TR 26-Deep Excavation - 2010 [PDF]

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TECHNICAL REFERENCE TR 26 : 2010 (ICS 93.020)

TECHNICAL REFERENCE FOR

Deep excavation

Published by SPRING Singapore 2 Bukit Merah Central Singapore 159835 SPRING Singapore Website: www.spring.gov.sg Standards Website: www.standards.org.sg

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TECHNICAL REFERENCE TR 26 : 2010 (ICS 93.020)

TECHNICAL REFERENCE FOR

Deep excavation

All rights reserved. Unless otherwise specified, no part of the Technical Reference may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying and microfilming, without permission in writing from SPRING Singapore at the address below: Head Standardisation Department SPRING Singapore 2 Bukit Merah Central Singapore 159835 Telephone: 62786666 Telefax: 62786667 Email: [email protected]

ISBN 978-981-4278-44-7

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TR 26 : 2010

First published, 2010

NOTE 1.

Users of this Technical Reference should refer to the relevant professional or experts for any technical advice on the subject matter. SPRING Singapore shall not be liable for any damages whether directly or indirectly suffered by anyone as a result of reliance on this Technical Reference.

2.

Compliance with this Technical Reference does not exempt users from legal obligations.

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TR 26 : 2010

Contents Page Foreword

7

CLAUSES Section One – General 1.1

Scope

8

1.2

Normative references

8

1.3

Terms and definitions

9

Section Two – Site Investigations 9

2.1

General

2.2

Extent of investigation

10

2.3

Determination of wall toe-in

11

2.4

Presence of boulders

11

2.5

Investigation for tie-back design

12

2.6

Sampling and in-situ tests

12

2.7

Existing building conditions

13

2.8

Geotechnical model

13

Section Three – Design requirements 3.1

General

13

3.2

Water pressures

13

3.3

Basis for design

14

3.4

Design considerations

14

3.5

Ultimate limit state

16

3.6

Unplanned excavation

17

3.7

Design checks

17

3.8

Serviceability limit states

19

3.9

Computer software using numerical methods

19

3.10

Structural design

20

3.11

Material traceability and reusability of strutting materials

25

Section Four – Ground treatment 4.1

General

25

4.2

Methods of ground treatment

26

4.3

Ground treatment for specific requirements

26

3

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TR 26 : 2010 Page Section Five – Ground anchors 5.1

General

29

5.2

Terms and definitions

29

5.3

Factor of safety

30

5.4

Design situations and actions

31

5.5

Design of the anchorage

32

5.6

Checking of earth retaining wall movement

33

5.7

Investigative tests

33

5.8

Suitability tests

33

5.9

Acceptance tests

34

5.10

Tests on anchors

34

5.11

Pre-loading

34

5.12

Supervision and monitoring

34

5.13

Corrosion protection of steel components of anchorage

34

5.14

Maintenance of anchorages during service life

34

Section Six – Impact assessment 6.1

General

35

6.2

Prediction of ground deformation

35

6.3

Damage assessment

35

6.4

Masonry structures

37

6.5

Reinforced/pre-stressed concrete structures

37

6.6

High-rise buildings

38

6.7

Piled foundations

38

6.8

Utilities

38

6.9

Protective measures

38

6.10

Limiting values of structural deformation and foundation movement

38

Section Seven – Instrumentation and monitoring 7.1

General

39

7.2

Considerations for instrumentation

39

7.3

Instrumentation and monitoring of structures

40

7.4

Monitoring the performance of excavations

41

7.5

Reading frequency of monitoring instruments

42

7.6

Review levels and interpretation of monitoring results

42

7.7

Multi-tier level monitoring and reviews

43

7.8

Full design reviews

43

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TR 26 : 2010 Page Section Eight – Safety of construction 8.1

Risk assessment

44

8.2

Risk registry

44

8.3

Permit-to-excavate

44

8.4

Site inspection

45

8.5

Verification of site findings with designer

45

8.6

Training and supervision

46

ANNEXES A

Guidance on descriptions and weathering classifications

47

B

Classification of brickwork or masonry building damage

53

C

Informative references

54

TABLES 1

Sampling and in-situ tests

12

2

BS 8002 mobilisation factors for soil parameters (γm)

16

3

Load combination factors for limit states design of structures

20

4

Minimum safety factors recommended for design of individual anchorages

30

5

Recommended number of field tests

34

6

Damage category for masonry buildings

37

7

General guidelines for instrumentation and monitoring

41

A.1

Description of soil and rock types

48

A.2

Rock weathering classification

49

A.3

Bukit Timah granite and Gombak norite

50

A.4

Jurong formation

51

A.5

Old alluvium

52

B.1

Classification of visible damage to walls with particular reference to ease of repair of plaster and brickwork or masonry

53

5

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TR 26 : 2010 Page FIGURES 1(a)

Walls not penetrating into competent soil

18

1(b)

Walls penetrating into competent soil

18

2

Stability against hydraulic uplift

18

3 (a)

DPL diagrams for sand

21

3 (b)

DPL diagrams for stiff clay

21

3 (c)

DPL diagrams for soft clay

22

4 (a)

Example of excavation with stable base

22

4 (b)

Example of excavation with “unstable” base

22

5

Considering the forces due to eccentricity in the design of strut and waler

24

6

Length of stiff bearing for strut-to-waler connection

25

7 (a)

Treatment for utility gap

28

7 (b)

Treatment for TBM entry

28

8

Monitoring zone (minimum 2H) for buildings, structures, roads or utilities

40

6

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TR 26 : 2010

Foreword This Technical Reference was prepared by the Working Group on Deep Excavation appointed by the Technical Committee on Civil and Geotechnical Works under the direction of the Building and Construction Standards Committee (BCSC). The BCSC endorsed the Technical Reference on 3 February 2010. Deep excavations are complex due to the following: a)

There is always an element of uncertainty concerning in-situ conditions because the ground is a product of nature;

b)

Limitations in sampling and testing;

c)

The intrinsic soil and rock behaviour is complex;

d)

Limitations in modelling, e.g. on interfaces;

e)

Methods of construction can be varied and difficult to anticipate;

f)

Predicting building response is complex;

g)

Nature and condition of existing foundations and structures;

h)

Complex soil-structure interaction problems.

All deep excavations should be structurally safe and robust. The planning, design and construction processes in deep excavation projects are often not straightforward, involving many project parties and specialists. It is associated with higher risks, especially when implemented in urban built-up areas and in difficult ground conditions. While this Technical Reference is not meant to be a design guide or manual on deep excavation, it aims to draw attention and provide references to the key aspects of design, construction and practices. This Technical Reference is not to be regarded as a Singapore Standard; it is made available for provisional application over a period of two years but does not have the status of a Singapore Standard. The aim is to use the experience gained to modify the Technical Reference so that it can be adopted as a Singapore Standard. Users of the Technical Reference are invited to comment on its technical content, ease of use and any ambiguities or anomalies. These comments can be submitted using the feedback form provided at the end of the Technical Reference and will be taken into account in the review of the publication. At the end of two years, the Technical Reference will be reviewed by the WG to discuss the comments received and to determine its suitability as a Singapore Standard. Submission for approval by the Standards Council as a Singapore Standard will be carried out only upon agreement after review. Acknowledgement is made to CIRIA for permission to reproduce in this TR, Figure 7.12 of CIRIA 517 – Temporary propping of deep excavations – Guidance on design (London, 1999), www.ciria.org. At the time of publication, this Technical Reference is expected to be used by parties involved in deep excavation works, including designer, developer, owner and builder. Attention is drawn to the possibility that some of the elements of this Technical Reference may be the subject of patent rights. SPRING Singapore shall not be held responsible for identifying any or all of such patent rights.

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TR 26 : 2010

Technical Reference for deep excavation Section One – General 1.1

Scope

The Technical Reference is specific to the design and construction of deep excavations. Deep excavation refers to any excavates which has a retained height or excavation depth of 6 m or more. This includes shafts, trenches, cofferdams, marine or land retaining structures with walls, both temporary and permanent, ranging from free-standing gravity walls to multi-braced or anchored embedded walls. For a sloping ground behind the retaining wall, the height is taken to be from the excavated level to the top of slope. The excavation depth includes smaller but separate excavations or holes which extend beyond the main excavation level for construction of pile caps, pump sumps, lift pits etc. This Technical Reference is also applicable to situations where the excavation depth or retained height is less than 6 m if any of the following conditions is met: a)

There are adjacent structures within a horizontal distance of less than the excavation depth from the excavation face that are vulnerable to or likely to be adversely affected by the excavation works;

b)

Ground conditions are poor; or

c)

Lowering of groundwater table will likely lead to significant consolidation settlements in surrounding ground.

1.2

Normative references

The following referenced documents are indispensable for the application of this Technical Reference. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

1.2.1

Code of practice for site investigations

BS 5930 : 1999

1.2.2

Code of practice for site investigations

Laboratory and field tests

BS 1377 : 1990

Methods of test for soils for civil engineering purposes

BS 4019 : 1993

Rotary core drilling equipment

BS ISO 14686 : 2003

Hydrometric determinations – Pumping tests for water Considerations and guidelines for design, performance and use

1.2.3

Geotechnical structures or elements or processes

1.2.3.1 Codes of practice BS 5950-1 : 2000

Structural use of steelwork in building Part 1: Code of practice for design – Rolled and welded sections

BS 8002 : 1994

Code of practice for earth retaining structures

BS 8081 : 1989

Code of practice for ground anchorages 8

wells

–

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TR 26 : 2010 BS EN 1997-1 : 2004

UK National Annex to Eurocode 7: Geotechnical design Part 1: General rules

CP 4 : 2003

Code of practice for foundations

1.2.3.2 Standards and Execution of special geotechnical works BC1 : 2008

Design guide on use of alternative steel materials to BS 5950, BCA Sustainable Construction Series - 3

BS 5400-2 : 2006

Steel, concrete and composite bridges. Specification for loads

BS EN 1536 : 2000

Execution of special geotechnical work. Bored piles

BS EN 1537 : 2000

Execution of special geotechnical work. Ground anchors

BS EN 1538 : 2000

Execution of special geotechnical works. Diaphragm walls

BS EN 12063 : 1999

Execution of special geotechnical work. Sheet pile walls

BS EN 12699 : 2001

Execution of special geotechnical work. Displacement piles

BS EN 12715 : 2000

Execution of special geotechnical work. Grouting

BS EN 12716 : 2001

Execution of special geotechnical works. Jet grouting

BS EN 14199 : 2005

Execution of special geotechnical works. Micropiles

BS EN 14475 : 2006

Execution of special geotechnical works. Reinforced fill

BS EN 14679 : 2005

Execution of special geotechnical works. Deep mixing

BS EN 14731 : 2005

Execution of special geotechnical works. Group treatment by deep vibration

BS EN 15237 : 2007

Execution of special geotechnical works. Vertical drainage

BS EN 10025-1 : 2004

Hot rolled products of structural steels. conditions

Pr EN 14490

Execution of special geotechnical works. Soil nailing

General technical delivery

The informative references for the application of this Technical Reference are listed in Annex C.

1.3

Terms and definitions

For the purpose of this Technical Reference, the terms and definitions are given under the respective sections where applicable.

Section Two – Site investigations 2.1

General

Proper and adequate site investigations should be carried out for the design and construction of deep excavations so as to gain a thorough understanding and enable determination of the type and character of the ground conditions and groundwater conditions.

2.1.1

Site investigations in a wider sense refers to activities ranging from exploration of the ground, to testing and acquiring knowledge of the characteristics of the site that may affect the design and construction of deep excavation work and the security of neighbouring land and property.

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TR 26 : 2010 2.1.2

Guidance on the site investigation and geotechnical data for the design is given in Section 2 of BS 8002.

2.1.3 Desk study should include information from existing boreholes, geological map of Singapore (PWD, 1976; DSTA, 2009), site inspections and observations. Based on the information available and the intended purposes, a site investigation programme is carried out to obtain the essential field and geotechnical data to be used in design and construction. NOTE – See also Annex A

2.2

Extent of investigation

2.2.1

The extent and number of exploration or investigation points including boreholes, in-situ tests, geophysical means etc should provide the information required to adequately establish the ground conditions and water regime, and their variability along the length of the proposed retaining wall or excavation boundaries and within the influence zones of excavation work for the purposes of design and construction of deep excavations. 2.2.1.1 Site investigations for deep excavation work normally proceed in stages as follows: a)

A desk study and site reconnaissance;

b)

Ground investigation should be carried out in stages, i.e. preliminary and detailed investigation stages to obtain field and geotechnical data for design and construction purposes;

c)

Verification, and, if necessary, follow-up investigations during construction.

