–121 cold-forming sheet piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 collapse conditions design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 124, 127, 135, 143 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

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ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147, 153 limit equilibrium analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 57, 62, 69–75, 81 limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21–22, 127, 135, 143 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193, 205 combi walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29, 86–87, 222, 290–293 communications construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 11–14, 18, 24 excavations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 risk assessment/management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 compaction pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 comparative risk assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 competent soils, definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 complementary investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104–105 compression-type grouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245–246, 248 compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134, 144, 189–190, 197, 200 computer-based analytical methods see software computer hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 concrete structures see also cast in situ systems aesthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 autogeneous healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 cages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 33, 35, 222–224 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 28, 30–33, 35–43, 46, 48–49 diaphragm walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223–224, 316–321 EC2 design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37, 40, 184, 220, 224–230, 265–266 pile walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221, 223, 293–300 reinforced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–43, 46–49, 85–86, 197–198, 202–204, 220–230, 251–252, 265–266 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193, 197–198, 202, 204, 209, 220–230, 316–321 watertightness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 Young’s modulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85–86, 385–386 connection types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48–53 consistency index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 consolidated deposits design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399–401 design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101–102, 108–124, 126–127, 133, 139, 141–144 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55–57, 60, 69–71, 81, 84, 89–90 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306–321 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 147–154, 171, 345, 349–350 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241–243 construction considerations see also construction sequences; top-down construction sequences Ab initio observational method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47–48, 53, 247–248 basements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29, 39–40, 43, 48, 382–411 berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46–47, 243 bottom-up methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41, 43–45, 53, 153, 194–196, 231 cantilever walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41, 44–45, 47, 52–53 carbon emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 28–29, 34, 37–39 cased methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30–33, 36, 43 concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–43, 46–49 constructability requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36–37 constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 10–11, 26, 37, 45, 53, 145, 203 contaminated ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 continuous flight auger walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30–31, 35–36 costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 34–37, 39, 41, 44–45, 47–48, 52–53 decision making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 26–27, 34, 40–45, 53, 105–106 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 10–53, 145, 203, 382–411 diaphragm walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 29–30, 32, 35–39, 44–45, 48, 50, 300–302 driving techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 32, 36 economic design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–28, 31, 34–48, 52–53 environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–30, 34–35, 37–39

