Showing papers in "Geotechnique in 2003"

Soil deformation measurement using particle image velocimetry (PIV) and photogrammetry

Abstract: A deformation measurement system based on particle image velocimetry (PIV) and close-range photogrammetry has been developed for use in geotechnical testing. In this paper, the theory underlying this system is described, and the performance is validated. Digital photography is used to capture images of planar soil deformation. Using PIV, the movement of a fine mesh of soil patches is measured to a high precision. Since PIV operates on the image texture, intrusive target markers need not be installed in the observed soil. The resulting displacement vectors are converted from image space to object space using a photogrammetric transformation. A series of validation experiments are reported. These demonstrate that the precision, accuracy and resolution of the system are an order of magnitude higher than previous image-based deformation methods, and are comparable to local instrumentation used in element testing. This performance is achieved concurrent with an order of magnitude increase in the number of meas...

Coupling of hydraulic hysteresis and stress–strain behaviour in unsaturated soils

Abstract: Consideration of the different roles of pore air pressure, pore water pressure within bulk water and pore water pressure within meniscus water suggests that the degree of saturation will have a significant influence on the stress–strain behaviour of an unsaturated soil, in addition to any influence of suction. This suggestion is supported by experimental evidence. In the light of this, a new elasto-plastic framework for unsaturated soils is proposed, involving coupling of hydraulic hysteresis and mechanical behaviour. Within the proposed framework, plastic changes of degree of saturation influence the stress–strain behaviour, and plastic volumetric strains influence the water retention behaviour. A specific constitutive model for isotropic stress states is proposed, and model predictions are compared with experimental results, in order to demonstrate some of the capabilities of the new framework. Forms of behaviour that can be represented include proper transitions between saturated and unsaturated types ...

An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour

Abstract: The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress, which includes the average fluid pressure acting on the soil pores, and an additional scalar constitutive variable, ξ, related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. The formulation of the model in terms of variables closely related to specific behaviour mechanisms leads to a remarkable unification of experimental results of tests carried out with different suctions. The analysis of experimental isotropic compression data strongly suggests that the quotient between the void ratio, e, of an unsaturated soil and the void ratio es, corresponding to the saturated state at the same average soil sk...

Mechanical behaviour of heavily compacted bentonite under high suction changes

Abstract: The paper reports the results of an experimental study carried out on a bentonite compacted to a dry density of up to 1·7 Mg/m3, a high value for this type of soil. The soil fabric has been studied using a variety of techniques, revealing a clear bimodal pore distribution that corresponds to two distinct structural levels: a microstructural one and a macrostructural one. The main testing programme has been performed using oedometers especially designed to apply a very large range of suctions. By applying the axis-translation technique (using nitrogen as the gas fluid), it has been possible to reach suctions up to 15 MPa. The higher suction range has been achieved by applying a controlled atmosphere where the relative humidity has been fixed by a solution of sulphuric acid or salts. In this way suctions up to 550 MPa could be reached. The maximum vertical stress that could be applied in the apparatus was 10 MPa. Two types of test have been carried out: (a) tests in which a combination of loading paths at constant suction and drying/wetting paths at constant load were applied; (b) swelling tests under constant-volume conditions in order to determine the swelling pressure and the stress path followed during wetting. The results of the experimental programme are examined, taking into account the role of the soil fabric in controlling observed mechanical behaviour. In addition, the results of the laboratory tests are reproduced and interpreted using a generalised plasticity model that considers explicitly the interaction between macrostructure and microstructure. In this way, it is possible to achieve a more complete understanding of the mechanisms that underlie observed behaviour, and in particular the interplay between the two structural levels.

Science and empiricism in pile foundation design

Abstract: Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon empirical correlations. Improvements have been made in identifying the processes that occur within the critical zone of soil immediately surrounding the pile, but quantification of the changes in stress and fabric is not straightforward. This paper addresses the degree of confidence we can now place (a) on the conceptual and analytical frameworks for estimating pile capacity, and (b) on the quantitative parameters required to achieve a design. The discussion is restricted to driven piles in clays and siliceous sands, with particular attention given to extrapolating from design approaches derived for closed-ended piles of relatively small diameter to the large-diameter open-ended piles that are used routinely in the offshore industry. From a practical viewpoint, we need design approaches that minimise sensitivity...

