Soil-Structure Interaction. [PDF]

Zitiervorschau

Soil-Structure interaction - Stabilization of ground for geotechnical structures.

Ahmad Safuan A Rashid

Contents • Introduction • Soil Structure Interaction (SSI) • Wave propagation in soil • Stabilization of Ground for Geotechnical Structures

Geotechnical Engineering

*All Civil Engineering begins with Geotechnical Engineering

Typical Geotechnical Project Laboratory ~ for testing

soil properties

construction site 4

Design Office ~ for design & analysis

Introduction • Traditional soil mechanics and geotechnical engineering design was predominantly concerned with soil strength and stiffness so that the design engineer could define the failure state of the soil and/or control excessive deformation of the soil/structure. • In these circumstances, the loading is considered static and the strain imposed within the soil may vary from about 10–3 (in service) to a few percent (at failure). • Many circumstances where cyclic (dynamic) loads are applied to the soil either by natural forces such as earthquakes (seismic), wind and water waves, or from manmade sources such as bomb blasts, traffic loads and machine foundations.

Introduction • Serviceability – settlement, tilted • Failure - collapse

Introduction • The magnitudes of these dynamic loads are generally much smaller than most static loads and generate strains within the soil as low as 10–6. • Although the magnitudes of these dynamic loads are often much smaller than static loads, inertial forces may become important and must be considered in geotechnical design.

Introduction • Inertia: • The tendency of an object to resist being moved or , if the object is moving, to resist a change in speed or direction. • Newton’s first law says that objects do not accelerate spontaneously. • This property of matter, which causes objects to resist acceleration , has been named “inertia”. • Newton’s First Law is often called the Law of Inertia.

Introduction • Most of the civil engineering structures involve some type of structural element with direct contact with ground. • When the external forces, such as earthquakes, act on these systems, either the structural displacements or the ground displacements, are dependent of each other. • Even though the foundation structure and the supporting soil are two different physical entities, they form a system, and in terms of the mechanics of behaviour of such systems, one component influences the behaviour of the other, the end product being the result of this mutual action, or interaction, between the two. • This is the essence of soil-structure interaction.

Introduction • The process in which the response of the soil influences the motion of the structure and the motion of the structure influences the response of the soil is termed as soilstructure interaction (SSI). • Conventional structural design methods neglect the SSI effects. Neglecting SSI is reasonable for light structures in relatively stiff soil such as low rise buildings and simple rigid retaining walls. • The effect of SSI, however, becomes prominent for heavy structures resting on relatively soft soils for example nuclear power plants, high-rise buildings and elevated-highways on soft soil.

Introduction • Damage sustained in recent earthquakes, such as the 1995 Kobe earthquake, have also highlighted that the seismic behavior of a structure is highly influenced not only by the response of the superstructure, but also by the response of the foundation and the ground as well. • Hence, the modern seismic design codes, such as Standard Specifications for Concrete Structures: Seismic Performance Verification JSCE 2005 stipulate that the response analysis should be conducted by taking into consideration a whole structural system including superstructure, foundation and ground.

Soil-Structure Interaction • The process in which the response of the soil influences the motion of the structure and the motion of the structure influences the response of the soil is termed as soilstructure interaction (SSI). • Stiffness relates increments of stress and increments of strain. A knowledge of soil stiffness is required to calculate ground movements and to obtain solutions to problems of soil–structure interaction, such as loads on retaining walls.

Soil-Structure Interaction • When an earthquake occurs, the building and the ground vibrate with influencing each other. • This phenomenon is called “Dynamic Soil Structure Interaction” and is recognized as being very important for seismic design of structure.

Soil-Structure Interaction • Role of Foundation • Under normal condition • Supporting the dead weight and the live load of the building. • Transmitting these loads to the ground.

• During an earthquake • Transmitting the ground motion to the building • Bearing the building vibrations and transmitting them to the ground.

• The ground and the building influence each other through the foundation and this is called the Dynamic Soil Structure Interaction.

Soil-Structure Interaction • Degree of influence of SSI on Response of Building depends on: • Stiffness of Ground • Dynamic characteristic of building itself, that is natural period, damping factor. • Foundation types.

a) Spread Foundation b) Pile Foundation without basement

c) Basement without d) Basement with pile piles

Soil-Structure Interaction • The position where the SSI takes place. • a) through the bottom surface of the foundation. • b) through the pile foundation. • c) through the bottom surface and side wall surface. • d) through the basement surface and the piles. • The influence of the SSI becomes remarkable and more complicated as the amount of contact between the ground and the foundation increases sequentially in the order from (a) to (d)

Soil-Structure Interaction • Interaction between ground and building during earthquake event. • When the seismic wave, E0 generated by the earthquake fault reaches the bottom of the foundation, they divided into two types: • The wave E1 entering into the building and the waves F0 being reflected back into the ground. • The wave E1 is called the transmission wave while the wave F0 is called the reflection wave.