Guidance on embedded retaining wall design

419

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

excavations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21–22, 26, 28–30, 33–36, 40–47, 53 grab techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 36 ground conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–43, 45–53, 153, 345–381 groundwater conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–30, 35–37, 40, 48 headroom restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 27–28, 31, 34–35, 42 height aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 30, 41 king post piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–28, 30 life cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 manager’s roles and responsibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12 mill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 36 noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 35–37 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203–219 obstructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28, 35–36, 47–48, 53 permanent works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–30, 34–37, 40–53 piles/piling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–33, 35–41, 43, 45, 48–49, 220–222 plant loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 propping systems/walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41–53, 231–238 quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 recycling/reusing materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37–38 reinforcement requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–43, 46–49 removal aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37, 42–44, 46–48 rotary boring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30–32, 35–36, 39, 43 secant walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–33, 36, 44–48, 51 selection processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10, 26–27, 34–45, 53 site-specific constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 27–28, 31, 34–37, 42 slabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41–43, 48–53 soil support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40–53, 231–253 steel walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–32, 35–49, 220–222 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–28, 30, 34–37, 40–53, 231–253 tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 28–33, 35–36, 43, 48 vibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 35–37 wall depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34–35, 71–73, 81 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192–196, 199–225, 229–230 wall supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 26–53 water retention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–30, 34–37, 40, 44, 48 watertightness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 22, 28, 34, 40–41 workmanship quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Construction (Design and Management) Regulations (CDM 2015) . . . . . . 12–14, 24, 26, 225, 232, 254, 264, 283, 287–289 contamination aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 37, 103, 105, 220, 300 contiguous piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293–296, 317, 320 see also bored pile walls analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73, 85 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–28, 30, 32, 36, 40 design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317, 320 ground/wall movements . . . . . . . . . . . . . . . . . . . . . . . . 146, 154, 162, 347–348, 356, 358, 363–368, 370, 372, 374, 380 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 contingency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206–207, 210, 213, 215–216 continuity aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12 continuous flight auger (CFA) piling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30–31, 35–36, 146, 203, 222–223, 293–299, 347 contractors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12, 217 control(s) basement design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391, 395 cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176–177, 182–183 quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 43, 248, 250, 266 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 CoP see codes of practice corner aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 46, 149, 214, 244, 298–301, 377–378 corrosion anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247–248, 250 corrosion rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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inspections/monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258, 262 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85–88, 220–222, 225–227, 230, 291 costs benefit assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 construction considerations/wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 34–37, 39, 41, 44–45, 47–48, 52–53 economic design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 11, 24 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264–265 inspection, monitoring and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231–232, 234, 253 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–185, 203, 205, 211, 216, 221–225, 230 Coulomb analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57–59, 62–64, 67, 75 couplers, wall/slab connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48, 50–51 cracks see also tension cracks atmospheric zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 autogeneous healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 dry zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63–65, 73, 89 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177–180 parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128–129, 137–139 reinforced concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225–228 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196–198, 200–203, 217–218, 225–230 watertightness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225, 228, 230 wet zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 width calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227, 230 cross-sectional areas, second moment of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 85–90, 151–152, 233, 408 see also wall flexural stiffness cumulative ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 DA1 see Design Approach 1 calculations damage assessments buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 154–155, 176–183, 202, 409 design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115, 124, 343 environmental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20–21 deadman anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47, 244–245 decision making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 26–27, 34, 40–45, 53, 105–106, 287 deep inward ground movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 deflection ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181–182 deflections ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 149–152, 155, 164–167, 170–177, 181–183, 345–378 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137, 231–234, 239, 241–243 deformations determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99, 101–102, 110, 126–127, 141, 143 ground movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149, 151–153, 163–166, 172–173, 177, 182 degree of restraint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412–414 density indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95–96, 123–125 Design Approach 1 (DA1) calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 88, 235–237 see also design calculations DA1C1 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334–342 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185–189, 191–195, 201–202, 208–209, 212–214, 217–219, 224, 229 worked design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396–406 DA1C2 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334–342 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185–188, 190–195, 201–202, 208–209, 212–214, 217–219, 224, 229 worked design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396–406 design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–144, 184–209, 212–230, 343–344 see also Design Approach 1 calculations; design considerations Ab initio observational method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207–209 characteristic values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–144, 343–344, 388–398, 409–411 clays . . . . . . . . . . . . . . . . . . . . . . . . . 93–94, 97, 100–102, 108–121, 126–129, 133, 139–144, 389–390, 393–394, 410

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cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128–129, 137–139 damage assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115, 124, 343 drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 95, 100–101, 107–114, 117–125, 127–144 EC7 geotechnical design . . . . . . . . . . . . . . . . . . . . . . . . . . 17–19, 25, 91, 96, 103–104, 111, 113, 123, 132–136, 143–144 embedment depths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128, 185–188, 191, 201–209, 212–214, 217–219, 228–229 excavations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 99, 120–121, 128–129, 133–134, 137, 141, 143 geological conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 94, 99–104, 107, 120, 124, 127, 135, 141, 343 Geotechnical Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 103–105, 109, 126–127, 141 ground conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 95, 100–113, 117–125, 127–142 groundwater conditions . . . . . . . . . . 91–92, 99, 102–105, 111, 118, 127–131, 134, 137–141, 390–391, 395, 397–401 hydrogeological conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 107, 127, 141 in situ earth pressure coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 105, 113–117, 141–142 loading conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 99, 107–108, 116, 122–123, 129–137, 141 Mohr-Coulomb models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 124–126, 343–344 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 138, 144, 207–209, 213–215, 219 parameter determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–144, 185–230, 343–344, 386–411 partial factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134–135, 137, 143–144, 185–202, 205–209, 212–230 permanent works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 102–103, 129, 140, 144 permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 100, 107–108, 111–114, 127–131, 139, 141 pore water pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104, 114–123, 127–129, 142 rock parameters determination and selection . . . . . . . . . . . . . 91–92, 95–100, 102, 105, 107–111, 115–127, 134, 141–144, 343–344 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–205, 208–209, 212–221, 224–230 serviceability limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 126–127, 143, 198 soil parameters determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . 91–96, 99–102, 105–124, 126–144, 395–401, 407–411 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–205, 208–209, 212–221, 224–230 stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 104, 107–108, 111–112, 127, 141 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100–102, 108–109, 113–129, 134, 138–144 temporary conditions/works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 102, 108, 111, 117, 128–129, 137–140, 144 unplanned excavations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 99, 133–134, 137, 141 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–144, 184–209, 212–230, 343–344 water availability . . . . . . . . . . . . . . . . . . . . . . . 94, 104–105, 107–108, 110–111, 114–118, 122, 127–131, 135, 138–144 weak rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95–99, 102, 107–111, 115–116, 120–121, 124–126, 141–142 worked design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384–411 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–25 see also accidental design situations; design calculations, Design Approach 1 calculations; designers; EC7 geotechnical design; economic design anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231–232, 244–253 assumption aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14–15, 25 basements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 382–411 berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231, 239–243, 252–253 carbon emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 28–29, 34, 37–39 CDM Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12–14, 24, 26, 225, 232, 254, 264, 283, 287–289 communication continuity aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12 concept aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14–19 construction/constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 10–53, 145, 203, 382–411 costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 11, 24 design lap lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224–225 design life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19–20, 27, 37, 256–257 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21–22, 25 examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382–411 execution aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–4, 14, 184, 224–225, 231, 247–250 experience aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18, 21–22, 25, 185–188, 199–203, 206–209, 212–214, 217–219, 229 Geotechnical Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 15–17, 24 ground conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–11, 17, 20 groundwater conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 20 health and safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–14 hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 20 inspection, monitoring and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255–257 limit equilibrium analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185, 187–188, 201–202, 208, 229 limit states . . . . . . . . . . . . . . . . . . . . 10–11, 17–25, 62, 88, 100–101, 134–137, 141, 144, 184–202, 205–219, 224–230 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–185, 199, 203–219, 228–230 parameter selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–144, 343–344, 388–398, 409–411