Modelling the variation of degree of saturation in a deformable unsaturated soil

Abstract: An improved relationship for the variation of degree of saturation in an unsaturated soil is presented, incorporating the influence of changes of void ratio. When combined with an elasto-plastic stress-strain model, this is able to represent irreversible changes of degree of saturation and changes of degree of saturation caused by shearing. Experimental data from tests on compacted Speswhite kaolin are used to demonstrate the success of the proposed new expression for degree of saturation. The experimental data involve a wide variety of stress paths, including wetting, isotropic loading and unloading under constant suction, constant suction shearing, and constant water content shearing. Improved representation of the variation of degree of saturation has important consequences for numerical modelling of coupled flow-deformation problems, where the expression used for the degree of saturation can influence significantly the suction generated within the soil and hence the predicted stress-strain behaviour.

Discrete element simulation of crushable soil

Abstract: Computer simulations of crushable agglomerates were performed using the PFC3D computer code, which adopts the distinct element method (DEM). Agglomerates were made by bonding elementary spheres in ‘crystallo-graphic’ arrays, and by giving each sphere an existence probability of 0·8. Weibull statistics of the crushing strength of the resulting agglomerates, when tested singly between parallel platens, matches that of real silica sand grains. Triaxial tests on a cubical sample made of 389 agglomerates were then simulated. Curves of isotropic compression are shown, and the effect of loading rate on the position of the compression curves is discussed in relation to the practical question of performing DEM simulations as fast as possible without creating inertia errors. Other stress paths, including conventional ‘drained’ triaxial compression, constant mean effective stress, and constant-volume paths, were also simulated from different initial stresses along the virgin e–log p′ curve. The numerical results are...

Effect of strength non-homogeneity on the shape of failure envelopes for combined loading of strip and circular foundations on clay

Abstract: The capacity of surface foundations on clay under pure vertical (V), horizontal (H) or moment (M) loading may be expressed in non-dimensional form through the use of appropriate bearing capacity factors, with values that will be affected by the shape of the foundation and also any variation of undrained shear strength with depth. A common assumption has then been that the shape of the complete failure envelope in three-dimensional loading space (V, M, H) will be similar regardless of foundation shape and soil non-homogeneity, once scaled to the appropriate apex points. The appropriateness of this assumption has been explored by means of two- and three-dimensional finite element analyses of strip and circular footings, for a simple Tresca soil model where the shear strength varies linearly with depth. With a view to applications involving partially embedded foundations, such as offshore skirted foundations, full suction and ‘bonding’ with the underlying soil has been assumed. The paper documents the normal...

Estimation of active earth pressure against rigid retaining walls considering arching effects

Abstract: It is known that the distribution of active earth pressure against a translating rigid wall is not triangular but nonlinear, owing to arching effects in the backfill. In the present paper, a new formulation is proposed for calculating the active earth pressure on a rigid retaining wall undergoing horizontal translation. It takes into account the arching effects that occur in the backfill (or retained soil mass). In order to check the accuracy of the proposed formulation, the predictions from the equation are compared with both existing full-scale test results and values from existing equations. The comparisons between calculated and measured values show that the proposed equations satisfactorily predict both the earth pressure distribution and the lateral active earth force on the translating wall. In order to facilitate calculation of active earth pressures using the proposed equations, a modified active pressure coefficient and an effective height of application of the lateral active force are also prov...

Total suction measurement of unsaturated soils with a device using the chilled-mirror dew-point technique

Abstract: The total suction of a soil consists of two components: matric suction and osmotic suction. There are different techniques for measuring the total, matric and osmotic suction. The total suction of the soil is related to the water vapour pressure in the air space of the soil, and can be measured by devices capable of measuring relative humidity. A range of devices is available to measure relative humidity. In this paper, a device using the chilled-mirror dew-point technique with a selectable temperature environment was used to measure the relative humidity of a soil sample. Two residual soils of Singapore were used in the evaluation of the device for total suction measurement. The residual soils were compacted at various water contents and with three compaction efforts to provide soil samples of various suction values. The total suction of the compacted soil was measured and compared with its matric and osmotic suctions as independently measured using the null-type axis-translation apparatus and the pore f...

Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration

Abstract: A major infrastructure project, the South-to-North Water Transfer Project, has been proposed to carry potable water from the Yangtze River region in the south to many arid and semi-arid areas in the northern regions of China, including Beijing. The proposed 1200 km ‘middle route’ of the project is likely to be an open channel with a trapezoidal cross-section. At least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils. An 11 m high cut slope in a typical expansive clay with medium plasticity in Zaoyang, close to the ‘middle route’ in Hubei, was selected for a comprehensive and well-instrumented field study of the effects of rainfall infiltration. Two artificial rainfall events were created during a month of field investigation and monitoring. Prior to the rainfall events, high soil suction was measured within the top 1 m of soil. This high initial soil suction created an upward flux of water and moisture. The abundant cracks and fissures near the ground surface p...