2.2.1.2 The site investigation should be planned to address the following: a)

The probable ground conditions at site;

b)

Significant design and construction issues to be considered e.g. values of geotechnical parameters, permeability conditions and adequacy of data;

c)

Potential areas of design and construction risks;

d)

Selection of appropriate investigation methods, tools and techniques.

2.2.2

In general, investigation points or boreholes should be located along each excavation boundary according to the complexity of the site. At critical areas and excavation in difficult ground, the spacing and location of investigation points/boreholes should be more closely related; and additional boreholes and/or cone penetration tests should be conducted between boreholes to establish any ground variability and to delineate the penetration depth of the retaining walls. The number of boreholes for each design section is dependent on the size of the excavation and should increase according to the variation of subsoil strata between boreholes and the complexity of geological formation.

2.2.3

For most embedded walls, particularly for cast-in-place concrete walls e.g. diaphragm walling, secant or contiguous bored piles, site investigations should consider the following: a)

The investigation points should be spaced as evenly as practicable around the full periphery of the wall.

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TR 26 : 2010 b)

Sufficient boreholes should extend to at least 5 m below the level that meets the design criterion for wall toe. For example, if the wall toe is designed to penetrate 3 m into a layer with SPT-N ≥ 50, the boreholes have to be taken to at least 5 m beyond that level. This criterion is not intended to prevent the designer from specifying deeper boreholes as he may consider it necessary.

2.2.4

Use of cone penetration tests (CPT)

Cone penetration tests (CPT) are useful for soil profiling and evaluation of engineering soil properties. Owing to the limitations of CPT e.g. in getting deeper, particularly in relatively harder materials or limited by presence of boulders, their use is usually restricted to supplementing investigative boreholes. Sufficient boreholes should be carried out to confirm that the CPT/CPTU (CPT with measurement of pore pressure) are being pushed to or beyond the interface level as appropriate, such as interface between Kallang formation and old alluvium.

2.2.5

Additional site investigations

During construction, ongoing analysis and interpretation of excavated ground conditions, and construction observations and monitoring data should be carried out to check or verify the design assumptions which are derived from previous investigations. Additional site investigation during construction is usually necessary for verifying the design assumptions e.g. thickness of soil strata. Any uncertainty in the site investigation results need to be factored into the design. Where necessary, redesign and rectification should be undertaken to comply with the relevant codes, standards and statutory requirements to ensure that the recommended safety margins are present throughout the excavation.

2.2.6

Quality of site investigation

Site investigations, including field and laboratory works, should be undertaken with due care and diligence as they have a direct impact on the reliability of all test results, which in turn can affect the design and construction works.

2.3

Determination of wall toe-in

2.3.1

Wall penetration is typically specified either as a penetration below final excavation level or as penetration into a particular stratum or material (e.g. SPT-N ≥ 50). It is not possible to accurately or reliably identify the interface between different strata or SPT simply by inspecting the spoils from bored piles or diaphragm walling. It is generally possible to identify the difference between soil and rocks by inspecting the spoils as the excavation progresses. However, the vertical continuity of such rocks cannot be reliably ensured in an environment of tropical weathering.

2.3.2

The design toe levels of an embedded wall should be determined by investigative boreholes which should be spaced relatively closely so that the design requirements for the embedded wall depth in the specified soil or rock stratum is fulfilled in the wall installation or construction work. This is especially so for cast-in-place concrete embedded walls/piles e.g. diaphragm walling, and secant or contiguous bored piles. In difficult or erratic soil layering, it is a good practice to have at least an investigative borehole at each diaphragm wall panel.

2.4

Presence of boulders

The depth of investigative holes should be such that they can appropriately establish the boulders, rock head, or competent strata. The depth of boreholes should go beyond the expected depth of the retaining system or embedded walls, kingposts or soldier piles. It is common practice to drill into rock for a depth of at least 5 m to establish whether a boulder or bedrock is present. In some cases where investigation requires the determination of socketing into rocks, it is necessary to drill deeper than 5 m.

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TR 26 : 2010

2.5

Investigation for tie-back design

For use of soil nails, tie-backs or ground anchors, the investigation points should be extended laterally from the alignment at critical sections so that sectional interpretations can be made for design. The investigation should also cover the area of the expected fixed anchor zone.

2.6

Sampling and in-situ tests (see Table 1)

2.6.1

Sampling

To ensure the quality of undisturbed soil samples, sampling procedures, techniques, methods and tools, e.g. thin-walled tube sampler, piston sampler and Mazier sampler, should be selected based on the type and stiffness of soil.

2.6.2

In-situ tests

In-situ tests include standard penetration tests (SPT), field vane tests (FVT), cone penetration tests (CPT), permeability tests, packer tests and pressuremeter tests (PMT). The tests should be appropriate for the soil types. Test results should not be derived from extrapolated data. Section 4 of BS 5930 provides detailed test procedures and their limitations. Table 1 – Sampling and in-situ tests Type of ground Fill / Reclaimed sand / F1

Sampling methods/tools Thin-walled tube sampler

Techniques Hydraulic pressing

Block sample

Trial pit and soil trimming

Thin-walled tube sampler

Hydraulic pressing

Piston sampler

Hydraulic pressing with suction

Thin-walled tube sampler

Hydraulic pressing

Mazier sampler

Rotary drilling

Bouldery clay (FCBB)

Triple-tube core barrel

Diamond coring

Rock (JF & BT)

Triple-tube core barrel

Diamond coring

Marine deposits (Ka - E, M & F2)

OA / Residual soil (JF & BT)

2.6.3

In-situ tests Standard penetration test Cone penetration test Field permeability test Standard penetration test (F2) Cone penetration test Field vane shear test Pressuremeter test Standard penetration test Pressuremeter test Field permeability test Standard penetration test (Clay matrix) Pressuremeter test Pressuremeter test Field permeability test

Groundwater conditions

Water levels encountered during boring operations or from records of boreholes are known to be unreliable and they seldom represent equilibrium conditions. Standpipes and piezometers should be installed to determine the groundwater conditions and pore water pressures at the excavation site. Where seepage is crucial, in-situ testing should be conducted to establish the field permeability.

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TR 26 : 2010

2.7

Existing building conditions

Pre-construction surveys should be carried out to establish the condition of adjacent properties, including obtaining the plans of existing buildings and structures. Special attention should be paid to buildings or structures that are sensitive to settlements.

2.8

Geotechnical model

2.8.1

One of the end-products of site investigation should be one or more geotechnical models. Such geotechnical models, which may be in two-dimensional or three-dimensional representation of the excavation site, shows how the stratigraphic units, geotechnical characteristics and the groundwater conditions relate to the deep excavation, the structure being designed and constructed, and the surroundings. The appropriate sections to be used for the detailed design of the excavation work may be derived from the geotechnical model taking full consideration of the variability and uncertainty involved in modelling. The design should consider the case with the most adverse or worst anticipated ground conditions.

2.8.2

The geotechnical models should be updated and verified during construction as and when additional information becomes available. This includes: a)

Additional site investigation results;

b)

Logs from installation of walls, kingposts, piles, dewatering/pressure relief wells, monitoring instruments and any grouting or other geotechnical processes;

c)

Mapping of exposed ground during excavation.

Section Three – Design requirements 3.1

General

3.1.1

The support system for deep excavations should be safe and robust to ensure stability and to control ground and wall deformations. It should be designed and constructed with appropriate factors of safety and load factors that are not less than those required for permanent works in accordance with the relevant standards, codes of practice and statutory requirements.

3.1.2

The excavation support system should be designed for appropriate forces obtained from analysis which considers the full construction sequence through to the removal of the supporting elements for the excavation or construction works. The analysis should consider appropriate boundary conditions and progressive changes in pore water pressures throughout the excavation and construction stages.

3.2

Water pressures

3.2.1 The water pressure regime used in the design should be the most onerous that is considered to be reasonably possible for active pressure and passive resistance. The design groundwater level should be considered at the ground level as loading for each stage of construction unless alternative levels are adequately justified, taking into account tidal and probable flooding conditions. Where artesian condition exists, additional water pressure should be considered. 3.2.2

The groundwater flow pattern around an excavation can affect water pressures and earth pressures on the active and passive sides of the wall, and piping as well as heave potential of the excavation. The most adverse groundwater condition that can reasonably be anticipated should be used in the estimation of passive resistance, taking into consideration the influence of upward seepage and potentially high pore water pressure within the passive zone of the soil. 13

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TR 26 : 2010

3.3

Basis for design

The design of the deep excavation wall system should consider both the ultimate limit states (ULS) and the serviceability limit states (SLS). As a minimum, the following limit states should be considered: a)

Loss of overall stability;

b)

Failure of a structural element e.g. anchor, waler, strut or connection between such elements;

c)

Combined failure in the ground and structural element whereby behaviour of one, when it fails, has an adverse effect on the other which leads to progressive failure;

d)

Failure in basal heave, toe kick-in, hydraulic heave and piping;

e)

Movements of the retaining structure or surrounding ground which may cause collapse, partial or otherwise, or affect the appearance, serviceability or functionality of the structure, nearby structures or services;

f)

Failure due to rotation or translation of the wall or parts thereof;

g)

Failure due to lack of vertical equilibrium;

h)

Any combination of the above.

3.4

Design considerations

3.4.1

The design of retaining structure and its supporting structural elements for deep excavations should take into account the following key design considerations: a)

Adequate site investigation;

b)

Proper selection of the soil parameters for design;

c)

Effects due to onerous water pressures and seepage forces, on both active and passive sides of the wall, unless justified otherwise;

d)

Effects of soil permeability (drained and undrained conditions), drainage boundary, rain water infiltration and time rate of construction (consolidation).

e)

Effects of surcharge loads, including incidental loads, construction loads, vehicular traffic etc;

f)

Varying load conditions and movement conditions during stages of construction or excavation;

g)

Design robustness and redundancy considerations which should include one-strut failure, accidental loads, thermal loads, eccentricity etc;

h)

Construction material deficiencies and construction imperfections or tolerances;

i)

Adequacy of wall embedment;

j)

Overall wall stability, basal heave, toe kick-in, hydraulic heave and piping;

k)

Structural adequacy of supporting system e.g. walers, struts, anchors, including provision of adequate stiffeners;

l)

Provisions of restraints in structural members’ connections, ties and bracings;

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TR 26 : 2010 m)

Sensitivity analyses on the performance of retaining structures;

n)

Effects due to groundwater lowering;

o)

Effects of ground deformation on neighbouring properties;

p)

No allowance for any material overstress in the design;

q)

Factors of safety for designing retaining structures and all supporting structural members for deep excavations should not be less than those for permanent works;

r)

Unplanned excavation;

s)

Preloading of struts/anchors.

3.4.2

Factors of safety

The retaining structures for deep excavations should be designed with an adequate safety factor that is not less than that required of permanent works and the calculations should take full account of the material deficiencies, construction imperfections and tolerances adopted. No allowance for any material overstress should be made in the design. Among other considerations, the factor of safety should take into account abnormal risks, unusual/difficult ground, loading conditions, soil characteristics, extreme soil and groundwater conditions, the need to restrict deformation, consequence of failure and impact on surrounding properties.

3.4.3

Robustness of design

As a minimum, all types and conditions of loading on retaining structure for deep excavations should be identified for design. The retaining structure has to be robust and sufficient redundancy should be provided to avoid catastrophic collapse resulting from an isolated case of overloading or failure of any particular element.

3.4.4

In addition to the earth pressures under all identifiable conditions and surcharge/building loads, the design should consider the following: a)

Accidental load;

b)

One-strut or one structural member failure;

c)

Material deficiencies and construction imperfections;

d)

Abnormal loads, particularly from construction surcharges, and exceptionally high groundwater levels caused by flooding or prolonged heavy rainstorms, or water-filled tension cracks;

e)

Eccentric loads or out-of-balance forces and reactions from the support systems, both temporary and permanent e.g. due to inclined anchors or struts;

f)

Temperature effects.