422

CIRIA, C760

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

party walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 250 performance criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–11, 19–20 philosophy aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–185 predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 28 project life cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12 project-specific requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 20, 24 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231–238, 241, 243–244, 252–253 recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24–25 requirement establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 19 risk assessment/management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 13–14, 24 safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–15, 20, 24–25, 255–257 site-specific constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 18, 20, 24 stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 subcontractors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 137, 232, 247, 249 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231–253 third parties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 13, 232, 250 wall design/types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–25, 91–144, 184–230, 343–344 whole-life costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 11, 24 worked example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382–411 workmanship quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 25 designers construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 30–32, 34–37, 40, 45, 48, 52–53 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 roles and responsibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–12, 24–25, 232, 249–250, 254–255 desk studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103–106, 141 diameters cast in situ concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 30–33, 39, 43, 45, 48 diaphragm walls cages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29, 223–224, 300–302 concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223–224, 316–321 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 29–30, 32, 35–39, 44–45, 48, 50, 300–302 DIN 4126:2013-09 stability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155, 285 flexural stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85–86 geomechanics/quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316–321 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146–162, 169–170, 345–352, 356–359, 363–375, 378–381 DIN 4126:2013-09 stability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155, 285 Directive 89/391/EEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 Directive 92/57/EEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 283 distributed prop loads (DPL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236, 412–415 drainage design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382, 387–394, 399–401, 408 groundwater pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127–131 parameter determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 95, 100–101, 107–114, 117–125, 127–144 drained ground behavior parameter determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 100–101, 107–113, 119–125, 127–142 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–57, 61–63, 66–67, 69, 71–81, 84, 88–90, 306–307, 314–316 drilled in bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 driven piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60, 146, 162 drive-sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 driving techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 32, 36 dry zones, cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 durability aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 35, 37, 41, 220–221, 225–227, 230, 291, 296 earth berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46–47, 231, 239–243, 252–253 earth pressure analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–90, 316–348 EC0 structural design design life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21–22 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235–236 undertaking design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–185, 192, 199–200, 224, 227, 230 EC1 actions on structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 196 EC2 concrete structure design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37, 40, 184, 220, 224–230, 265–266 EC3 steel structure design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87, 90, 184, 220–222, 229–230, 235, 291–293