Soil-Structure Interaction • The transmission wave , E1 entering into the building travel towards the top of the building with subjecting the building to vibration. • And then, they reflected at the top and travel back down to the foundation. Here, a crucial phenomenon occurs for consideration of the SSI.

Soil-Structure Interaction • When the waves that are reflected at the top of the building and travelling downwards, F1 reach the foundation, a part of them is transmitted into the ground, while the rest is reflected back again and once starts to move upwards through the building. • The former waves, escaping into the ground are called “Radiation waves”, R1.

Soil-Structure Interaction • When the amount of these radiation waves are small, the seismic waves once transmitted into the building, and the building continues to vibrate for a long time. • The apparent vibration condition becomes the same of the small damping of the building. • The damping caused by escape of the seismic waves, which have been transmitted into the building back into the ground is called “Radiation damping”

Soil-Structure Interaction • When the building foundation is forced to vibrate vertically, a stress called “Contact Earth Pressure” is caused at the boundary between the bottom surface of the foundation and the ground. • The distribution of this contact earth pressure over the

bottom surface of the foundation is called the contact earth pressure distribution. Three type of distribution (Rigid – foundation rigidity, Uniform – soil classification and Parabolic distribution – soil nonlinearity) are observed.

Soil-Structure Interaction • For foundation and substructure solutions with an increased complexity more involved methods of calculating the forces within, and displacements of, structural elements are required. • The use of the more basic approaches that rely heavily on hand calculation or ‘rules of thumb’ may not be appropriate in such situations as there may be too many degrees of indeterminacy within the problem or they may simply be too time consuming to use. • The use of finite element models allows the user to create a soil–structure model of the whole or part of a problem with relative ease.

Soil-Structure Interaction • This model can generally then be modified to respond to changes in a design or undertake parametric studies that might otherwise take a great deal of time. • Soil–structure models can also provide us with a means of predicting the impact of one structure on another. Displacements and stress changes can be calculated for different construction phases.

Wave propagation in soil • For soils, three types of waves are readily encountered and are of importance. The first two are termed body waves, which are propagated within the soil and comprise the compressional wave (P-wave) and shear wave (S-wave). • In relatively soft saturated near surface sediments (which is likely to be the case for a large number of geotechnical problems) the P-wave (Vp) is dominated by the bulk modulus of the pore fluid (the water is ‘hard’ compared to the soil) and the resultant Vp may be close to that of the pore fluid. If the soil is unsaturated, Vp can range from that of the soil matrix (with no pore fluid) to that for the saturated case. Therefore the use of Vp to determine the properties of soils is problematic.

Wave propagation in soil • However, pore fluids do not carry shear stresses, so the velocity of S-waves (Vs) is only influenced by the soil and not by the pore fluid. It can also be shown that during most dynamic loading events, it is cyclic changes in shear stresses that influence the behaviour of the soil. From the theory of propagating waves it can be shown that the wave velocity through the soil is related to the stiffness of the soil by G = ρVs2 (1) • where G is the shear modulus of the soil, and ρ is the soil density. Therefore changes in Vs within a soil can be used to determine the small strain stiffness, and measurement of Swaves is commonly employed to determine the dynamic behaviour of soils at small strains.

Wave propagation in soil

Wave propagation in soil • Shear Wave: A wave in which the disturbance is an elastic deformation perpendicular to the direction of motion of the wave. (Shear waves are also called 'transverse waves.') Compression Wave: A wave in which the disturbance is a compression of the medium. (Compression waves are also called 'longitudinal waves.')

Wave propagation in soil • The third wave that is frequently encountered in soils is the Rayleigh wave, which travels along the ground surface. • Rayleigh wave measurements have gained in importance in recent years since ground surface measurements can be readily and easily undertaken without the need for any intrusive investigations, such as those required for measuring P-waves or S-waves. There is no direct link between the velocity of Rayleigh waves (VR) and soil stiffness (Young's modulus, E or G), however, it has been shown that for nearly all values of Poisson’s ratio, Vs ≈ 1.09VR (Richart et al., 1970). Therefore measurements of Rayleigh waves can give a good determination of shear wave velocities (Hiltunen and Woods, 1988) from which G can be derived.

Wave propagation in soil

Stabilization of Ground for Geotechnical Structure • In both seismically active and inactive areas, soil improvement techniques are commonly used a sites where the existing soil conditions are expected to lead to unsatisfactory performance. • Unsatisfactory performance can take many forms, but usually involve unacceptably large soil movements. • The movements may include horizontal or vertical (or both) components and may take place during and/or after earthquake shaking. • In the absence of the earthquake shaking, unacceptable movement usually result from insufficient soil strength and/or stiffness.