3.4.5

Soil parameters

In assessing the shear strength, influence due to factors such as stress level imposed on the soil, strain rate effects, large strain situation, time effects, and sensitivity should be considered. The representative strength values should make due allowance for the influence of sampling and the method of testing as well as for possible softening during excavation.

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TR 26 : 2010 3.4.6

Design drainage conditions

Soils with field permeability greater than 10-6 m/s can be considered as free draining. Drained conditions should be adopted for these materials. Soils with permeability less than 10-8 m/s can be considered as undrained in most cases. If the rate of construction is very low or there is a long idling period, consolidation analysis may be more appropriate. For soils with permeability between 10-6 and 10-8 m/s, two options should be considered. The first option is to conduct a consolidation analysis. The second option is to conduct two analyses, i.e. drained and undrained.

3.4.7

Sensitivity analyses

For deep excavations, especially those in difficult or poor ground conditions, the designer should not rely merely on ‘one-off’ analysis, in which a single set of geotechnical parameters is used, and the results of the analysis then taken as ‘the prediction’ of deformations, loads and stresses. The analyses should include variations in the input parameters within a reasonable range corresponding to those determined from site investigation and ground conditions, and to critically examine the effects of such variations on the computed deformations, loads and stresses. Sensitivity analyses should be performed as part of the design to demonstrate that the design and models are not unduly sensitive to variations in any of the input parameters such as shear strength, soil stiffness and reduced wall stiffness due to cracking. It should also cover the effect of time on the soil conditions and its impact on the performance of retaining structures.

3.5

Ultimate limit state

3.5.1

Design factors

Mobilisation factors are to be used in the ultimate limit state (ULS) design of the excavations system. The design values of the geotechnical parameters Xd should be derived using Xd = Xk / γm

Equation (3a)

where Xk is the moderately conservative estimate of the soil parameter; and γm is the reduction factor for the parameter. The mobilisation factors based on BS 8002 are shown in Table 2. Table 2 – BS 8002 mobilisation factors for soil parameters (γm) Soil parameter Angle of shear resistance (tan φ) Effective cohesion Undrained shear strength Unconfined strength Weight or density

Symbol γφ’ γc’ γcu γqu γγ

Min value 1.2 1.2 1.5 1.5 1.0

3.5.2

The term “moderately conservative” is taken to mean the “cautious estimate” of the value relevant to the occurrence of the limit state as specified in CIRIA C580. It is also considered to be equivalent to “representative value” as specified in BS 8002.

3.5.3

“Worst credible” value is the worst value which is reasonably believed to occur. considered to be equivalent to “conservative” value as specified in BS 8002.

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TR 26 : 2010 3.5.4

The ULS design should be based on the more onerous of the following approaches:

a)

Approach 1: Earth pressures derived from design values as defined in this section in which the mobilisation factors γm in Table 2 are appropriately applied to the moderately conservative parameters;

b)

Approach 2: Earth pressures derived from the worst credible parameters.

3.6

Unplanned excavation

The design for excavations should consider an additional depth of 0.5 m of unplanned excavation in front of the wall. The minimum values should be reviewed for each design and more adverse values adopted in particularly critical or uncertain circumstances. The requirement for an additional or unplanned excavation as a design criterion is to provide for unforeseen and accidental events. Foreseeable excavations such as for waler support brackets, services or drainage trenches in front of a retaining wall, which may be required at some stage, should be treated as planned excavation.

3.7

Design checks

As a minimum, design checks should be carried out for, but not limited to, the following.

3.7.1

Against toe kick-in

The design of the retaining wall for deep excavations should demonstrate that the system is safe against toe kick-in. For the system shown in Figure 1 (a), which has adequate factor of safety against basal heave (i.e. FS ≥ 1.5), it may not be necessary to check toe stability. For walls penetrating into the competent stratum [(see Figure 1 (b)], the toe stability has to be checked using the equation below. PP LP + Mp FS = --------------------- ≥ 1.5 PA LA where

Pp LP PA LA Mp

Equation (3b)

is the total passive resistance, inclusive of the water force, in front of the wall; is the distance from the lowest strut level to Pp; is the total active force, inclusive of the water force, behind the wall; is the distance from the lowest strut level to PA; is the ultimate moment capacity of the wall.

PP and PA should be determined based on unfactored strength parameters. The interface wall friction angle and adhesion should be taken into consideration. A competent stratum may consist of very stiff to hard clay or silt and medium dense to dense sand.

3.7.2

Against base heave failure

The retaining wall for deep excavations should be designed with appropriate adequate precautions against base heave for every stage of the construction. In cases of long and wide excavations where excavation width (B) is greater than excavation depth (H), i.e. B > H, methods proposed by Terzaghi (1943), Goh (1994), or Wong and Goh (2002) may be used to check against base heave. For narrow excavations, i.e. H ≥ B and/or excavations of finite length, the methods of Bjerrum and Eide (1956), NAVFAC DM-7.2 (1982) or Eide et al. (1972) may be used to check against base heave. For the design check against base heave of an excavation, the minimum factor of safety is 1.5.

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Figure 1 (a) – Walls not penetrating into competent soil

3.7.3

Figure 1 (b) – Walls penetrating into competent soil

Against base failure due to hydraulic uplift and seepage

Hydraulic uplift check should be carried out to ensure that the base of the excavation will not blowout. This check should be carried out in accordance with Figure 2. The minimum factor of safety as defined in Equation (3c) is 1.2. Factor of safety = (γ DB + factored side resistance)/ U where

γ D B U

Equation (3c)

is the saturated unit weight of the soil; is the silt or clay thickness; is the width of excavation; and is the hydraulic uplift force.

The prerequisites for blowout are as follows: a)

The permeability k2 is much higher than k1 such as marine clay above F1 sand;

b)

Ample water supply in the k2 soil layer.

Where the factor of safety is less than 1.2, the wall needs to be deepened to cut off the water supply else relief wells should be provided. The reduction factor on side resistance has to be correctly determined.

Standpipe

Figure 2 – Stability against hydraulic uplift

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TR 26 : 2010 Where necessary and appropriate, hydraulic uplift has to be checked at intermediate levels to ensure that there is adequate factor of safety. The design should also check for piping and quick conditions, where appropriate; this check should be based on seepage analysis.

3.7.4

Against one-strut/anchor/structural member failure

The design for deep excavations should accommodate possible failure of any individual strut, tie rod, ground anchor, structural member or connection at each stage of the construction works. The wall and remaining supporting members, including walings and connections, should be capable of redistributing the load from the failed member. The remaining structural system and wall should continue to be safe without causing any danger to surrounding adjacent structures and properties.

3.8

Serviceability limit states

3.8.1

The retaining wall system for deep excavations should be designed to keep deformation of the wall and surrounding ground to a minimum to prevent damage to neighbouring structures, utilities and properties.

3.8.2

Wall deflection is commonly adopted as a performance indicator for serviceability limit state consideration. The limit depends on soil condition, depth of excavation and proximity to surrounding structures and utilities.

3.8.3

For reinforced concrete retaining walls, the effect of cracking on wall deflection should be checked. This can be achieved by using a reduced wall stiffness equal to 70 % of the full stiffness, i.e. 0.7EoI where Eo is the short-term Young's modulus of concrete and I is the second moment of area of the reinforced concrete wall section. The bending moment envelopes of the walls obtained from 1.0Eo and 0.7Eo should be used for the ultimate limit state design. For sheetpile walls utilising U-shape sheetpile such as FSP-III, there is a possibility of slippage at the clutches between sheetpile resulting in a reduction in wall stiffness. If no action is taken to prevent such slippage, analyses should be conducted to assess the effect of using full and reduced stiffness on bending moment and wall deflection.

3.8.4

Seepage analysis

In cases where changes in groundwater conditions could be expected from excavation activities, seepage analysis should be performed with sensitivity study of the results to variations in permeability values, to demonstrate that the ground deformation would not damage surrounding buildings and properties.

3.9

Computer software using numerical methods

Computer software using numerical methods in geotechnical analysis and design for deep excavations should be able to model soil-structure interaction problems in staged construction, pore pressures, ground stresses and strains, and perform both total and effective stress analyses. Where numerical modelling is used, e.g. finite element methods in geotechnical design, experienced users with fundamental understanding of geology and soil mechanics, and clear understanding of numerical modelling, particularly the limitations, should supervise the use of such analyses. Regarding use of computer software, users are advised to follow the guidelines in: 1)

The use of computers for engineering calculations published by The Institution of Structural Engineers, UK (Mar 2002);

2)

Guidelines for the use of advanced numerical analysis edited by David Potts, Kennet Axelsson, Lars Grande, Helmut Schweiger and Michael Long, published by Thomas Telford (2002);

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TR 26 : 2010 3)

“CIRIA C580: Embedded retaining walls – Guidance for economic design” AR Gaba, B Simpson, W Powrie and DR Beadman, published by CIRIA London (2003);

4)

Numerical analysis: A virtual dream or practical reality? Rankine lecture by David Potts, Geotechnique 53, No.6, pp 535-573 (2003).

For undrained analysis involving soil of low permeability such as clays and silts, there are several ways to analyse this problem depending on the soil model adopted and whether total or effective stress is used. When using any finite element or finite difference software it is essential that the inherent assumptions of the program are fully understood, particularly for the modelling of soft clays with the Mohr-Coulomb model.

3.10

Structural design

3.10.1 The strutting system should be designed to ensure lateral stability of the excavation and assist in limiting wall and ground deformations. The design should ensure that the structural failure of the strutting and its connections will not occur and that deflections are restricted so as to avoid damage to any structure and adjacent property.

3.10.2 Minimum load factor and load combinations The strutting system and the retaining wall should be designed with a minimum load factor of applied to excavation load which includes earth, water and surcharge load using the results of most onerous design based on moderately conservative soil parameter. The design values of geotechnical parameter should be derived from Equation (3a) assuming γm = 1.0. Table 3 shows typical load combination factors for use in the design of strutting and retaining wall structure.

1.4 the the the

Table 3 – Load combination factors for limit states design of structures Load combination

Load factor Excavation load

Dead load

Live load

Temperature load

Impact load

Normal working condition

1.4

1.4

1.6

1.2

-

One strut failure

1.05

1.05

0.5

-

-

Accidental impact

1.05

1.05

0.5

-

1.05

3.10.3 Kingposts and decking structures Kingposts and decking structures should be designed with the appropriate loads and load factors in accordance with the relevant codes of practice to achieve robustness and adequate factor of safety such that no disproportionate catastrophic collapse would occur. Where appropriate, anticipated retaining wall movements under the most onerous conditions should be considered in the design and detailing of kingposts and decking structures.

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TR 26 : 2010 3.10.4 Excavation loadings Loading from earth pressure can readily be estimated from the apparent pressure diagrams developed by Terzaghi and Peck (1967). These are maximum envelopes from field measurements. In 1999, CIRIA updated these diagrams with more case records. The results are a series of revised diagrams which are termed characteristic diagrams of the distributed prop loads (DPL) method. These diagrams are not maximum envelopes. They are cautious estimate of the distributed strut loads and have approximately a 5 % chance of being exceeded. These diagrams (see CIRIA, 1999) are reproduced in Figures 3 (a) to 3 (c). Figure 3 (a) shows the DPL diagrams for excavations in granular soil. If the water table is below the final excavation depth, the total unit weight is to be used to compute the earth pressure. If the water table is above the excavation level, the total and buoyant unit weights should be used above and below the water table respectively. In this case, the hydrostatic water pressure above the formation level has to be taken into consideration. Figure 3 (b) is for excavations in stiff to very stiff clay. The earth pressure on the wall is inversely proportional to the amount of wall displacement. Since a flexible wall deflects more than a stiff wall, the soil behind the flexible wall is given more freedom to expand and hence a greater reduction in soil pressure. Figure 3 (c) shows the DPL diagrams for excavations in soft to firm clay. A stable base refers to an excavation with adequate factory of safety against basal heave. In the computation of safety factor, Terzaghi’s method for wide excavation and Bjerrum and Eide’s method for narrow excavation should be used. These methods do not consider the contribution from wall penetration below the formation level. It should be noted that the FS value has to be satisfactory at all stages of excavation, not only at the final level of excavation. Unstable base refers to situations where the wall needs to penetrate into the competent layer (the depth to which the slip surface would not exceed) to improve the basal heave stability. Figure 4 shows examples of excavations with stable and “unstable” base. The struts are the most important element in an excavation. It is wise to err on the safe side hence be conservative in choosing an appropriate DPL diagram in the design. It should be noted that the DPL diagrams are derived from case studies involving mainly flexible walls. Chang and Wong (1996) highlighted that the apparent earth pressure on “rigid” walls such as diaphragm wall can be higher than those indicated in the DPL diagrams. Therefore the application of these DPL diagrams is limited to flexible wall systems without soil improvement. An alternative to the DPL diagrams is to conduct finite element analysis (FEM). If done correctly, the FEM will generally produce more realistic strut load magnitudes and distributions.