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EC7 geotechnical design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–4, 6, 14–25 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 application rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18–19, 25, 91, 96, 103–104, 111, 113, 123, 132–136, 143–144 designer’s role . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254–255 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56, 62–63, 83, 88, 321 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Geotechnical Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 15–17 good practice guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–197, 199, 201–219, 224, 228–230 ground movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 inspection, monitoring and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254–255, 260 limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18–19, 83, 88, 135–136, 144, 334 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 24, 184–185, 199, 203–219, 228–230 strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62–63 stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 worked design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382, 391, 396–406, 410 economic design construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–28, 31, 34–48, 52–53 costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 11, 24 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264–265 inspection, monitoring and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 props/temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231–232, 234, 253 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–185, 203, 205, 211, 216, 221–225, 230 wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–28, 31, 34–48, 52–53 worked design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 EFFC-DFI (2013) carbon calculator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 effective cohesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388–394 effective normal stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 effective stress analysis design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 design parameter calculations . . . . . . . . . . 100–102, 108–109, 113–117, 119–124, 128–129, 134, 139–140, 142, 144 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–57, 61–63, 66–67, 69, 71–81, 84, 88–90 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304, 308–313, 331–339 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189–190, 193–194, 197, 200 effects of method of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88–90, 331–339 E/G see longitudinal stiffness to shear stiffness elastic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 elasto-plastic models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 126, 178 embedment depths berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128, 185–188, 191, 201–209, 212–214, 217–219, 228–229 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 41 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398–399 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74, 76–77, 79–80 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334, 337, 342 wall deflections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356–375 embodied energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 empirical ground movement estimations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155–170 empirical permeability rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 EN see Euronorm design standards (Eurocodes) end-of-life aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37, 42–44, 46–48 environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13, 26–30, 34–35, 37–39 estimating ground movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146–147, 151–152, 154–176, 179–183 Euronorm (EN) design standards (Eurocodes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–3, 283–285 see also EC0 structural design; EC7 geotechnical design; national annexes EC1 actions on structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 196 EC2 concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37, 40, 184, 220, 224–230, 265–266 EC3 steel structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87, 90, 184, 220–222, 229–230, 235, 291–293 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185–192, 196–197, 199–200, 203–208, 220–230 evolution of earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–69 excavation of formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 99, 133–134, 137, 141 excavations construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21–22, 26, 28–30, 33–36, 40–47, 53 corners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149, 214, 377–378

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design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 99, 120–121, 128–129, 133–134, 137, 141, 143 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382–384, 387–388, 391, 395–401, 408–411 distributed prop loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412–415 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56–57, 59, 81–83 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149, 382–383, 387 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145–156, 162–178, 182–183, 376–381 limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196–199 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205–207, 209–216, 219 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189–200, 203–207, 209–216, 219, 223 wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 28–30, 33–36, 40–47, 53 extreme events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 128, 132, 143, 197, 200–202, 225, 257–259 extremely high strength clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101–102 extremely low strength clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100–101 factor of safety (FoS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 142, 149–153, 155, 165–167, 171, 352–355 failures calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101–102, 120–126, 138, 141 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 46 costs of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 design considerations . . . . . . . . . . . . . . . . . . . . . . 18, 21–22, 25, 185–188, 199–203, 206–209, 212–214, 217–219, 229 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 57–59, 62, 72–80, 89 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304–306, 309–315 ground movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147–153, 165–167, 171, 408 maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Mohr-Coulomb models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62, 100, 124–126, 311–312, 343–344, 408 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412 rock strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343–344 strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 57–59, 62, 72–80, 89, 304–306 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232, 235, 241–242, 252–253 financial risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 see also costs; economic design fine grained soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93–95, 100–102, 108–121, 125–127, 138, 142, 196–200, 214 fines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 finite difference methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70–72, 77–80, 84–89, 100, 116, 126, 129, 142, 171–172 finite element analysis/methods see also pseudo-finite element methods basement design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202, 386–388, 398–411 berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231, 239–243, 253 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170–172 parameter determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 116, 126, 129, 142 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235, 331–339, 415 quasi-finite element analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194–196 soil-structure interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70–72, 77–80, 84–89 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331, 336–341 wall deflections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241–243 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194–196 firm clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149–150, 155–156, 165–167, 172 flexible walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70–71, 81–90, 151–153, 172–174, 180–183, 236, 354, 412–413 flexural stiffness see wall flexural stiffness flownnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129, 131 FoS see factor of safety frequency of inspections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257–258 FREW program design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72, 88 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 parameter determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 126 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331, 335–336, 338–341 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57–62, 67, 73–75, 81, 89, 193–197, 200–202, 387–388 front of wall excavations construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21–22, 33