Stabilization of Ground for Geotechnical Structure • Consequently, most soil improvement were developed to the increase the strength and stiffness of soil deposits. • During the earthquake, the build-up of excess pore water pressure can lead to very large deformation. • The most common soil improvement technique can be divided into 4 majors categories: • • • •

Densification Techniques Reinforcement Techniques Grouting and Mixing Techniques Drainage Techniques

Densification Techniques • The particles that comprise a particular soil can be arranged in many different ways. • The strength and stiffness of the soil is higher hen the particles are packed in a dense configuration that when they packed loosely. • Also the tendency to generate positive excess porewater pressure due to cyclic loading is lower when the soil is dense that when it is loose. • As a result, densification is one of the most effective and commonly used means of improving soil characteristic for mitigation of seismic hazard.

Densification Technieques • The most common approaches to densification include vibro techniques, dynamic compaction, blasting and compaction grouting.

Vibro techniques • Vibro techniques use probes that are vibrated through a soil deposit in a grid pattern to densify the soil over the entire thickness of the deposit. • Vibro techniques can be divided into those based on horizontal vibration (vibroflotation) and those based on vertical vibration (vibro rod systems). • Vibro techniques are among the most commonly used techniques for mitigation of seismic hazards.

Vibro techniques

vibro rod systems

vibroflotation

Dynamic compaction • Dynamic compaction is performed by repeatedly dropping a heavy weight in a grid pattern on the ground surface. • The weight usually constructed of steel plates generally range from 6 to 30 tons. • Drop height usually range from about 10 to 30 m.

Blasting • Loose granular soils have also been compacted by blasting. • Blasting densification involves the detonation of multiple explosive charges vertically spaced (3 to 6 m) apart in drilled boreholes. The boreholes are usually space between 5 to 15 m) apart and backfilled prior detonation.

Compaction Grouting • Soft or weak soils can be densified by injecting a very low slump (2.5 cm) grout into the soil under high pressure. • Because the grout is highly viscous, it forms an intact bulb or column that densifies the surrounding soil by displacement. • Compaction grouting may be performed at a series of points in a grid or along a line. • Ground point spacing ranging from 1 to 4.6 m have been used. • Compaction grouting may be used to remedy foundation settlement problem as well.

Reinforcement Techniques • In some cases it is possible to improve the strength and stiffness of an existing soil deposit by installing discrete inclusions that reinforce the soil. • These inclusion may consist of structural materials such as steel, concrete, or timber and geomaterials such as densified gravel.

Stone Column • Soils deposits can be improved by the installation of dense columns of gravel known as stone columns. • Stone columns may be used in both fine and coarse grained soils. • In fine grained soil, stone columns are usually used to increase shear strength beneath structures and embankment by accelerating consolidation and introducing column of stronger material. • For mitigation of seismic hazards, they are commonly used for improvement of liquefiable soil deposits.

Compaction Pile • Granular soils can be improved by the installation of compaction piles. • Compaction piles are displacement piles, usually prestressed concreted or timber, that are riven into a loose sand or gravel deposit in a grid pattern and left there. • Compaction piles improve the seismic performance of a soil deposit by three different mechanism. • The flexural strength of the piles themselves provides resistance to soil movement. • The vibrations and displacement produced by their installation cause densification. • The installation process increases the lateral stresses in the soil surrounding the piles.

Grouting and Mixing Techniques • The engineering characteristics of many soil deposits can be improved by injecting or mixing cementitious materials into the soil. • These materials both strengthen the contacts between soil grains and fill the void space between the grains. • Grouting techniques involve the injection of such materials into the voids of the soil or into fractures in the soil so that the particle structure of the majority of the soil remains intact. • Mixing techniques introduce cementitious materials by physically mixing them with soil, completely disturbing the particle structure of the soil.

Grouting and Mixing Techniques • The mixing can hydraulically.

be

accomplished

mechanically

or

• Both techniques tend to be expensive but can often be can often be accomplished with minimal settlement or vibration. • As a result, both techniques can be often be used in situations where other soil improvement techniques cannot work.

Grouting • The term grouting is used to describe a variety of processed by which cementitious material is introduced into the ground. • Grouting techniques are often classified according to the method by which the grout is placed in the ground. • A special pump with a high pressure is used to inject the particulate grout aqueous suspension of cement based into a desired depth.

Mixing • Localized improvement of soil columns can be achieved by in situ mixing of the soil with cementitious materials. • Deep mixing is carried out in situ using a machine equipped withmixing blades mounted at the end of a tube that has a nozzle at the lower end. • The stabilizer agent is injected (dry or wet) into the soil via the nozzle using a pumping system so that it mixes with the soil as the blades are rotated.

Drainage Technique • Unacceptable movement of slopes, embankment, retaining structures and foundations can frequently be eliminated by lowering the groundwater table prior to earthquake shaking. • A number of dewatering techniques have been developed and proven useful in engineering practice. • These standard techniques may be used to increase the stiffness and strength of a soil deposit for mitigation of seismic as well as nonseismic hazards. • The installation of stone column introduce a column of freely draining gravel into a liquefiable soil deposit.

Drainage Technique • The installation of Prefabricated Vertical Drain also could accelerate the consolidation process of soft soil area.