No ground water

High ground water table

Figure 3 (a) – DPL diagrams for sand

Flexible wall

Stiff wall

Figure 3 (b) – DPL diagrams for stiff clay 21

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TR 26 : 2010

Firm clay with stable base

Soft clay with stable base

Soft clay with unstable base

Figure 3 (c) – DPL diagrams for soft clay Figure 3 is reproduced from CIRIA C517, 1999

Figure 4 (a) – Example of excavation with stable base

Figure 4 (b) – Example of excavation with “unstable” base

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TR 26 : 2010 3.10.5 Additional design considerations In addition to the excavation load under all identifiable conditions, the following loads also have to be considered in the design of the excavation support system: a)

Abnormal loads, particularly from loads caused by flooding and construction loads, for example construction cranes, heavy vehicles etc, which are considered to be in excess of minimum surcharge of 10 kPa. Where there is vehicular traffic, a design surcharge load of 20 kPa should be used. Higher surcharge load (> 20 kPa) may be required if heavy construction equipments are employed.

b)

Change of strut force due to temperature difference of ± 10 °C should be considered.

c)

Change of strut force due to the installation and removal of struts at any level.

d)

Change of strut force induced by wall rotation and relative displacements between the supported ends.

e)

Accidental load of 50 kN to be applied normal to the strut at any point in any direction, unless otherwise demonstrated by risk assessment.

f)

Axial force on the waler due to the inclined struts (in plan) imposing force along, rather than orthogonal to, the waler.

Besides the loading, the design of the support system should also consider the following: a)

Material deficiencies, construction imperfections and tolerances;

b)

Robustness of connection to avoid brittle failure;

c)

Accidental removal or failure of one strut/anchor or its connections.

3.10.6 Temperature effects An increase or decrease in the temperature of a strut from its installation temperature will cause a change of strut force according to the relationship:

ΔP=kαΔtEA where

E A Δt

α k

Equation (3d)

is Young's modulus of the strut material; is the cross-sectional area of the strut; is the change in temperature from the installation temperature force; is the thermal coefficient of expansion for the strut material, typically 1.2 x10-5 per °C for steel; and is reduction factor due to the degree of end restraint.

In Equation (3d), k = 1.0 is for a fully restrained strut where both ends are prevented to expand freely. If the degree of restraint of the strut allows some expansion, lesser strut loads due to temperature effects will result. In the absence of rigorous analysis, k = 0.6 is recommended for flexible sheet pile walls and k = 0.8 for stiff wall with stiff soil condition. Temperature effects are normally added to the predicted strut loads after the analysis is completed. A rigorous soil-structural interaction analysis may be used to evaluate the strut force by considering the temperature effects, wall movement and soil condition. However, the best way to observe the temperature effect on strut force is to monitor the strut force together with the temperature variation and make comparison with the predicted results.

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TR 26 : 2010 3.10.7 Struts, walers, kingposts and connections The axial load capacity of strut may be reduced due to loss of end fixity or when the loss of an intermediate support results in an increase in the effective length of the strut over that assumed in design.

3.10.8 The design of struts, walers and strut/waler connections should take into account eccentricity of load transfer from the waler to the strut. The eccentricity (e) should be taken as 0.1d but not less than 30 mm, where d is the depth of the strut. Figure 5 shows the forces due to the eccentricity for the design of the strut and waler. For strut supported by a single waler beam, both the strut and the waler’s web should be designed to resist additional moment, M = Fe, where F is the applied strut force. For strut acting on double waler beams, each waler should be designed to resist additional force of magnitude 0.5Fe/D where D is the spacing between the two walers. If necessary, stiffener(s) should be provided to prevent side sway of the waler beam(s). 3.10.9 Buckling and bearing checks should be carried out for all strut-to-waler connections and the effect due to load eccentricity should be considered as specified in 3.10.8. Suitable stiffeners should be provided to prevent brittle failure of waler connection. Where the waler flange through which the load is applied, including the effect of eccentricity, is not effectively restrained against lateral movement relative to the other flange, proper effective length considering the effect of side sway should be used to check the buckling capacity of the stiffened web. 3.10.10 All restraints to axially compressed struts should be capable of resisting a force of not less than 1.0 % of the axial force in the strut and transferring it to the adjacent points of positional restraint. A restraint should have adequate strength and stiffness to inhibit movement of the restrained point in position or direction, as appropriate. Positional restraints should be connected to an appropriate shear diaphragm or system of triangulated bracing. 3.10.11 A bracing system that provides positional restraint to more than one member should be adequately designed to resist the restraint forces from each member restrained.

Single waler

0.5F+Fe/D D

0.5F-Fe/D Double waler

Figure 5 – Considering the forces due to eccentricity in the design of strut and waler

3.10.12 Kingposts should be designed as an unbraced column if no triangulated bracings are provided. The effective length of an embedded kingpost should be determined from analysis to derive the position of fixity below the ground. The analyses should also include construction stages when the temporary support members or struts are removed.

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TR 26 : 2010 3.10.13 In calculating the bearing capacity of the beam web, the length of stiff bearing to be considered should be taken as b1 = t + 2T, where t = thickness of the web of strut and T = thickness of end plate, as shown in Figure 6. Web of strut

t

End plate

T Flange of waler

b1 = t +2T Web of waler Concrete packing Figure 6 – Length of stiff bearing for strut-to-waler connection

3.11

Material traceability and reusability of strutting materials

A quality assurance plan has to be established to ensure material traceability and the use of reusable strutting materials in construction. Guidelines to establish a quality plan for material traceability and reusability of steel strutting system in accordance with BC1 : 2008: Annex A (2009). Steel materials other than class 1 and class 2 are not to be used.

Section Four – Ground treatment 4.1

General

Although ground treatment is not required in excavation projects, there are situations where ground treatment becomes an essential part of the excavation support system for stability, ground movement control or water flow control purposes.

4.1.1

In excavation works, ground treatment may be used to:

a)

Enhance passive support to the retaining wall;

b)

Provide adequate resistance against base heave;

c)

Prevent boiling at the excavation base;

d)

Retain earth behind wall openings;

e)

Enhance trench stability of a diaphragm walling;

f)

Form gravity mass for shallow excavations;

g)

Prevent soil and water ingress between wall members in case of contiguous bored pile wall;

h)

Form water cut-off;

i)

Make up for ground movement and loss as compensation grouting;

j)

Minimise settlement in the ground adjacent to excavation. 25

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4.2

Methods of ground treatment

4.2.1 Ground treatment related to deep excavations is done either by grouting or mechanical mixing. Depending on the type of ground and purpose of the treatment, grouting may take the form of permeation, fracture, displacement (compaction) or replacement (jet) grouting. Some of the common methods listed below or a combination of them may be used: a)

Single or double packer grouting;

b)

Tube-A-Manchette (TAM);

c)

Mechanical deep mixing;

d)

Jet grouting.

4.2.2 Methods of grouting depend on the ground conditions and requirements of the treated ground. Ground treatment to reduce permeability and control water flow is mainly done by permeation grouting whereas strength applications are mainly done either by mechanical deep mixing, jet grouting or a combination of both.

4.2.3

In the case of high pressure grouting (permeation, fracture or jet grouting), the effects of applied pressure and possible ground movement have to be assessed and kept within safe limits to avoid damage to the retaining wall, buildings, structures and utilities in the vicinity.

4.2.4

When mechanical deep mixing is used, guidelines given in BS EN 14679 : 2005 should be followed.

4.2.5

When mechanical deep mixing is employed to enhance passive support below the final excavation level, the upper part of the ground is weakened. Unless a certain amount of hardening agent is used to restore in-situ shear strength, the design has to consider a weakened soil for the disturbed zone within the excavation depth.

4.2.6

When jet grouting is used, installation and testing should follow ‘BS EN 12716 : 2001 – Execution of special geotechnical works – Jet grouting’ and the design may follow the recommendations in ‘Jet grout’ (Column jet – Technique materials) of the Japan Jet Grout Association or equivalent guide document.

4.2.7

Although extreme pressures are used in jet grouting, they are meant to generate the high velocity fluid jets required to disintegrate soil into particles but not to subject the surrounding ground to such high pressures. The pressure in the grouting space is governed by the back pressure offered by the escaping effluent in the chamber. Therefore, unobstructed flow of effluent in the annulus between the drill rods and the soil has to be ensured at all times during grouting to minimise ground movements, including ground heave.

4.3

Ground treatment for specific requirements

4.3.1

The design and methods of ground treatment depend very much on the existing soil and requirements of the treated ground.

4.3.2

Passive support to retaining wall

4.3.2.1 Deep excavations in soft ground often require some form of ground treatment to improve passive support to the wall. Usually jet grouting or deep mixing is done to enhance passive support below the final excavation level. It may also be done at intermediate levels and excavated away as the excavation proceeds and structural supports are put in place.

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TR 26 : 2010 4.3.2.2 The treated ground can only be considered as an improved soil and not as a structural prop in excavations. The design also has to consider the ‘brittle’ nature and rapid strain softening of the treated ground as opposed to the ‘ductile’ behaviour of naturally occurring soils. The strength adopted in the design needs considerations such as field variability and possible voids in parts of the grouted mass and should be based on adequate trials. 4.3.2.3 The design and design checks of all supporting structural members and walls for deep excavations should include the full consideration of the transfer of forces under the most onerous conditions if the treated material is excavated away or removed, partially or wholly, at any excavation stage. Analyses and design checks should be done with strength and stiffness at the upper end of measured values of treated material as this would yield higher forces on the structural members. 4.3.2.4 The grout pile size adopted to form mass treatment needs to be substantiated adequately based on all trial measurements and tests. Allowances have to be made for positioning and drilling inaccuracies, e.g. where drilling tolerances cannot be achieved, usually within 1:100. 4.3.2.5 The required thickness of treated soil has to be evaluated based on its strength and stiffness properties and the design requirement of the wall with the proposed support system. As a guide, owing to construction tolerances, the thickness of the treatment should not be less than 2 m.

4.3.3

Support against base heave

Ground treatment to resist base heave should have sufficient thickness or depth to resist and transfer the up-heave forces to the wall or supporting piles without developing significant tensile forces within the grouted mass. The thickness also has to be checked with the bond strength developed between the treated ground and the wall as well as foundation piles. The treatment may be done in a manner to form an inverted arch to minimise bending moments within the treated mass.

4.3.4

Prevent boiling at excavation base

Permeation grouting may be required to prevent boiling at the base of an excavation in saturated granular soils. The grout holes have to be evenly spaced, preferably in a hexagonal arrangement to achieve uniform treatment. The spacing among grout holes depends mainly on the permeability of the ground, type of grout, grout flow rate, setting time, grout volume and grout pressure. Since numerous grouting parameters are involved and the selected combination may affect the results, the success of treatment needs to be determined by field permeability tests. However, it is important to remember that in saturated granular soils, ground treatment against boiling usually increases the potential for base blow up.

4.3.5

Retaining earth behind wall openings

It is sometimes not possible to relocate utilities away from the proposed excavation. The construction of a retaining wall directly below the pipes or cables of utilities becomes unfeasible. Under these circumstances, ground treatment may be considered to bridge the gap in the wall to prevent soil and water inflow during excavation. Figure 7 (a) illustrates one possible application involving both vertical and inclined grout piles and Figure 7 (b) illustrates another possible application for the entry of a tunnel boring machine (TBM).

4.3.6

Prevent localised collapse during wall trenching

In the installation of cast-in-place concrete walls such as diaphragm walls, contiguous or secant piled walls, it is usually inevitable that slurry is needed to maintain stability during trenching or boring operations. Trench stability design calculations need to be done to determine the required slurry level to safeguard against potential trench collapses. It may be necessary to prevent localised collapses by using ground treatment on either side of the trench. The slurry level has to be controlled and maintained at all times throughout wall trenching or boring operations.