Guidance on embedded retaining wall design

425

Licensed copy:Lancashire County Council, 13/06/2018, Uncontrolled Copy, © CIRIA

design parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120–121, 128–129, 133–134, 143 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56–57, 59, 81–83 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147–153, 163–170 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264–266 geological condition design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 94, 99–104, 107, 120, 124, 127, 135, 141, 343 Geological Strength Index (GSI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 102, 124, 343 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303–342 geometry aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91, 99, 133–134, 137, 141, 382–383, 387 geostructural ground movement mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173–174 Geotechnical Categories Category 1 walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 15–16, 92, 103, 126–127 Category 2 walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6–7, 16–20, 92, 103–105, 126–127, 170 Category 3 walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 16–17, 92, 104–105, 127 design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 103–105, 109, 126–127, 141 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 15–17, 24 geotechnical design see EC7... geotechnical risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 geothermal piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 German standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .155, 285 glacial soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 30, 108, 118–119, 348, 356–358, 364 global stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187, 201–202 good practice guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184–197, 199, 201–254 grab techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 36 grading curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111–112 grain size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97–98 granular materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56, 61, 97–98, 151, 388, 413–414 gravel deposits construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93–94, 97, 110–112, 118, 122–123 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382, 388–397, 409–410 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156, 162, 345–351, 356–360, 366–370 ground anchorages see anchorages ground behavior/conditions see also ground movements boring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104–105, 107, 115–117, 126–127, 142, 146 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–43, 45–53, 153, 345–381 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–11, 17, 20 desk studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103–106, 141 distributed prop loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412–415 excavation in front of wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149–151 ground fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17–20, 24, 92, 107–108, 112, 127–128, 141 parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 95, 100–113, 117–125, 127–142 pore water pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104, 114–123, 127–129, 142 preliminary investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104, 106, 109 shear strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117–124 soil-structure interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83–88, 100–101, 116, 126–128, 141–142 stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100–102, 105–109, 115–116, 122, 126–127, 141–143, 149–151 strain analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306–307 stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 65, 91–92, 104, 107–108, 111–112, 127, 141, 345–381 strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117–124, 149–151 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–57, 61–67, 69, 71–81, 84, 88–90, 303–321, 331–339 wall types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26–43, 45–53 ground engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6, 53 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–11, 40–47, 145–183, 345–381, 407–411 see also heave construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28, 35–37, 40–43, 45–47, 53, 153, 345–381 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56–57 predictions . . . . . . . . . . . . . . . . . 145–147, 155–158, 169–175, 183, 198–205, 208–211, 214–219, 229, 350, 407–411 serviceability limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170, 198–199 wall installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56–57, 145–183, 345–378 wall movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145–183, 198–199 groundwater conditions see also groundwater pressures

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berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241–242 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–30, 35–37, 40, 48 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 20 desk studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103–106, 141 determination and selection . . . . . . . 91–92, 99, 102–105, 111, 118, 127–131, 134, 137–141, 390–391, 395, 397–401 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 147–153, 156, 162–163 hydraulic cut-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 limit states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191–193, 197 temporary conditions/works/support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137, 139, 241–242 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191–193, 197, 200–203, 208–211, 214–220, 226 groundwater pressures see also groundwater conditions aquifers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105, 108, 127, 140 determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 105, 111, 118, 127–131, 134, 139–143 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 147–153, 156, 162–163 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191–193 grouting anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245–248, 261 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36, 47–48 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 protective measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 GSI see Geological Strength Index hand soil tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94–95, 117 hangers, slabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 hard/firm secant walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28–30, 294, 296–299 hard/hard secant walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28–30, 294, 299–300, 382, 408 hard/soft secant walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28–30, 32, 294–295 hardware, future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 hazard identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13–14, 35, 67, 103–104, 137 headroom restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 27–28, 31, 34–35, 42 head tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111–113 health and safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10–14, 26, 35, 48, 53, 225, 248, 258 heave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111, 133, 147, 149–150, 156, 163–164, 350–355 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 construction consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36, 43, 48 design consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 height aspects cantilever heights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 378–381 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 30, 41 design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104, 133, 143 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 15, 21 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391, 408 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69, 73, 77–78, 82–83 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151–153, 173, 182 headroom restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 27–28, 31, 34–35, 42 temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234, 239–243 wall design/types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225, 291, 293 high modulus walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290–293 high strength clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101–102 highway traffic loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65, 132 hinged joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48, 52 Hoek-Brown failure criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62, 102, 124–125, 343–344 Hoek, E. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102, 124–125 hogging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155, 177, 181 horizontal effects see lateral... hot-rolled U-piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222, 290, 292 hydraulic cut-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151, 187, 201, 228 hydraulic grab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 hydrogeological conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92, 107, 127, 141 hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17, 20