Isotropic yielding in an artificially cemented soil cured under stress

Abstract: The work simulates, in the laboratory, the formation of a cemented sedimentary deposit in which cement bonding occurs after burial and under geostatic stresses. Isotropic compression tests were carried out on artificially cemented specimens made with variable cement contents. After consolidating the samples to the uncemented normal compression line, the specimens were allowed to cure for 48 h before testing. The curing confining stresses ranged from 50 to 2000 kPa, and were intended to represent soil elements at different depths in the fictitious sedimentary deposit when the cementing occurred. The contribution of the cement bonds to soil compression and the changes in the isotropic yield stress as a function of void ratio and cement content were analysed. The results showed the importance of the void ratio during the formation of cement bonds and also of the degree of cementation for the compressive behaviour of the cemented soil, and demonstrated that the variation in yield stress with void ratio and ce...

Undrained bearing capacity factors for conical footings on clay

Abstract: INTRODUCTION The bearing capacity of circular foundations on undrained clay is of fundamental importance in many geotechnical problems. In particular there are a number of designs of offshore foundations where the foundation can be treated approximately as a large circular footing, for instance some gravity bases, the spudcan foundations of jack-up units, and the more recently developed caisson foundations. In most cases the footing is not placed at the ground surface, and it is important to take into account the depth of embedment. Furthermore, the base of a spudcan is generally not flat, but approximates a shallow cone. For foundations on soft clays, the effect of the increase of strength of the soil with depth needs to be taken into account, and this is particularly important for large foundations. The purpose of this note is to present calculations of bearing capacity factors for shallow circular foundations, accounting for embedment, cone angle, rate of increase of strength with depth, and surface roughness of the foundation. The results have widespread application, particularly in the offshore industry. The soil is assumed to be rigid-plastic, with yield determined by the Tresca condition with an undrained strength su. The method of characteristics is used for the bearing capacity calculation, as described by Shield (1955), Eason & Shield (1960), Houlsby (1982) and Houlsby & Wroth (1982a) for application to undrained axisymmetric problems. Some previous results have been published for this problem using similar numerical techniques (e.g. Houlsby & Wroth, 1982b; Salencon & Matar, 1982; Houlsby & Wroth, 1983; Tani & Craig, 1995; Martin, 2001), but the study presented here involves a much more comprehensive coverage of the parameters. Where comparisons can be made with the previous solutions, the factors differ by up to about 0·5%, which gives some indication of the level of accuracy attainable with this numerical technique. Exceptionally, the rough footing results given by Tani & Craig (1995) are higher by up to about 5%, but this may be due to a problem with their numerical integration procedures (see Martin & Randolph, 2001).

Suction effects on a compacted clay under non-isothermal conditions

Abstract: The paper presents the results of an experimental study on the effects of suction and temperature on the volumetric behaviour of an unsaturated soil. Statically compacted Boom clay samples have been used in the testing programme, which has been performed using an oedometer apparatus with simultaneous control of suction and temperature. Suctions up to 0·45 MPa and temperatures up to 80°C have been applied. Special measures have been taken to control and minimise the problems associated with phase changes and vapour migration at high temperatures. A comprehensive experimental programme has been carried out, including isothermal tests with control of vertical stress, suction and temperature, non-isothermal tests under constant suction, and isothermal swelling pressure tests. Testing samples at two compacted soil densities provides observations concerning a wide scope of behaviour ranging from collapse to swelling behaviour on wetting. Test results obtained at 80°C have been compared with those of tests perfo...

Instabilities on moraine slopes induced by loss of suction: a case history

Abstract: In comparison with saturated conditions, negative pore water pressures arising from partial saturation increase the available shear strength on a potential slip surface. This additional contribution is lost progressively during infiltration of rainfall, leading to instabilities, sometimes before full saturation is reached. In such cases, reliable prediction of the safety factor may be achieved, by taking the suction history of the soil into account. A combination of field and laboratory tests was carried out recently to investigate the triggering effect of rainfall on shallow slips in alpine moraine slopes. The problem is complicated by the strong heterogeneity of these soils, with particle sizes varying from silts to boulders. The data highlight the influence of suction on the peak shear strength, and allow for the calibration of simple models, which take into account the dependence of shear strength on the saturation degree. An infinite slope stability analysis is performed. The results are compared wit...

Soil matrix suction: some examples of its measurement and application in geotechnical engineering

Abstract: Soil matrix suction is assuming an increasing importance in geotechnical engineering. It is now recognised that soil suction is not confined to arid or semi-arid regions of the world. It is frequently encountered in a great many geotechnical problems in temperate climates. For example, all fills and compacted soils have an inherent suction when they are formed; many slopes, both natural and man-made, have in-situ suctions; and all samples that are removed from the ground during a site investigation possess a soil suction in their unconfined state. Measurements of soil matrix suction can be used to: (a) identify disturbance and/or desiccation in samples removed from the ground; (b) estimate the average in-situ effective stress once the effect of disturbance and/or desiccation has been quantified; (c) apply more rigorous controls to the compaction of fill materials where swelling is likely to cause problems; and (d) assess the stability in natural and engineered slopes. This paper describes and compares thr...

The mechanical behaviour of a reconstituted unsaturated silty clay

Abstract: This paper describes a novel method of controlling suction by means of air circulation in combination with independent suction measurement. The technique has been incorporated in a standard triaxial stress path cell, and could easily be incorporated in other standard apparatus such as the direct shear box and oedometer. The application of the technique is demonstrated by means of an exploratory testing programme in which samples of a reconstituted saturated clayey silt were progressively dried to various predetermined suctions, which were then maintained constant during isotropic compression followed by shearing to failure. In this way the drying and stress history of the soil was carefully controlled in a way that could be made representative of the genesis of natural unsaturated soils. Though exploratory, the experimental investigation described in this paper has provided some interesting and thought-provoking results. Prior to desaturation the clayey silt behaves in accordance with the classical critic...

Numerical analysis: a virtual dream or practical reality?

Abstract: The development of numerical analysis and its application to geotechnical problems over the past 20 years have provided geotechnical engineers with an extremely powerful analysis tool. However, the use of such analysis is still not widespread, and when it is used there is all too often evidence of bad practice. Part of the reason for this is a lack of education and of guidance, especially from codes of practice, as to the appropriate use of such methods of analysis. Clearly, some form of initiative is required to promote good practice and allow the full potential of this analysis tool to be realised, both from a safety and an economy perspective. This lecture begins by reviewing the key advantages of numerical analysis over conventional analysis tools, and then debates whether or not it can replace the conventional analysis tools in the design process. Examples from engineering practice are used extensively to illustrate the arguments both for and against the use of numerical analysis.

A state-dependent elasto-plastic model for saturated and unsaturated soils

Abstract: A state-dependent elasto-plastic constitutive model for both saturated and unsaturated soils is presented in this paper. The model, which is developed under an extended critical-state framework, uses two independent stress state variables: net normal stress and matric suction. In addition, the influence of a simplified non-linear soil-water characteristic curve on mechanical behaviour is incorporated in the model by using a bi-linear function. Most importantly, a state-dependent dilatancy formulation is introduced to account for the effects of stress level or stress ratio, internal state (density) and soil suction. In this paper, details of the model formulation and the determination of model parameters are described and reported. By using the new model, numerical simulations of triaxial tests have been carried out on both saturated and unsaturated fine-grained (sandy silt) and coarse-grained (gravelly sand) weathered soils. The simulated triaxial tests include undrained shear tests on saturated specimens...

Stability of seismically loaded slopes using limit analysis

Abstract: Numerical limit analysis is used to assess the stability of slopes subjected to seismic loading. The soil is assumed to follow the Mohr–Coulomb failure criterion. The lower and upper bound theorems are formulated as linear problems to be solved using linear programming techniques. Based on finite element discretisation of the slope, the velocity field is optimised to find the lowest upper bound, and the stress field is optimised to obtain the highest lower bound. Limit equilibrium computations and log-spiral upper bound solutions were also performed for comparison purposes. Additionally, finite element analyses were done for selected cases. Results from the limit equilibrium and finite element methods are in excellent agreement with the rigorous lower and upper bounds for all cases studied. The slip surfaces obtained from both the limit equilibrium and log-spiral upper bound methods lie within the plastic zones obtained for the slopes from both finite element and numerical limit analysis. Plots are presen...

Source near-field effects and pulse tests in soil samples

Abstract: Shear modulus measurement using bender elements in laboratory samples has become very popular. However, the test results are hard to interpret. Simple plane wave test models are too coarse and result in substantial measurement uncertainty: near 100% in G0. A possible refinement of the test model is based on Stokes's fundamental solution for an isolated source. Near-field effects appear, and they have been regularly quoted as a major source of uncertainty in tests. There are some criteria in use for avoidance of near-field distortions, but they are signal dependent and not always successful. After some consideration of Stokes's fundamental solution the authors propose a new frequency domain signal-independent criterion to avoid near-field effects. Time-domain criteria are also given, but they are shown to be signal dependent. Applying this criterion to some experimental results the authors then also show how near-field induced errors are not responsible for much of the observed signal distortion. Uncertain...

Piled rafts in overconsolidated clay: comparison of in situ measurements and numerical analyses

Abstract: This paper presents the results of detailed back-analysis, using three-dimensional finite-element analysis, of three piled raft foundations on overconsolidated clay. Comparisons of overall settlement, differential settlements and the load carried by the piles show reasonably good agreement, although the finite-element analyses generally show a higher proportion of the overall load being carried by the piles than estimated from the field measurements. Three main performance indicators of the piled raft are proposed: the proportion of load carried by the piles, and the maximum settlement and maximum differential settlement, both as a proportion of the corresponding quantity for an unpiled raft foundation. The last indicator, in particular, suggests that improved layout of the pile support can lead to a reduction both in the maximum differential settlement and in the overall quantity of piles. This is illustrated for one of the case histories.

Critical-state parameters for an unsaturated residual sandy clay

Abstract: This paper presents experimental data from constant water content triaxial tests on a residual soil from Singapore, tested under unsaturated conditions with measurements of matric suctions. The data for critical-state conditions from these tests are presented within a framework previously proposed by the first author. The unsaturated critical state requires five parameters: Ma, Mb, Γab, λa and λb. At high degrees of saturation (in a non-aggregated condition) Ma = Mb = Ms (the saturated critical-state stress ratio). At lower degrees of saturation, the value of Mb drops considerably and reaches values near to zero at degrees of saturation below 60%. Conversely, the value of Ma appears to increase as the degree of saturation reduces. A similar pattern is observed for λa and λb. The functions relating the critical-state parameters to degree of saturation (or volumetric water content) can be expressed in normalised form by referencing them to the saturated state. The same normalised functions for Ma/Ms, Mb/Ms,...

Tensiometer saturation and the reliable measurement of soil suction

Abstract: Recent advances in the technology for measuring matric suction have permitted the direct measurement of gauge pore water pressures lower than −100 kPa—that is, absolute tensions. This significant a...

Generalised framework of limit equilibrium methods for slope stability analysis

Abstract: A generalised framework is proposed in this paper incorporating almost all of the existing limit equilibrium methods of slices for slope stability analysis with general slip surfaces. The force and moment equilibrium equations are derived in terms of the factor of safety and the initially assumed normal stress distribution over the slip surface, multiplied by a modification function involving two auxiliary unknowns. These equations are then analytically solved to yield explicit expressions for the factor of safety. Various assumptions regarding the interslice forces can be transformed into a unified form of expression for the normal stress distribution along the slip surface. An iterative procedure is developed to expedite the convergence of the solution for the factor of safety. Experience to date indicates that the process generally converges within a few iterations. Computation schemes are suggested to avoid numerical difficulty, especially in computing the factor of safety associated with the rigorous...

Seismic bearing capacity of foundations on slopes

Abstract: The effect of pseudo-static horizontal earthquake body forces on the bearing capacity of foundations on sloping ground has been assessed using the method of stress characteristics. Two failure, mechanisms were considered, based on the extension of the characteristics from the ground surface towards the footing base from either one side or both sides. The magnitude of N-gamma based on the both-sides failure mechanism, for smaller values of earthquake acceleration coefficient (alpha(h)), has been found to be significantly smaller than that obtained using the single-side mechanism; however, in the presence of alpha(h) the both-sides mechanism becomes kinematically inadmissible in many cases for higher values of phi. Only the single-side mechanism was found statically admissible for computing the bearing capacity factors N-c and N-q on sloping ground. All the bearing capacity factors reduce considerably with increase in ah for various ground inclinations

Elastic response of circular footings embedded in a non-homogeneous half-space

Abstract: This study uses a semi-analytical technique (the scaled boundary finite-element method) to evaluate the load–displacement response of rigid circular footings embedded in a non-homogeneous elastic half-space. Results are presented in terms of dimensionless elastic stiffness coefficients. The footings are subjected to vertical, horizontal, moment and torsional loads. The cross-coupling between the horizontal and moment load case is also evaluated. The variation of soil shear modulus with depth may correspond to normally consolidated sand or clay, and the footing geometries considered cover a range of footings commonly encountered in engineering practice.