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TR 26 : 2010

Figure 7 (a) – Treatment for utility gap

4.3.7

Figure 7 (b) – Treatment for TBM entry

Prevent soil and water ingress behind CBP walls

When contiguous bored piles (CBP) are used as retaining walls, it is often required to seal the gaps between the piles to prevent soil or water inflow. The clear distance between the piles is usually set between 75 mm to 100 mm. However, the true gap may be wider or narrower depending on pile installation process. In firm soils, the water ingress is initiated first, followed by the weakening of soil due to water absorption which eventually leads to soil slumps through gaps between piles. Loss of soil behind the wall may lead to cavities which eventually form sink holes at the surface where heavy machines operate. In firm soils, ground treatment to prevent water flow, i.e. permeation grouting, may be sufficient whereas in weak soils strength may be required additionally to prevent material squeeze. Jet grouting, pre-installed deep mixing piles or other pre-drilled ‘soft’ piles may serve as barriers preventing water and soil inflow between bored piles.

4.3.8

Water cut-off

4.3.8.1 When an excavation is carried out with a non-water tight retaining wall and/or there exists a permeable soil layer, the pore pressure drop could result in consolidation settlement and damage to nearby structures. Ground treatment to form water cut-off in the permeable layer becomes necessary. This treatment is usually done by permeation grouting. However, jet grouting or grouting with the overlapping insertions of a steel I-section is common in forming cut-off grouting. The effectiveness of cut-off grouting can be tested by forming enclosed space or cells with cut-off grouting and performing pumping tests within the cells. 4.3.8.2 When a permeable layer is present at a short depth below the final excavation level beneath an impermeable layer, the potential for base heave is high during excavation. Water cut-off of this permeable layer along the perimeter of the excavation is required to avoid the risk of base heave. Relief wells may also be used to reduce heave potential.

4.3.9

Make up of volume loss

Compensation grouting is done to make up for the volume loss associated with wall displacement during an excavation to protect tunnels, structures and utilities. Compensation of lost volume is achieved either by compaction or fracture grouting. Fracture grouting done with thin seams of grout in impermeable soft soils may show initial increase in volume but part of the increase may reduce subsequently with time. Field trials are required to assess the performance of compensation grouting done with thin grouts in soft ground.

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TR 26 : 2010 4.3.10 Minimise ground settlement Compensation grouting is done as an ‘observe-and-do’ process during excavation, whereas overall ground treatment in the active side of the wall may be done in advance to control ground settlement during deep excavation. Evenly spaced jet grout piles, grout mixed piles of mechanical deep mixing are most suited for this application. Such an overall treatment not only decreases the ground settlement but also decreases the active pressure on the retaining wall and thus the wall displacement.

Section Five – Ground anchors 5.1

General

This section applies to the design of temporary and permanent ground anchorages used for deep excavations to: a)

Support a temporary earth retaining structure;

b)

Support a permanent earth retaining structure;

c)

Provide the stability of slopes or cuts.

This is achieved by transmitting a tensile force to a load-bearing formation of competent soils or rocks. The following standards should be referred to: a)

BS EN 1537 : 2000 Execution of special geotechnical works – Ground anchors;

b)

BS 8081 : 1989 British standard code of practice for ground anchorages;

c)

BS EN 1997-1 : 2004 Eurocode 7: Geotechnical Design – Part 1: General Rules.

5.2

Terms and definitions

5.2.1

The main terms are used in common with BS 8081 : 1989 or Eurocodes, where applicable.

5.2.1.1 Permanent anchorage Anchorage with a design life of more than two years. 5.2.1.2 Temporary anchorage Anchorage with a design life of up to two years. 5.2.1.3 Removable anchorage Anchorage system that allows its components, i.e. strands, to be removed after its service life. 5.2.1.4 Acceptance test Load test on site to confirm that each anchorage meets the design requirements. 5.2.1.5 Suitability test Load test on site to confirm that a particular anchor design is adequate in particular ground conditions.

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TR 26 : 2010 5.2.1.6 Investigative test Load test to establish the ultimate resistance of an anchor at the grout/ground interface and to determine the characteristics of the anchorage in the design working load range. 5.2.1.7 Proof load The maximum test load to which an anchor is subjected. 5.2.1.8 Working load Safe load of an anchor. 5.2.1.9 Lock-off load The load transferred to an anchor head immediately on completion of a stressing operation. 5.2.1.10 Anchor holding piece Used in a removable anchorage. Component of anchor that is bonded directly to the grout and capable of transmitting the applied tensile load. 5.2.1.11 Unit anchors Term used in a removable anchorage to denote a pair of strands which loops around an anchor holding piece.

5.3

Factor of safety

5.3.1

The factor of safety to be used should not be less than those given in Table 4. Table 4 – Minimum safety factors recommended for design of individual anchorages Minimum safety factor

Proof load factor to working load (WL)

Anchorage category

Tendon failure

Ground/ grout interface

Grout/ tendon or grout/ encapsulation interface

Temporary anchorage with a service life of, say, up to two years where although the consequences of failure are quite serious, there is no danger to public safety without adequate warning*

1.6

2.5

2.5

1.25

Permanent anchorages and temporary anchorages where corrosion risk is high and/or the consequence of failure is serious*

2.0

3.0

3.0

1.5

* NOTE – There has to be adequate proper instrumentation and monitoring of wall movements and anchor loads.

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5.4

Design situations and actions

5.4.1

When selecting design situations, consideration should be given to:

a)

All circumstances during the construction of the structure.

b)

All anticipated circumstances during the design life of the structure.

c)

All pertinent limit states, both individually and in their combinations including, but not limited to, the following: i)

Structural failure of the tendon caused by applied stresses;

ii)

Distortion or corrosion of the anchor head;

iii)

Failure at the interface between the body of grout and the ground;

iv)

Failure of the bond between the steel tendon and the grout;

v)

For temporary anchor, failure by insufficient resistance of the holding piece;

vi)

Loss of anchorage force by excessive displacements of the anchor head or by creep and relaxation processes;

vii)

Failure or excessive deformation of parts of the structure due to the applied anchorage force;

viii)

Loss of overall stability of the retained ground and the retaining structure;

ix)

Interaction of groups of anchorages with the ground and adjoining structures.

d)

The groundwater and water pressures.

e)

The consequences of failure of any anchorage.

f)

The possibility that the forces applied to the anchorage during pre-stressing (anchorage load) may exceed the forces required for the design of the structure.

5.4.2

Design and construction considerations

5.4.2.1 The design of the anchorage and the specification for its execution should take into account any adverse effects of tensile stresses transmitted to ground beyond the vicinity of the anchorage. 5.4.2.2 The zone of ground into which tensile forces are to be transferred should be included in site investigations. 5.4.2.3 The anchor head should allow the tendon or rod to be stressed, proof-loaded and locked-off and, if required by the design, released, de-stressed and re-stressed. 5.4.2.4 For all types of anchorage, the anchor head should be designed to tolerate angular deviations of the anchor force (see BS EN 1537 : 2000) and be able to accommodate deformations which may occur during the design life of the structure. 5.4.2.5 Where different materials are used in an anchorage, their design strengths should be assessed with due account of the compatibility of their deformation performance.

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TR 26 : 2010 5.4.2.6 The only anchorage systems that should be used are those assessed by investigative tests (see BS EN 1537 : 2000) or for which successful comparable experience is documented in terms of both performance and durability. 5.4.2.7 The direction of the tendon should normally be such as to provide self-stressing with deformations due to potential failure mechanisms. In case this is not feasible, adverse effects should be taken into account in the design. 5.4.2.8 The characteristic resistance of anchor should be determined and verified on the basis of suitability tests. The design resistance should be checked by acceptance tests after execution. 5.4.2.9 The performance of the tendon free length of pre-stressed ground anchorages should be checked in accordance with BS 8081 : 1989 and BS EN 1537 : 2000.

5.5.

Design of the anchorage

5.5.1 The design of the anchorage should satisfy both the structural and geotechnical requirements. The structural capacity of the anchorage should be designed to support its design load. The number of strands should be calculated using Equation (5a), as follows: N = (Fs x WL) / (UTS) where N Fs UTS WL

Equation (5a)

= number of strands; = factor of safety, structural; = Ultimate tensile strength of strand, kN; = working load of anchor, kN.

Fs should be no less than the factor of safety given in Table 4.

5.5.2

The fixed length for the anchorage should be calculated using Equation (5b). Lfix = (Fg x WL) / (π x D x fs) where

Lfix D Fg fs

Equation (5b)

= required fixed length for each anchorage, m; = diameter of borehole, m; = factor of safety, geotechnical; = ultimate skin friction, kPa.

Fg should be no less than the factor of safety given in Table 4.

5.5.3

For temporary anchors which are to be removed after their service life, the strands are usually in loops. The bending of the strand at the end of such loop will result in reduction of strength of the strands. This reduction should be taken into account when calculating the number of strands required. The number of strands for removable anchor using loop system should be calculated as follows: N = (Fs x WL) / (UTS x Rd) where

N Fs UTS Rd WL

Equation (5c)

= number of strands; = factor of safety, structural (see Table 4); = Ultimate tensile strength of strand, kN; = reduction factor due to bending of strand; = working load of anchor, kN.

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TR 26 : 2010 5.5.4

Rd should not be more than 0.8. This value should be verified by appropriate tests on site for every batch of strand delivered to site.

5.5.5

The removable anchor using loop system usually consists of one or more unit anchors. The fixed length for each of this unit anchor should be calculated using Equation (5d). Lfix,i = (Fg x WLi) / (π x D x fsi) where

Lfix,i D Fg fsi WLi

Equation (5d)

= required fixed length for each unit anchorage, m; = diameter of borehole, m; = factor of safety, geotechnical (see Table 4); = ultimate skin friction at the location of the anchor holding piece, kPa; = working load of each unit anchor.

5.5.6

As skin friction may not be uniformly mobilised, the design of the fixed length of the unit anchor should take into consideration efficiency factor and should generally be not more than 10 m as recommended in BS 8081 : 1989.

5.5.7

For good practice, the investigative test anchor should be representative of the actual length of working anchors to be used in similar ground condition.

5.6

Checking of earth retaining wall movement

5.6.1

The most adverse combination of the minimum or maximum anchorage stiffness and minimum or maximum pre-stress should be selected when analysing the design situation.

5.6.2

When considering a non-prestressed anchorage as a (non-prestressed) spring, its stiffness should be selected to achieve compatibility between calculated displacements of the retained structure and the displacement and elongation of the anchorage.

5.6.3

Account should be taken of the effects of any deformations imposed on adjacent foundations by the anchorage pre-stress force.

5.7

Investigative tests

Investigative tests should be specified for anchorages and should be carried out before the installation of any working anchor. The size, depth and angle of borehole should be representative of the designed working anchor.

5.8

Suitability tests

5.8.1 Suitability tests should be specified for anchorages. The performance of the test should comply with BS 8081 : 1989 and BS EN 1537 : 2000, as appropriate. 5.8.2

At least three suitability tests should be performed for each distinct condition of ground and structure to determine or verify the characteristic resistance of the anchor for design.

5.8.3

The proof load of a suitability test of anchor should comply with BS 8081 : 1989 and BS EN 1537 : 2000, as appropriate.

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5.9

Acceptance tests

5.9.1

It should be specified in the design that all grouted anchorages should be subjected to acceptance tests prior to lock-off and before they become operational.

5.9.2

The procedure for acceptance tests should follow the rules given in BS 8081 : 1989 and BS EN 1537 : 2000.

5.9.3

The proof load of an acceptance test of anchor should comply with BS 8081 : 1989 and BS EN 1537 : 2000, where relevant.

5.9.4

Where groups of anchorages cross with tendon bond lengths at spacing of less than 1.5 m, random control tests should be made after completion of the lock-off action.

5.10

Tests on anchors

Depending on the number of ground anchors and site conditions, the number of tests to be carried out should not be less than those indicated in Table 5: Table 5 – Recommended number of field tests Type of test

5.11

Frequency

Investigative test

Minimum 2

Suitability test

Minimum 3 for each distinct ground condition

Acceptance test

Every working anchor

Pre-loading

Pre-loading for the anchorages should be based on the design value of pre-load assumed in the earth retaining wall analysis. The anchor lock-off load should be increased by 10 % of the design pre-load value to allow for losses. Consideration has to be given to the verification or confirmation of the actual lock-off load at the site.

5.12

Supervision and monitoring

Supervision and monitoring should follow the rules given in BS 8081 : 1989 and BS EN 1537 : 2000, where appropriate.

5.13

Corrosion protection of steel components of anchorage

The tendon of anchorages used for permanent earth retaining structure should be provided with at least two protective barriers to corrosion such that if one barrier is damaged during installation or anchor loading, the second barrier would remain intact. The anchor head and bearing plate of permanent anchor should be coated with anti-corrosion paint. The anchor head should be protected by a protective cap secured to the bearing plate with suitable seal. The void in the cap should be infilled with corrosion protection compound.

5.14

Maintenance of anchorages during service life

Provision, e.g. instruction manuals, should be made for the maintenance of the anchorages to verify and ensure that its structural capacity is not affected throughout its design service life. The anchor head and its components should be accessible at all times for maintenance, e.g. necessary restressing, periodic inspections and monitoring. 34

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Section Six – Impact assessment 6.1

General

Pre-construction building or condition surveys should be carried out to establish the condition of surrounding properties including obtaining the plans of existing buildings, structures, foundations and buried utilities where available.

6.2

Prediction of ground deformation

6.2.1

Each soil or rock type has its own problems and assessing the problems requires knowledge and understanding of the soils, soil profile and groundwater conditions. Movements in soils can be time-dependent. The prediction of the deformation of surrounding ground due to excavation should consider, but not limited to, the following: a)

All stages of work including wall construction or installation, and installation and removal of the support system;

b)

Construction tolerances;

c)

Potential loss of fines through the walls e.g. arising from flow of water;

d)

Movements during excavation;

e)

Drained, undrained and consolidation deformation;

f)

Effects of grouting, soil improvement, drilling, piling, or any other construction activities which may cause or induce ground movements.

6.2.2

Determining ground and wall movement is complex. Even advanced numerical methods such as finite element or finite difference may give misleading results for the development of ground movements outside the excavation. The predictions are likely to remain only approximate until further numerical analyses are validated and calibrated against field experience. It is not prudent to step outside the bounds of experience without full justification and careful observation of performance.

6.2.3

All buildings, structures, utilities, roads and any properties that may be or are likely to be affected by the excavation should be assessed for damage.

6.3

Damage assessment

6.3.1

A three-stage damage assessment may be adopted.

6.3.1.1 Stage 1 Assessment For preliminary assessment, a very simple approach may be adopted based on consideration of both maximum slope and maximum settlement of the ground surface at the location of each building. This approach uses ground surface rather than foundation level, displacements, and neglects any interaction between the stiffness of the buildings and the ground. According to Rankin (1988), a building which experiences a maximum slope of 1/500 and a settlement of less than 10 mm has negligible risk of damage. As such, no further assessment is required. However, this approach is not applicable for building/structure under mixed foundations or building/structure/utility or any property sensitive to ground deformation. They warrant more stringent movement criteria and a Stage 3 assessment should be carried out.

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TR 26 : 2010 6.3.1.2 Stage 2 Assessment The building or any structure is assessed based on “greenfield” assumption, i.e. the building or structure is assumed to have no stiffness so that it deforms with the ground or conforms to the “greenfield” ground deformation profile. For masonry structures, the building is represented by a simple beam whose foundation is assumed to follow the displacement of the ground in accordance with the “greenfield site” assumption. The maximum resultant tensile strains are calculated for both hogging and sagging settlements. The potential category of damage is then obtained from Table 6. 6.3.1.3 Stage 3 Assessment Detailed evaluation is carried out, taking into account building/structure stiffness and foundation. Stage 3 assessment is carried out for cases including: a)

Those buildings or structures on mixed foundations;

b)

Buildings/structures/utilities or any property that is unusually sensitive to ground deformation e.g. old masonry structures;

c)

Those deemed unsatisfactory as a result of Stage 2 assessment.

6.3.2

The approach is a refinement of the Stage 2 assessment in which the particular features of the building and the excavation and construction scheme are considered in detail. Each case is different and has to be treated on its own merits. Factors that should be taken more closely into account include: a)

Detailed approach: In the detailed evaluation in Stage 3 or in a structural appraisal of a building. Where applicable, the negative skin friction on the piles should be evaluated in the assessment. The framing of the superstructures, layout of the foundations and tie beams, reinforcement details in the structural members and connection details should be considered as a whole.

b)

Excavation or construction sequence: the sequence and method of excavation and construction should be given detailed consideration.

c)

Structural continuity: Buildings with structural continuity such as those of steel and concrete frame construction are less likely to suffer damage than those without structural continuity, e.g. masonry buildings or load-bearing masonry walls.

d)

Orientation of buildings: Deformation of a building oriented at a significant skew to the main ground may be subjected to warping or twisting effects.

e)

Soil-structure interaction: The predicted greenfield movements will be modified by the stiffness of the building. The detailed analysis of the problem should include soil-structure interactions. Analysis can include established procedures such as those published by the UK Institution of Structural Engineers.

f)

Previous movements: The building may have undergone or is still undergoing movements due to various causes. These effects should be assessed as they may reduce the tolerance of the building to future deformation. This is particularly so when the buildings or parts of its structures have cracks or exhibit signs of deformation.

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6.4

Masonry structures

6.4.1

The classification of damage to masonry structures or buildings should be based on the work of Boscardin and Cording (1989), Mair, Taylor and Burland (1996) and Boone et al. (1999) in accordance with Table 6. Table 6 – Damage category for masonry buildings Normal degree of severity

Limiting tensile strain (εlim) (%)

Negligible

0 - 0.05

Very slight

0.05 - 0.075

Slight

0.075 - 0.15

Moderate

0.15 - 0.3

Severe to very severe

> 0.3

6.4.2

The description of the damage associated by degrees of severity by Burland, Broms and De Mello (1977) is given in Annex B.

6.4.3

It should be noted that the classification in Annex B and Table 6 was developed for brickwork or blockwork and stone masonry. It may be adapted for other forms of cladding. It should not be applied to reinforced/pre-stressed concrete, steel or composite elements.

6.5

Reinforced/pre-stressed concrete structures

6.5.1

For reinforced/pre-stressed concrete structures, such as underpasses, underground structures, bridges and structural members of a building, the procedures outlined in 6.3.1 may be adopted. In addition, a serviceability performance review may be carried out.

6.5.2

Each building differs and has to be considered on its own merits. The existing condition of the building should be considered and the following factors included in the serviceability performance review: a)

Basement configuration;

b)

Cladding system;

c)

Construction sequence;

d)

Foundation system;

e)

Orientation to alignment;

f)

Soil-structure interaction;

g)

Structural continuity.

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6.6

High-rise buildings

Where foundation movement occurs, a building may suffer from both distortion and tilt. With high-rise buildings, however, the relatively large ratio of height to length of the structure usually ensures a predominantly rigid body rotation. High-rise buildings should be assessed on an individual basis to determine whether the tilt affects the serviceability of the building, which depends on purpose of the building, type of building superstructure and the type of foundation system.

6.7

Piled foundations

The construction of an excavation adjacent to existing piles requires consideration of the pile response to the ground movements as a result of the works, and an assessment of the consequential axial movement, lateral deflection, bending moment and rotation of pile head. The horizontal soil movement induced by the works will in turn induce additional lateral deflection and bending moment in the piles (Finno et al. 1991; Goh et al. 1996; Poulos and Chen 1997; Ong et al. 2002; Goh et al. 2003; see Annex C). Such effects should be appropriately considered and assessed.

6.8

Utilities

6.8.1. A damage assessment for every utility that may be affected should be carried out. The allowable values for settlement, deformation, joint rotation, joint slip or other such criteria as agreed with the utility agency should be established. The allowable values should be such that the utility can be kept fully functional during and after the works.

6.8.2 Particular attention should be given to the junction of pipes and spurs of pipes, as outlined by Attewell, Yeates and Selby (1996). For cast iron pipes, reference may be made to the methodology by Bracegirdle et al (1996).

6.9

Protective measures

6.9.1

Protective measures should be designed and installed with the aim to prevent damage and to satisfy the serviceability or functionality or acceptance criteria. Protective measures include underpinning, ground improvements, compaction or compensation grouting, jacking or building strengthening or some combination of these or other such means. Care should be taken to ensure that the selected method of protection does not do more harm than the original movements to the building.

6.9.2

Where there is presence of a layer of peat (soft and compressible peaty clay or soft soils with peat), judiciously designed water recharge wells should be put in place and operationally tested before any excavation work commences. This is particularly so where there are nearby buildings or surrounding structures which are on shallow foundations e.g. timber piles, footings or of mixed foundations, or any property which is sensitive to ground movements.

6.10

Limiting values of structural deformation and foundation movement

6.10.1 The components of foundation movement which should be considered include total settlement, relative or differential settlement, rotation, tilt, relative deflection, relative rotation and horizontal displacement. 6.10.2 Limiting values should be established for the foundation movements of neighbouring structures. Any differential movements of foundations leading to deformation in the supported structure should be limited to ensure that they do not lead to a limit state in the supported structure. The differential settlement should take account, among others, of the occurrence and rate of settlements and ground movements. 38

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TR 26 : 2010 6.10.3 The maximum acceptable relative rotations for open-framed structures, in-filled frames and load-bearing or continuous brick walls are unlikely to be the same. For such structures which deform in a sagging mode, they are likely to range from about 1/2000 to about 1/500, to prevent the occurrence of a serviceability limit state. A maximum relative rotation of 1/500 is acceptable for many structures. For a hogging mode, where the edge is settling more than other parts of the structure, the acceptable values should be half of those in sagging mode, i.e. less than 1/1000.

Section Seven – Instrumentation and monitoring 7.1

General

7.1.1

The instrumentation and monitoring scheme for the excavation works should be properly designed, supervised, coordinated and reviewed continuously by qualified and competent personnel who understand the objective of the monitoring and underlying principles for the purposes, including: a)

Verifying the assumptions made in the design;

b)

Providing confirmation of the predicted behaviour of the support system for the excavation and surrounding ground during excavation and construction works;

c)

Providing information and enabling assessment of the effects of the works on buildings including its own and other surrounding structures, utilities and other structures like roads;

d)

Providing a record of the performance of the works;

e)

Providing early warning, and to enable excavation and construction to be carried out safely without causing damage to any property at all stages as long as the excavation remains;

f)

Enabling appropriate protective and precautionary measures to be implemented in time so as to prevent any settlement or movement which may impair the stability of or cause damage to the whole or part of any surrounding premises or building, structures, roads and other properties.

7.1.2

The instrumentation and monitoring scheme should be appropriate to the objective and purposes of monitoring, particularly to safeguard the safety of persons such as workers within the site, persons like the public outside the site, and surrounding structures, and prevention of damage to any surrounding premises or buildings, structures, utilities, roads and other properties. The monitoring should extend around all sides of the excavation.

7.1.3

The choice of instruments should take into account the required accuracy of the measurements, reliability of the instruments and site conditions. Some allowance needs to be made for instrumentation failures during construction. Detailed guidance on geotechnical instrumentation is given in literature such as Hanna (1985) and Dunnicliff (1988).

7.1.4

Timely evaluation of results and assessments should be carried out at all appropriate stages, particularly where the data are required to provide an indication of safety. Changes in values between consecutive measurements should be examined.

7.2

Considerations for instrumentation

7.2.1

The accuracy and reading range of all monitoring instruments should be specified as part of the design of the retaining system for the excavation works.

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TR 26 : 2010 7.2.2

The depth of inclinometers should extend beyond the influence zone of the excavation to a position where negligible ground movement is anticipated to occur throughout the excavation and to where fixed toe conditions are expected to be achieved. The coordinates of the top of the inclinometer casing should be regularly surveyed as a check.

7.2.3

For monitoring the changes in ground water table, the depth of water standpipes/piezometers should extend beyond the expected drawdown level. In order to monitor the hydraulic uplift stability, some piezometers should be installed in the pervious soil layers beneath the final excavation depth within the excavation area. In areas where there is soft clay underlain by a pervious soil layer, piezometers should be installed in this layer to monitor the water drawdown which can potentially lead to consolidation settlement of the soft clay.

7.2.4

Tiltplates should be properly bonded to the surface of structural members on which they are installed after the removal of paint, tiles and plaster.

7.2.5

All instruments should be properly calibrated. Generally, instruments undergo a three-stage calibration, namely factory calibration, acceptance test and re-calibration. The factory calibration provides only a quality check for products in the manufacturing process which needs to be verified by acceptance test prior to installation in view of possible disturbance involved in the shipment works. Re-calibration will help to minimise possible instrument errors attributed to changes in instrument properties, e.g. misalignment of sensor, deviation in reference gauge reading (e.g. zero gauge reference) and elongation of measuring tape and cable.

7.2.6

The as-built condition of instruments including the co-ordinates, especially inclinometer casing and levels, e.g. piezometer tip level, of the installed instruments and any change which may be created to the installed instruments during monitoring period, such as change of top level of water standpipe and inclinometer, etc. should be properly captured, adjusted and considered in the monitoring and interpretation of results.

7.2.7

The design should include protection for all instruments to ensure that they are suitably protected against accidental damage, vandalism, and adverse climatic conditions.

7.3

Instrumentation and monitoring of structures

7.3.1 All buildings, structures, roads, utilities and any other property where any part thereof falls within the minimum monitoring zones defined in Figure 8, should be monitored. ≥ 2H

≥ 2H

H

Base of excavation

Figure 8 – Monitoring zone (minimum 2H) for buildings, structures, roads or utilities

7.3.2

In addition to instrumentation, monitoring of properties should include inspections, site observations, tests and records. Although the measurements usually include tiltmeters and precise levelling, for a building or structure which is more sensitive or subject to protective works, electro-level beam systems may also be used to monitor movements in the building or structure. Crack monitoring is carried out on the development of existing cracks and new cracks which may occur during the works, and on movement joints.

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TR 26 : 2010 7.3.3

For existing buildings, the pre-existing tilt and cracks in the structure, where appropriate, should be determined before the start of any construction and excavation activity.

7.3.4

All gas, water mains and sewer pipes which fall within a horizontal distance of less than the excavation depth from the excavation face should be more closely monitored. For more sensitive utilities, settlement points at closer intervals should be used, e.g. for electrical cables with voltage higher than 22kV and fibre optical systems.

7.4

Monitoring the performance of excavations

7.4.1

Excavations and their impacts on adjacent structures/utilities should be considered through monitoring of embedded retaining walls, ground and structural movements/deformations, basal heave, strut/anchor loads, groundwater table and piezometric pressures, etc. so as to provide data for design and review on the field performance shown in Table 7 as follows: Table 7 – General guidelines for instrumentation and monitoring Measurement of parameters

Instruments

Deformation of the ground (including any slope), retaining wall movements and supporting members

• • • • • • •

Deformation of structure

Inclinometers in the ground and/or in the wall Settlement points Magnetic extensometers Rod extensometers Prisms

• • • • •

Tiltmeters Settlement markers with precise levelling Crackmeters Electrolevel beams Prisms Tape extensometers Vibration sensors

Groundwater levels and pore pressures

• •

Piezometers Water standpipes

Loads on supporting structural members (struts), and anchors where appropriate

• •

Load cells Strain gauges

7.4.2

Control sections for excavation works should be identified and adequately instrumented with the validation or calibration between the design/predicted and actual values to be verified as early as possible during the construction and excavation stage.

7.4.3

Piezometers or water standpipes at active sides should be installed to monitor changes of pore water pressure or water level. Piezometers should be also installed on the excavation side to ensure toe stability, hydraulic uplift stability and piping stability etc when there are permeable soil layers below the excavation level.

7.4.4

When ground improvement is used to strengthen the earth retaining system for a deep excavation, heave and compression of the ground improvement layer should be monitored.

7.4.5

Struts and ground anchors should be monitored for load, preferably by using load cells. If struts are monitored with strain gauges only, the monitored struts should also be checked using load cells. Where strain gauges are used, a minimum of two strain gauges coupled with temperature monitoring should be installed at each monitoring location. 41

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TR 26 : 2010 7.4.6

There are several pitfalls for the load measurement in steel struts. The strain gauges for member forces should be properly installed, calibrated and checked for meaningful interpretation and monitoring as the readings could be affected by many factors such as non-uniform stresses, temperature, joints, strut installation and pre-loading.

7.4.7

Caution should be exercised when evaluating strut force by hydraulic jacking. In addition to the use of calibrated jacks, the loads should be independently confirmed by calibrated load cell.

7.4.8

The layout of the settlement markers should be able to reflect the differential settlement of the building.

7.5

Reading frequency of monitoring instruments

The design of the retaining system for the excavation works should include tables giving reading frequencies of all instruments. The frequency of reading the instruments may be varied, depending on the type of instrument and the relationship between the instrument and areas of current activity. Criteria for increasing the frequency of reading should also be given.

7.6

Review levels and interpretation of monitoring results

7.6.1

Instrumentation monitoring is provided to verify the design assumptions by comparing the actual response to excavations against predicted values. It also protects the functionality and safety of surrounding surfaces, roads, utilities, structures and any property by having preset acceptable limits on the stresses, loads, deformations, water levels and pressures measured.

7.6.2

Prior to the commencement of excavation, preset acceptable levels should be assigned to selected critical instrument as follows: a)

Check Level (CL) – 50 % of the Suspension Level;

b)

Alert Level (AL) – 70 % of the Suspension Level;

c)

Suspension Level (SL) – the limiting value allowed in the design.

7.6.3

Separate AL and SL values should be set for both positive and negative readings, where appropriate. These should include appropriate allowances for background reading fluctuations, tolerances etc.

7.6.4

The following should be taken into consideration when establishing the SL:

a)

Limits as given in relevant codes and regulations;

b)

Limiting loads and/or deflections in structural members;

c)

Limits on movement of the ground, surfaces, utilities or structures;

d)

Limits on water levels and/or pressures.

7.6.5

A limit on the lowering of water table outside the excavation should be set to minimise consolidation which, alone or in combination with other expected movements, could result in a settlement SL being exceeded.

7.6.6

Upper limits should be set for water pressures below the base of the excavation for each excavation stage such that design assumptions or parameters including hydraulic uplift, toe stability and piping should not be invalidated or stability in any other mode impaired.

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7.7

Multi-tier level monitoring and reviews

7.7.1 In addition to the SL values based on the most critical construction stage, preset limits should be determined for each stage of excavation and backfilling. These limits should be assigned to selected critical instruments that measure wall deflections, ground and building movements, loads or stresses in structural members and water pressures. The preset limits should usually be assigned at each stage of excavation, e.g. immediately prior to installation of struts, anchors or the base slab, and after removal of struts, immediately prior to backfilling. 7.7.2

Excavation and strut removal should only proceed if the monitored performance for the current stage is within established preset limits. Otherwise, the design should be reviewed with new analyses to estimate the final performance during all remaining stages and to establish that the SL will not be breached.

7.7.3

All the monitoring results should be reviewed in a timely manner to ensure that there is a reasonable agreement between the predicted and actual performance. At each review, the values, trends and patterns of readings and results should be compared against design predictions. All anomalous readings should be investigated and their significance evaluated and assessed. Where significant changes occur between readings, the reading frequency should be increased.

7.8

Full design reviews

7.8.1 If at any time before reaching an alert level or the alert level is breached and it becomes apparent from monitoring instrument readings that SL is likely to be exceeded, the instrumentation data, retaining wall and its supporting system design should be reviewed in detail and the works should be inspected on site for condition and compliance with the design intent. Remedial or protective measures should be implemented in time to ensure that the works are safe and that no breach of an SL will arise. In particular, the review should consider whether the site conditions are more onerous than those assumed in the design and whether the materials, specifications and workmanship in the earth retaining system fully satisfy all the design requirements. The review should also consider the likely trend of the readings to the end of the works and whether any SL is likely to be exceeded. 7.8.2

If the SL is reached, the related part of the works should immediately be made safe and all other works in that area stopped, pending the outcome of review. Construction should not proceed until proper remedial actions and all necessary measures are put in place. These have to fully ensure safety and robustness and entirely remove the possibility of exceeding the SL. Methods to compensate present and future excess loads or movements should be part of the remedial and protective measures undertaken.

7.8.3

The review should include full analyses with justifications and design validation with actual monitored behaviour to assess and check on requirements for stability of the excavations and construction works, and to ensure that all minimum mobilisation factors, load factors and safety factors, including robustness considerations of supporting system, are adequately met.

7.8.4

The back-analysis should be carried out to reflect the actual construction sequence and monitored behaviour. It should not be limited to curve-fitting of wall deflections.

7.8.4.1 The review of the design assumptions should include the appropriateness of the models used, soil parameters, wall stiffness, construction sequences, actual soil layering, drainage conditions, recorded pore water pressures, actual surcharges and applied preloads etc. Any changes or revisions made in the back-analysis should not be arbitrary. There should be a clear rationale, justification and substantiation of all changes adopted in the model and computation such as the results of field or laboratory tests or evidence from construction records. 7.8.4.2 If a back-analysis is carried out for one area of a site that results in major changes being made to any aspect of the modelling then the same changes have to be made to the analyses for all other areas of the site where the similar conditions apply. 43

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TR 26 : 2010

Section Eight – Safety of construction Safety of construction personnel is of paramount importance and should be prioritised when planning any excavation works. The management of risks and hazards of excavation works should not be left to the builder or the Workplace Safety and Health Officer (WSHO) but should be addressed right at the beginning of the design stage.

8.1

Risk assessment

In accordance with the Workplace Safety and Health (Risk Management) Regulations, a risk assessment should be conducted to assess the risk posed by the excavation works. Starting from the design stage, a risk assessment specific to excavation works should be done and the control measures determined to reduce the risk. The obligation to reduce the risk should be upon the person who creates the risk. The hazards identified should include, but not be limited to, the possible worst case scenarios such as: −

Collapse of excavated slope;

−

Failure or damage of soil retaining structure;

−

Damage of supporting structure;

−

Flooding;

−

Damage to surrounding structures.

In assessing the risks of the hazards identified, the risk should be reduced to an acceptable level through proposed control measures and these control measures should be implemented in the course of the work. The Risk Assessment should be reviewed in the event of a change of the design or in the scope of works.

8.2

Risk registry

The risk assessment should be kept in a risk register and the information made available to the builder. Vital information on safety, design and construction should be included in the risk register to enable effective communication of risks and hazards to the parties involved.

8.3

Permit-to-excavate

In accordance with the Workplace Safety and Health (Construction) Regulations: Clause 10, a permitto-excavate system should be implemented before the commencement of the excavation to ensure that: a)

The excavation work can be carried out with due regard to the safety and health of the workers;

b)

The workers are informed of the hazards associated with such work and the precautions they have to take;

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TR 26 : 2010 c)

The necessary safety precautions are taken and enforced when such work is being carried out.

8.4

Site inspection

Before any work commences, the site should be examined for signs of cave-ins, failure of earth retaining systems, toxic or hazardous substances or atmospheres. Inspections should be carried out as follows: −

Daily and before work commences;

−

When there are tension cracks, sloughing, undercutting, water seepage or bulging, or when the soil is disturbed;

−

When the size, location or placement of the spoil heap changes;

−

When any movement of the adjacent structures is noticed;

−

When movement of the retaining structure is beyond expected reading;

−

After every rainstorm or seismic activity.

Any possible signs of cave-ins, failure of earth retaining systems, toxic or hazardous substances or atmospheres should be thoroughly investigated before any work can commence. Site personnel responsible for the daily site inspections should be adequately trained to identify possible signs of slope failure or failure of structural support.

8.5

Verification of site findings with designer

Often, site personnel are endangered in excavation works because the design intent, assumptions and excavation methods are not verified with the designer. This could lead to a failure of the excavation site, resulting in accidents. To ensure a safe excavation, the person in charge of excavation should: −

Confirm design intent with the designer.

−

Confirm assumptions to be verified on site with the designer. The verification of the assumptions is usually done through instrumentation readings which should be obtained on a regular basis and submitted to the designer for verification.

−

Confirm excavation method with the designer. The person in charge of excavation should follow the step-by-step procedure as agreed with the designer for excavation to ensure safety of the site and hence construction personnel.

−

Confirm inspection findings with the designer if any sign of soil or structure failure is observed.

−

Confirm instrumentation readings with designer to ensure they are within the allowable limits.

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TR 26 : 2010

8.6

Training and supervision

All workers on an excavation site should be trained and supervised during their course of work. The training programme should include, but not be limited to: −

Safe methods of excavation;

−

Identification of hazards related to the use of excavation plant and equipment;

−

Use of personal protective equipment (PPE);

−

Emergency procedures.

Employers have to take reasonably practicable steps to ensure a safe working environment for their workers and that workers are adequately prepared for their course of work.

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TR 26 : 2010

Annex A Guidance on descriptions and weathering classifications A.1

General

A.1.1 This Annex provides guidance on the various ways of describing and classifying weathered rocks as outlined in BS 5930 : 1999. It is intended to ensure consistent application of BS 5930, but not to change any requirement of the Standard. Reference should also be made to ‘The description and classification of weathered rocks for engineering purposes’, Quarterly Journal of Engineering Geology, 28, 207 – 242 (1995), for further details of the working party report that was used in the preparation of BS 5930 :1999.

A.1.2 All site investigation work should comply with the current version of BS 5930. A.1.3 Personnel: The soil and rock descriptions which appear on final bore logs should be prepared by a geologist, engineering geologist or geotechnical engineer and based on inspection of the samples retrieved, driller’s logs, site supervisor’s records, in-situ tests, laboratory tests and laboratory descriptions. See also Section 17 on Personnel for Ground Investigation, BS 5930 : 1999.

A.2

Background

A.2.1 Before the Mass Rapid Transit (MRT) projects in 1983, the weathering classification system in Singapore closely followed Public Works Department (PWD) standards which were based on the BS standards and proposals made by the Geological Society Engineering Group Working Party in 1970 (Anon 1970). However, a simplified weathering classification system was developed in 1983 for the first Phase of MRT and became general use. At the end of 2001/early 2002, weathering classifications based on BS 5930 :1999 came into use.

A.3

Description and classification of rocks

A.3.1 All samples of rocks have to be fully described and classified as outlined in Section 6 of BS 5930. It provides different approaches for classifying material scale weathering in rocks. These are Approach 2 (for rocks that are moderately strong or stronger in a fresh state), Approach 4 (rocks that are moderately weak or weaker in a fresh state) or Approach 5 (special cases such as karst).

A.3.2 The factual description of the material, including comments on weathering, in Approach 1 in Table 19 of BS 5930 :1999 should be followed in all cases. A.3.3 Weathering grade classifications at a material scale should be shown on the borelog. These classifications should be based on Approach 2 (Bukit Timah granite, Gombak norite and Jurong formation) and Approach 4 (Old alluvium), as seen in Table 19 of BS 5930. There should be no attempt to assess heterogeneous mass classifications (Approach 3) on the borehole records.

A.4

Basis for assessing material scale weathering classifications for rocks in Singapore

A.4.1 Table A.1 gives a description of the soils and rock types in relation to their geological origins.

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TR 26 : 2010 Table A.1 – Description of soil and rock types Notations

Soil and rock types

General description

Geological formation (PWD, 1976)

B

Beach (Littoral)

Sandy, sometimes silty, with gravels, coral and shells

Kallang Littoral, possibly also part of all other members & Tekong

E

Estuarine (Transitional)

Peats, peaty and organic clays, organic sands

Kallang Transitional, possibly part of Alluvial and Marine

F

Fluvial (Alluvial)

Sands, silty sands, silts and clays

Kallang Alluvial, possibly part of all other members and Tekong

F1

Predominantly granular soils including silty sands, clayey sands and sandy silts

Bed of Alluvial Member of Kallang

F2

Cohesive soils including silty clays, sandy clays and clayey silts

Bed of Alluvial Member of Kallang

M

Marine

Very soft to soft blue or grey clay

Kallang Marine Member.

O

Old alluvium

Very weak to weak beds of sandstone and mudstone. See Table A.5 for weathering classification

Old alluvium

FC

Fort Canning boulder bed (also known as S3, bouldery clay or boulder bed)

A colluvial deposit of boulders in a soil matrix. The matrix is typically a hard silty clay but can be granular. The material is largely derived from the rocks and weathered rocks of the Jurong formation.

Not shown in Geology of Singapore, PWD (1976)

S

Sedimentaries (rocks & associated soils)

Sandstones, siltstones, mudstones, conglomerate and limestone. The rock has been subjected to a varying degree of metamorphism. See Table A.4 for weathering classification.

Jurong, Tengah, Rimau, Ayer Chawan and Queenstown Facies (and Pandan Limestone, which was not identified in Geology of Singapore, PWD (1976))

G

Granite (rock and associated residual soils)

Granitic rocks, including granodiorite, adamellite and granite. See Table A.3 for weathering classification.

Bukit Timah granite

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TR 26 : 2010 A.4.2 Table A.2 gives rock weathering classification. Over the past decades, various systems for the classification of the weathering of the old alluvium have been in use. Reference should be made to the corresponding reports for the basis of the classification systems. Table A.2 – Rock weathering classification Geological notations

Grade/ Class

Classification

Characteristics

SI & GI

I

Fresh

Unchanged from original state

SII & GII

II

Slightly weathered

Slight discoloration, slight weakening

SIII & GIII

III

Moderately weathered

Considerably weakened, penetrative discoloration, large pieces cannot be broken by hand

SIV & GIV

IV

Highly weathered

Large pieces cannot be broken by hand, does not readily disaggregate (slake) when dry sample is immersed in water

SV & GV

V

Completely weathered

Considerably weakened, slakes, original texture apparent

SVI & GVI

VI

Residual soil

Soil derived by in-situ weathering but retaining none of the original texture or fabric

OA

A

Unweathered

Original strength, colour, fracture spacing

OB

B

Partially weathered

Slightly reduced strength, slightly closer fracture spacing, weathering penetrating in from fractures, brown oxidation

OC

C

Distinctly weathered

Further weakened, much closer fracture spacing, grey reduction

OD

D

Destructured

Greatly weakened, mottled, ordered lithorelics in matrix becoming weakened and disordered, bedding disturbed

OE

E

Residual soil

Matrix with occasional altered random or ‘apparent’ lithorelics, bedding destroyed. Classed as re-worked when foreign inclusions are present as a result of transportation

A.4.3 Tables A.3 to A.5 for classifying weathered rocks provide the basis for establishing weathering descriptions under BS 5930 : 1999. However, it is not necessarily the case that all of the weathering grades will be present at a particular location.

A.5

Bukit Timah granite and Gombak norite

A.5.1 Approach 2 follows the system originally devised by Moye in 1955 for granite in Australia, and which has been used for many years for granite in Hong Kong. The igneous rocks of Bukit Timah granite and Gombak norite should be described using Approach 2 for classification.

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TR 26 : 2010 A.5.2 The Geotechnical Engineering Office (GEO) in Hong Kong produced a list of simple indicators for the assessment of weathering grades in Hong Kong. A slightly amended version is given in Table A.3. These indicators may also be used in the igneous rocks of Singapore, which have a similar strength when fresh.

A.5.3 It is particularly important to distinguish between grades III, IV, V and VI, where present, as there is a significant difference in the engineering behaviour of each of these grades. Typically, the weathered granite in Singapore has a thick mantle of residual soil, with only limited Grade V and/or Grade IV materials below the residual soil. In order to identify these during site investigation, careful and closer sampling has to be carried out once the SPT value reaches 30 blows/300 mm. The ‘Classifiers’ are provided in BS 5930 (see Table A.2). Table A.3 – Bukit Timah granite and Gombak norite Weathering classification for Bukit Timah granite and Gombak norite

A.6

Grade

Basis for assessment

I

Intact strength, unaffected by weathering. Not broken easily by hammer – rings when struck. No visible discolouration.

II

Not broken easily by hammer – rings when struck. Fresh rock colours generally retained but stained near joint surfaces.

III

Cannot be broken by hand. Easily broken by hammer. Makes a dull or slight ringing sound when struck with hammer. Stained throughout.

IV

Core can be broken by hand. Does not slake in water. Completely discoloured.

V

Original rock texture preserved, can be crumbled by hand. Slakes in water. Completely discoloured.

VI

Original rock structure completely destroyed. Can be crumbled by hand.

Jurong formation

A.6.1 The Jurong formation includes a variety of sedimentary rocks that have been subjected to a variable degree of metamorphism. Where observed, such evidence of metamorphism has to be described on the borehole record. In some cases, it is appropriate to use a term indicating a metamorphic rock, rather than a sedimentary rock, in the Jurong formation i.e. quartzite rather than sandstone.

A.6.2 The engineering properties are affected by factors such as lithology, stress history, degree of cementation/lithifaction and degree of metamorphism or silicification. The degree of weathering is therefore only one factor in assessing the engineering properties of the ground. It is essential that the description of the material covers all observable features, as required in the BS 5930 : 1999, and not just weathering. A.6.3 The rocks of the Jurong formation exhibit a wide range of strength in the fresh state, and weather in different ways. The generally weak mudrocks weather in a way that is best described by Approach 4, while the stronger sandstones and conglomerates weather in a way similar to Approach 2. Where the rock is thinly bedded, which is the case in much of the formation, it is considered impractical to apply different approaches. Approach 2 will thus be used for the classification of Jurong formation wherever it is thinly bedded, and in all cases to sandstone, quartzite, siltstone, shale and conglomerate. The formation also includes the pandan limestone. Where mudstone or pandan limestone is predominant in an area, then Approach 4 may be used for the mudstone and Approach 5 for the limestone. The methods for assessing the weathering grade under Approach 2 are given in Table A.4.

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TR 26 : 2010 A.6.4 For Approaches 4 and 5, reference should be made to BS 5930. Table A.4 – Jurong formation Weathering classification for Jurong formation Grade

A.7

Basis for assessment

I

Intact strength, unaffected by weathering

II

Slightly weakened, slight discolouration, particularly along joints.

III

Considerably weakened & discoloured, but larger pieces cannot be broken by hand. (Rock Quality Designation, RQD is generally >0, but RQD should not be used as the major criterion for assessment).

IV

Core can be broken by hand or consists of gravel size pieces. Generally highly to very highly fractured, but majority of sample consists of lithorelics. (RQD generally = 0, but RQD should not be used as the major guide for assessment). For siltstone, shale, sandstone, quartzite and conglomerate, the slake test can be used to differentiate between Grade V (slakes) and Grade IV (does not slake).

V

Rock weathered down to soil-like material, but bedding intact. Material slakes in water.

VI

Bedding destroyed

Old alluvium

A.7.1 The old alluvium is an alluvial deposit that has been variably cemented, often to the extent that it has the strength of a very weak or weak rock. The upper zone of the old alluvium has typically been affected by weathering, and it is important that this weathering is described and classified. The use of Approach 4 of BS 5930 is recommended. However, although weathering of feldspars and mottling may be observed in borehole samples, it is generally difficult to assess the weathering grade from the visual inspection of samples obtained from boreholes.

A.7.2 It is common practice in Singapore to use the blow count from SPT testing as an indicator of weathering classification. It has to be understood that the SPT result is influenced by factors other than weathering. These other factors would include depositional environment, degree of cementation and the equipment and method used for the testing. Table A.5 gives guidance on the typical SPT values for different weathering grades, although the final classification should be based on an assessment of both SPT and careful inspection of the samples recovered. Where possible, the correlation with SPTs should be confirmed by comparing with any nearby large scale exposures of the old alluvium. Care has to be taken in the SPT testing to ensure a representative test result, and classification should not be based on SPT alone. It should be noted that layers of hardpan can be found in the weathered old alluvium, and very high SPT values or refusal of the CPT may be due to hardpan rather than a change in weathering grade. Conversely, there may be a sudden drop in SPT blow count if a layer of uniform sand is encountered in a borehole which is not full of stabilising fluid. Unlike the Bukit Timah granite or the Jurong formation, there is little evidence of joints in the old alluvium. As a result, the weathering has typically penetrated as a discernible front from the surface. It is therefore unlikely that there will be more weathered beds under less weathered beds. Other factors, such as those given above, are likely to be the cause of a sudden drop in SPT as the hole is advanced.

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TR 26 : 2010 Table A.5 – Old alluvium Weathering classification for old alluvium Class

Classifier

Characteristics

Indicative SPT, blows/300 mm*

A

Unweathered

Original strength

B

Partially weathered

Slightly reduced strength

C

Distinctly weathered

Further weakened

30 to 50

D

Destructured

Greatly weakened, often mottled, bedding disturbed

10 to 30

E

Residual

Bedding destroyed