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igneous rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98–99, 125 inclinometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105, 211, 218–219, 261, 378–381 information recording . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 in situ earth pressure coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55–56, 81, 90 design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 105, 113–117, 141–142 geomechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317, 320–321 in situ stresses design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100, 105, 113–117, 141–142 lateral stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55–56, 69, 81, 84, 89–90, 317 in situ walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317, 320–321 inspections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254–258 installation see also wall installation testing and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 instrumentation selection and maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260–262 interface friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387–388 interlock shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 internal columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 internal walling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43, 251 International Organization for Standardization (ISO) . . . . . . . . . . . . . . . . . . . . . . . . . 5, 93–96, 98–99, 102, 110–111, 123, 285 international standards see British Standards; Euronorm design standards; German standards Ipso tempore observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204–206, 212–216, 219, 230 king posts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292–293, 413 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 construction considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27–28, 30 design calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79–81 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146, 154 groundwater pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79–80, 251–252 socketed into rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 laboratory tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–4, 17–18 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391, 408 ground and groundwater conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104–122 parameter selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126–127, 135, 141–142 permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111–113 lap lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224–225 lateral earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–90, 316–335, 342 lateral ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147–173, 176, 178, 181–182 lateral loadings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79–80, 236–238 lateral stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187–188, 194–196, 199–203, 214, 228–229 lateral stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–90, 187–188, 229, 239, 241, 316–348 lateral wall deflections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351–352, 355 lead designers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 24–25, 232–233, 253, 264 length to height ratios (L/H) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149, 181–182 life cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12, 37 limit equilibrium analysis calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 62, 69–84, 87–90, 100, 128 cantilever walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72–75, 78–79, 87–88, 90, 187–188, 331–339 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187–188, 231, 234–235, 253, 331–339 stress analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331–339 temporary support systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231, 234–235, 239–241, 253 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185, 187–188, 201–202, 208, 229 limiting lateral stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54–55, 57–59, 61–63, 65–70, 74, 79–80, 89 limit states see also serviceability limit states; ultimate limit states anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 design considerations . . . . . . . . . . . 10–11, 18–25, 62, 88, 100–101, 134–137, 141, 144, 184–202, 205–219, 224–230 earth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69–70, 79, 84–90 EC7 geotechnical design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18–19, 83, 88, 135–136, 144, 334 ground movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 propping systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235–236 structural failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20–22, 25, 75

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linear seepage method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128–129 line loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65–66 liquidity index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 117–118 literature aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264, 316–317 loadings see also surcharge loadings adjacent site-specific constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65–67, 132–133 anchorages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244–253 design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 design examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382, 388, 397–407 determination and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91–92, 99, 107–108, 116, 122–123, 129–137, 141 EC1 actions on structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132, 196 finite element methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415 future work and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 king post walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79–80, 251–252 load tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 permanent loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65–67, 88, 227, 235, 334, 398 props/temporary supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87, 231–238, 244–253, 412–415 reinforced concrete structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 temporary conditions/works/supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137–139, 231–238, 244–253, 412–415 thermal loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87, 234–236, 238, 412–414 transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251–252 wall design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187–203, 206–209, 212, 214–215, 217–224, 227–230 longitudinal stiffness to shear stiffness (E/G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 long-term groundwater conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 long-term pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 low strength clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100–101 made ground characteristic values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388, 391–393, 410 maintenance aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247, 252, 254–262 manager’s roles and responsibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11–12 maritime structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78, 133, 193 material behavior analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387–388 maximum retained heights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 30 medium strength clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101–102 MEFP see minimum equivalent fluid pressures metamorphic rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 method of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69–90, 331–339 mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 36, 300–301 minimal site investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388–391, 401–404, 407–411 minimum equivalent fluid pressures (MEFP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64–65, 89, 335–337 mobile construction sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 mobilisation, soil strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70, 81, 174 modelling see also individual models berms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239–243 modulus walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291–293 Mohr circles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54, 173, 303–306, 310–311, 313–314 Mohr-Coulomb models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62, 100, 124–126, 311–312, 343–344, 408 monitoring corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258, 262 extent/purpose of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 observational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .