bluuebunny/crossref_metadata_2025 huggingface dataset:
One-dimensional self-weight consolidation of a lumpy clay fill
Authors: Yang L.-A. , Tan T.-S. , Tan S.-A. , Leung C.-F.
Published: October 2009

The need to use a large amount of dredged and excavated materials as fill for land reclamation necessitates an examination of the consolidation of a lumpy fill. Experimental results have shown the existence of two systems of voids in such a fill: inter-lump voids and intra-lump voids. The consolidation of this fill is highly complicated, and a simple box-spring model analogy is used to provide a better interpretation of the consolidation of this fill. To model this consolidation, the dualporosity model developed for fissured clay is used to model the consolidation of the lumpy fill; the primary motivation is the similarity between fissured clay and a lumpy fill with inter-lump voids. In the paper, this theory is re-derived using a two-phase flow analogy. Self-weight is explicitly included in the derivation: this is considered to be more relevant than the case of surcharge only, as the fill is often left to stand for a long period before sand fill is placed on top. A previously derived analytical solution is now modified to include self-weight. Using the analytical solution, salient features of the consolidation are demonstrated. In a lumpy fill, owing to the large inter-lump voids, the permeability of the inter-lump system is much larger than that of the intra-lump system. Parametric analyses using the analytical solution show that the inter-lump permeability dominates the overall consolidation behaviour in two ways. First, it will cause a rapid dissipation of the pore pressure in the inter-lump voids, and this in turn will cause a greater difference between the pore pressure in the inter-lump and intra-lump systems, thereby accelerating the fluid transfer from the intra-lump voids to the inter-lump voids.
A sand model with state-dapendent dilatancy
Authors: Li X. S.
Published: October 2009

A comprehensive bounding surface sand model has been formulated within a critical-state framework. This model differs from some existing models in that it incorporates the concept of state-dependent dilatancy in its formulation. The concept of state-dependent dilatancy states that the dilatancy of a granular soil depends not only on the stress ratio η = q/p, where q and p are the deviatoric and mean effective normal stresses respectively, but also on the current material internal state in reference to the critical state in e–p–η space, where e is the void ratio. By adopting such a state-dependent dilatancy, the present model simulates, with a single set of model constants, both the contractive and the dilative responses of granular soils over a wide range of variations in stress and material internal states. The model is formulated in general three-dimensional stress-strain space, and works for both monotonic and cyclic loading paths, and under either drained or undrained conditions. The formulations and the underlying concepts are presented in this paper.
The impact of confining stress on the rock mass deformation modulus
Authors: Asef M. R. , Reddish D. J.
Published: October 2009

The paper presents the results of a literature review combined with additional laboratory testing to determine the significance of confining stress on the true stiffness properties of rocks. Classical elasticity theory is introduced to set the scene for what is precisely meant by stiffness increase with confining stress. Previous laboratory-based stiffness testing on both intact and jointed rock specimens is reviewed, and the experimental results and leading equations and theories are summarised. The authors then introduce their own laboratory testing results concerning the same phenomena and present a new expression, which is directly compared with both the previous and the newly conducted test data. In conclusion it is seen that the rock types tested display significant increase in stiffness under confinement (commonly up to 3 times for typical intact rocks and up to 20 times for weak specimens), and that the implications for property definitions for numerical modelling are significant and relevant.
Collapse behaviour of a residual soil
Authors: Rao S. M. , Revanasiddappa K.
Published: October 2009

Residually derived red soils occur in Bangalore District of Karnataka State, India. The porous and unsaturated nature of the red soils makes them susceptible to collapse on wetting under load. The present study analyses the collapse behaviour of an unsaturated bonded (undisturbed) red soil from Bangalore referenced to tests on samples in an unbonded (remoulded) state. A filter paper method was used to determine the matric suction of the bonded and unbonded specimens, and mercury intrusion porosimetry (MIP) was used to determine their soil structure. Analysis of the experimental results shows that bonding plays an important role in the collapse behaviour of the unsaturated residual soil. The results of the study also provide insight into the volume change behaviour of unsaturated bonded soils on wetting within and beyond the yield locus.
The effect of strength anisotropy on the behaviour of embankments on soft ground
Authors: Zdravković L. , Potts D. M. , Hight D. W.
Published: October 2009

The need to build embankments on soft clay deposits is widespread, including flood defences, rail and motorway embankments. The difficulties in constructing embankments on these deposits are well known. The permeability of the ground is usually low, so that construction is generally performed under undrained conditions and, as a consequence of the low undrained shear strength of the clay, these structures are of a limited height, usually only 3–4 m. The classical design of an embankment is often based on a limit equilibrium approach, in which the soil is considered as an isotropic material. However, soft clays are almost always anisotropic, having both stiffness and strength dependent on the orientation of principal stresses. This complicates any design analysis, and as sufficient data are not usually available to define the magnitude of anisotropy, empirical factors are introduced into conventional design procedures. This paper attempts to demonstrate the effect of soil strength anisotropy on the behaviour of soft clay embankments through finite element analysis using an anisotropic soil model. The finite element predictions are compared with data from a full-scale test embankment brought to failure. The dangers of ignoring anisotropy, or treating it empirically, are highlighted.
Shear strength and small-strain stiffness of a natural clay under general stress conditions
Authors: Callisto L. , Rampello S.
Published: October 2009

In this paper the results of an experimental investigation into the mechanical behaviour of a natural stiff clay are presented. A set of drained true triaxial tests was performed, following rectilinear stress paths with constant mean effective stress, having different directions in the deviatoric stress plane. In addition, the small-strain shear stiffness of the soil in the vertical and horizontal planes was measured in the triaxial apparatus, during isotropic and anisotropic compression. To this purpose, three pairs of differently oriented bender elements were used. True triaxial failure data made it possible to plot the failure envelope in the deviatoric plane, and showed that the clay is slightly anisotropic with respect to strength. The analysis of the stress–strain response showed that the soil stiffness depends on the stress-path direction. For shear strains smaller than about 0·01% it was observed that, for monotonic loading conditions, the stress–strain relationships can be described with reasonable accuracy using a non-linear cross-anisotropic elastic model. Bender element test results confirmed that the clay is anisotropic with respect to stiffness, and that the small-strain shear modulus in the horizontal plane is larger, by about 22%, than that measured in the vertical plane. Such results also demonstrated that the small-strain stiffness increases with increasing values of the effective stress ratio experienced by the samples during constant-η compression paths, though this effect was not large because of the overconsolidated state of the soil. A reasonable agreement was finally observed between the shear stiffness of the clay obtained, at small strains, from true triaxial and bender element tests.
Discrete framework for limit equilibrium analysis of fibre-reinforced soil
Authors: Zornberg J. G.
Published: October 2009

A methodology is proposed for the design of fibre-reinforced soil slopes using a discrete framework. The analysis of fibre-reinforced soil using traditional composite approaches requires the implementation of laboratory testing programmes on composite fibre-reinforced soil specimens to characterise the material properties. Instead, the analysis of fibre-reinforced soil using a discrete approach can be conducted by independent characterisation of soil specimens and of fibre specimens, since the contributions of soil and fibres are treated separately. A fibre-induced distributed tension can be defined for use in limit equilibrium analysis using the proposed discrete framework. The fibre-induced distributed tension is a function of the volumetric fibre content and tensile strength of individual fibres when failure is induced by fibre breakage. Instead, when failure is induced by fibre pullout, the fibre-induced distributed tension is a function of the volumetric fibre content, interface shear strength and fibre aspect ratio. A critical normal stress, which defines whether the reinforced soil behaviour is governed by pullout or by breakage of the fibres, can be defined analytically using the proposed framework. An experimental testing programme involving tensile testing of fibres as well as triaxial testing of unreinforced and fibre-reinforced specimens was undertaken to validate the proposed framework. As predicted by the discrete framework, the fibre-induced distributed tension was observed to be proportional to the fibre content and fibre aspect ratio when failure was characterised by pullout of individual fibres. The discrete framework predicted accurately the contribution of randomly distributed fibres for the various soil types, fibre aspect ratios and fibre contents considered in the experimental testing programme.
Coupling of hydraulic hysteresis and stress–strain behaviour in unsaturated soils
Authors: Wheeler S. J. , Sharma R. S. , Buisson M. S. R.
Published: October 2009

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 of response, the occurrence of irreversible compression during the drying stages of wetting–drying cycles, and the influence of a wetting–drying cycle on subsequent behaviour during isotropic loading.
Suction effects on a compacted clay under non-isothermal conditions
Authors: Romero E. , Gens A. , Lloret A.
Published: October 2009

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 performed at room temperature (22°C). Based on these comparisons, thermal effects on a variety of features of behaviour such as swelling potential, swelling pressure, collapsibility, shrinkage strains, compressibility, stress path dependence and degree of strain reversibility are presented and discussed. Although the basic behaviour is similar at high and low temperatures, various significant differences are identified concerning mainly swelling strains developed during wetting and compressibility with respect to loading. Swelling pressure test results indicate that the high-temperature samples yield at a lower stress than that observed in room-temperature samples, suggesting that the stress domain bounded by the main yield surface reduces as temperature increases—a behaviour feature shared with saturated soils.
Critical-state parameters for an unsaturated residual sandy clay
Authors: Toll D. G. , Ong B. H.
Published: October 2009

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, λa/λs and λb/λs fit the experimental data for the residual sandy clay from Singapore and a lateritic gravel from Kenya previously tested. This suggests that the form of these functions may be common to a range of soil types.
Source near-field effects and pulse tests in soil samples
Authors: Arroyo M. , Muir Wood D. , Greening P. D.
Published: October 2009

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. Uncertainty in signal arrival lasts well beyond the end of the Stokes source near field.
A state-dependent multi-mechanism model for sands
Authors: Fang H. L.
Published: October 2009

A state-dependent multi-mechanism model for sands in three-dimensional space is proposed within the framework of critical-state soil mechanics. The mechanical deformation behaviour of sands is characterised with a macroscopic volumetric mechanism and a number of one-dimensional equivalent microscopic shear mechanisms (EMSMs) in various orientations. Each one-dimensional EMSM includes a shear deformation and a volumetric deformation due to dilatancy, which are described by a microscopic shear stress–strain relationship and a microscopic stress–dilatancy relationship respectively. The detailed relationships between the macroscopic and microscopic model parameters are established, and all microscopic model parameters can be defined explicitly or implicitly by soil parameters with a clear and easily understandable physical meaning. Moreover, a state parameter, ψ, defined by the difference between the current and critical void ratios at the same confining stress is used, relating the dilatancy and peak stress ratios to the critical stress ratio. Successful simulations of the responses of sands under drained, undrained, monotonic and cyclic loading conditions and during rotation of principal stress direction are obtained.
The middle surface concept and its application to the elasto-plastic behaviour of saturated sands
Authors: Yang Y. , Muraleetharan K. K.
Published: October 2009

An elasto-plastic constitutive model for the monotonic behaviour of saturated sands under triaxial loading conditions is presented in this paper. This model is capable of simulating the behaviour of sands under different densities and confining pressures using a single set of model parameters. The critical-state concept and the state parameter concept for sands are used in developing the model. A novel modelling technique, named the middle surface concept, is proposed to properly incorporate the state parameter into the model within the framework of critical-state soil mechanics. The underlying concept of this new modelling technique is that the true stress–strain response is a combined effect of two pseudo responses. One pseudo response incorporates the concept of the state parameter and the other pseudo response incorporates the concept of the critical state. The true yield surface is defined by the two pseudo yield surfaces and lies in between them: hence the name ‘middle surface concept’. By dividing the task between two pseudo yield surfaces, the problems faced by other constitutive models that use both the state parameter and critical-state concepts are avoided. The performance of the model under various conditions is first discussed. Comparisons between the model predictions and experimental results for Nevada sand and Toyoura sand are then presented; good agreement is achieved.
Discrete element simulation of crushable soil
Authors: Cheng Y. P. , Nakata Y. , Bolton M. D.
Published: October 2009

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 compared quantitatively with the testing data of the silica sand. They are also discussed in relation to characterising crushable soils during normal compression, yield, and critical states. Simulation of crushable soils using DEM provides valuable insights concerning the micromechanical origins of soil plasticity.
Experimental study of boundary effects in dynamic centrifuge modelling
Authors: Teymur B. , Madabhushi S. P. G.
Published: October 2009

Dynamic centrifuge modelling has been established as a powerful tool for studying soil–structure interaction problems under earthquake loading. Increasingly complex models are being tested in centrifuges all around the world in an attempt to understand real structure behaviour under earthquake loading. However, there is a need to model the field conditions correctly in these centrifuge models. In a geotechnical centrifuge, the space available to model real situations is not infinite, and it is necessary to enclose the model within the finite boundaries of a container. The boundary effects of the soil container are important and can lead to inaccurate simulation of a field situation that has infinite lateral extent. The equivalent shear beam (ESB) model container used in dynamic centrifuge testing is built to achieve the same dynamic response as the soil sample to minimise the boundary effects. A series of dynamic centrifuge tests involving loose and dense, dry and saturated models of homogeneous horizontal sand layers has been carried out to investigate the effects of the end walls of the ESB model container on soil behaviour. In both saturated and dry sand models it was seen that amplification of acceleration occurs at the base of the model close to the end wall. In saturated models, as excess pore pressures build up, the response of the centre of the model gets progressively more different from that near to the end wall. This is to be expected as the box stiffness was matched to that of the non-liquefied soil column. It was confirmed that when the relative density of the soil in the model was around 50%, close to that of the design soil layer, the change in behaviour towards the end wall was minimal.
Displacements ahead of an advancing NATM tunnel in the London clay
Authors: Van Der Berg J. P. , Clayton C. R. I. , Powell D. B.
Published: October 2009

This paper reports the results of monitoring carried out to study the patterns of ground displacement that develop ahead of an advancing sprayed concrete lined tunnel in soft ground. Extensive instrumentation was installed and monitored during the construction of the Heathrow Express platform tunnels forming part of the underground station at London Heathrow's Terminal 4. The construction of the concourse tunnel, built between the two completed platform tunnels, provided the opportunity to install additional instrumentation ahead of the tunnel drive, and to measure ground movements as the tunnel approached the instrumented sections. The paper describes the ground conditions at Heathrow, and the method of construction used for the concourse tunnel. The performance of the instrumentation and the pattern of ground displacements measured are reported. Finally, a synthesis of ground displacements measured is given, and the results are compared with the volume loss recorded during the concourse tunnel construction.
Progressive solidification of a liquefied sand layer during continued wave loading
Authors: Miyamoto J. , Sassa S. , Sekiguchi H.
Published: October 2009

A theoretical model for progressive solidification is described. It reproduces the experimental finding, from centrifugal wave tank testing with viscous scaling, that a layer of liquefied sand solidifies progressively from the base up while severe fluid-wave loading is imposed over a prolonged period of time. The solidification process reestablishes a grain-supported framework in the transitory fluid-like soil and is accompanied by the marked densification and gradual dissipation of excess pore pressures in a zone immediately below the upward-advancing solidification front. The continued disturbances to liquefied soil in the form of wave loading are an effective means for loosening the interlocking of solid particles, and permit rearrangement into a state of remarkably dense compaction due to contractancy. These aspects of soil behaviour are accentuated through comparison with consolidation in quiescent conditions following liquefaction. Furthermore, the proposed rigorous treatment of solidification as a moving-boundary problem provides a basis for reappraisal of an expression that was presented by Florin & Ivanov regarding the velocity of the solidification front.
Friction fatigue on displacement piles in sand
Authors: White D. J. , Lehane B. M.
Published: October 2009

Experiments with instrumented displacement piles have shown that the ultimate shaft friction that can develop in a given sand horizon decreases as the pile tip penetrates to deeper levels. This phenomenon, which is now commonly referred to as friction fatigue, is investigated here using centrifuge model piles equipped with lateral stress sensors, and by drawing on other experimental data from the laboratory and the field. It is shown that the primary mechanism controlling friction fatigue is the cyclic history imparted during pile installation to soil elements at the pile–sand interface. For a given installation method the stationary lateral stress acting at any given level on a displacement pile can be described as a relatively unique function of the cone penetration test end resistance and the number of cycles imposed during installation. The strong influence of cycling, which is also seen in cyclic constant normal stiffness interface shear tests, is attributed to contraction of a narrow shear zone at the shaft–soil interface that is surrounded by soil with a relatively high lateral stiffness.
Shear strength of assemblies of ellipsoidal particles
Authors: Ng T.-T.
Published: October 2009

A series of drained tests were performed numerically on specimens with ellipsoids of two different shapes. The drained tests include axial extension, constant mean stress compression, lateral compression, lateral extension, plane strain, triaxial compression, triaxial extension and six constant-b tests. The six constant-b tests were conducted by varying the intermediate principal stress with b = (σ2 − σ3)/(σ1 − σ3) = 0, 0·2, 0·4, 0·6, 0·8 and 1. These tests were performed using the discrete element method (DEM) with pressure boundaries. Two specimens composed of 1170 ellipsoids with two different aspect ratios were created. The two specimens were consolidated isotropically before being loaded along different stress paths. The effects of intermediate principal stress, mean stress, particle shape and stress path were investigated. The results show that the stress–strain behaviour of constant-b tests was affected by mean stress. Similar behaviours were observed for the drained tests with b = 0 regardless of the stress path. The friction angles of these tests were then compared with four failure criteria that have been proposed in the literature. Overall, the Lade and Ogawa models are better than the models proposed by Matsuoka & Nakai and Satake. The Ogawa model has a slightly better performance than the Lade model.
Triaxial tests on an unbonded locked sand
Authors: Cresswell A. , Powrie W.
Published: October 2009

This paper describes the results of a series of triaxial tests carried out to investigate the influence of fabric structure on the yield and failure behaviour of intact Reigate silver sand—a locked sand characterised by minimal cement content, large grain contacts, and a relative density index of 136%. Three distinct limiting stress envelopes are identified, based on (a) the start of dilation, (b) the maximum stress ratio during dilation, and (c) possible critical states. At effective cell pressures of 100 kPa and below, the stress ratio at the start of dilation is close to the maximum during subsequent dilation and much greater than at the critical state. At higher cell pressures the material behaves more conventionally, in that maximum stress ratios are more clearly linked to maximum rates of dilation. The role and significance of dilation in a locked sand are discussed with reference to the development of strain localisations into shear planes, thin sections from samples subjected to differing degrees of shear, and tests on unstructured sands.
Deformation characteristics of silty clay subjected to triaxial loading, by computerised tomography
Authors: Sun H. , Chen J. F. , Ge X. R.
Published: October 2009

The deformation characteristics of silty clay subjected to triaxial loading are studied using real-time computerised tomography scanning. Scanning is performed without unloading. The meso deformation characteristics are detected and described from scanning. Based on the CT number and CT image, it is shown that silty clay is typically a material that is originally damaged and inhomogeneous. Its deformation is anisotropic. The stress–strain curve of the soil is divided into four stages: (i) microdefects close up; (ii) new microcracks appear and develop slightly, and damage starts; (iii) local deformation and damage develop quickly; and (iv) softening failure and post damage develop rapidly. The real-time CT experiment provides an important experimental basis for studies on macro damage theory.
Contribution of fines to the compressive strength of mixed soils
Authors: Ni Q. , Tan T. S. , Dasari G. R. , Hight D. W.
Published: October 2009

The concept of granular void ratio, in which the volume of fines is treated as voids, was introduced to characterise mixed soils that contain both coarse (sand-size and above) and fine (clay and silt-size) soil particles. In using this concept, fines are assumed to make no contribution to the mechanical properties of the mixed soil. In this paper, this assumption is assessed through the analyses of a number of published results as well as data from tests conducted specifically for this study. The assessment shows that, in most instances, the relation between un-drained shear strength and granular void ratio of the mixed soil is not the same as that of the host coarse material. At the same granular void ratio plastic fines generally cause a reduction in undrained strength and, at best, act like voids, whereas non-plastic fines increase the undrained strength and, at worst, act like voids. A recently introduced equivalent granular void ratio, which makes use of a modification factor b to account for the contribution of fines, was evaluated. The equivalent granular void ratio was found to be able to synthesise the various results. The factor b has a different range of values, depending on the hardness of the fines, and reflecting the different ways these fines contribute to the behaviour. For non-plastic and thus relatively harder fines, b depends on the ratio of the average size of the fines to the average pore size space of the host coarse material. For plastic fines, this factor depends on the pore space of the host coarse material and the stress history, such as the overconsolidation ratio.
Characterisation of model uncertainties for laterally loaded rigid drilled shafts
Authors: Phoon K.-K. , Kulhawy F. H.
Published: October 2009

This paper presents a critical evaluation of model factors for laterally loaded rigid drilled shafts (bored piles). Both the lateral or moment limit and hyperbolic capacity are considered to make explicit the dependence of model factors on the criterion for interpreting ‘capacity’ from load test data. Although the hyperbolic capacity may be closest to the theoretical ultimate state or upper bound, results indicate that it generally does not produce a mean model factor of 1. When the measured capacity is interpreted consistently from load test data, the coefficient of variation (COV) appears to remain relatively constant between 30% and 40%. The range of the mean bias for the lateral or moment limit is 0·67 to 1·49, whereas that of the hyperbolic capacity is 0·98 to 2·28. Based on available data, a log-normal probability model appears adequate for reliability analysis. Laboratory-scale load tests conducted in uniform soil deposits prepared under controlled laboratory conditions are ideal for establishing benchmarks on the probable magnitude of uncertainty arising from model idealisations alone. However, the limited range of geometric and geotechnical parameters in a laboratory load test database may not produce a representative mean model factor. A field load test database typically contains more diverse geometric and geotechnical parameters, but it entails an unknown degree of extraneous uncertainties. A comparative study indicates that model statistics are surprisingly robust and appear not to be seriously affected by the above concerns (possibly because of normalisation). Model factors from drained analysis seem to be more variable than those from undrained analysis. A more detailed examination indicates that the higher COV of about 40% for these drained model factors arises because they are not completely random. There are reasons to believe that applying a more complete force system for drained analysis could minimise some of the undesired correlations and reduce the COV to a level comparable to undrained analysis.
Reliability-based design applied to retaining walls
Authors: Low B. K.
Published: October 2009

Geotechnical engineers have long used the lumped factor of safety approach in the design of foundations and retaining walls. A more recent alternative is the limit state approach using partial factors, for example as suggested in Eurocode 7. Yet another approach is perhaps more flexible and rational: design based on a target reliability index that reflects the uncertainty of the parameters and their correlation structure. Among the various versions of reliability indices, the Hasofer–Lind index and first-order reliability method (FORM) are more consistent. This paper illustrates practical reliability-based design procedures for retaining walls based on the Hasofer–Lind index and FORM. Correlated normal and non-normal random variables are considered. The efficient spreadsheet-based probabilistic approach, based on the work of Low & Tang, achieves the same result as the Hasofer–Lind method and FORM, but uses an intuitive expanding dispersion ellipsoid perspective that greatly simplifies the computations and interpretations. The differences between a reliability-based design and one based on partial factors will be discussed. Sensitivity information as conveyed in a reliability analysis will be studied. The probabilities of failure inferred from reliability indices will be compared with Monte Carlo simulations. This paper deals only with certain aspects of reliability, namely methodology and concepts, and not reliability in its widest sense.
Stochastic evaluation of static liquefaction in a predominantly dilative sand fill
Authors: Hicks M. A. , Onisiphorou C.
Published: October 2009

The paper investigates the potential for liquefaction in a predominantly dilative hydraulic sand fill, and, in particular, whether pockets of loose material could have been instrumental in the failure of the Nerlerk underwater berm. This has been achieved stochastically, by linking random field theory for modelling spatial variability with finite elements for computing geostructural response. Specifically, a double-hardening soil model has been calibrated against data from 74 triaxial tests on Erksak sand, for a wide range of sand states, while state parameter statistics have been derived from 71 cone penetration tests from two Beaufort Sea sand islands. Multiple realisations of structural performance have then been carried out, by generating univariate random fields of state parameter, from which the soil model parameters have been inferred. The results demonstrate that it may be possible for a predominantly dilative fill to liquefy, due to the presence of semi-continuous loose zones arising from deposition-induced anisotropy.
A deterministic/stochastic model to predict the variation in bulk modulus of chalk
Authors: Collin F. , Schroeder Ch. , De Gennaro V. , Bolle A.
Published: October 2009

Ekofisk, located 200 km west of the Norwegian coast, is one of the main oilfields in the North Sea. Since the early 1980s severe compaction of chalk reservoir layers has been observed as a consequence of reservoir depletion during oil production. Subsequently, this compaction has been amplified by assisted oil recovery using seawater flooding. The development of our understanding of the inherent mechanisms of this phenomenon has been the objective of extensive experimental investigations in the last two decades. Owing to the very high cost of cored material from the reservoir, experiments are usually performed on chalk samples from an outcrop in Belgium lying at the same stratigraphic level as the Ekofisk reservoir chalks. However, even at the laboratory sample scale, experimental variability of material response is observed. From a theoretical and numerical point of view, the determination of the mechanical properties of the material is of utmost importance for accurate modelling at both sample scale (laboratory tests) and reservoir scale. The aim of this paper is to introduce a stochastic approach within a deterministic constitutive model of chalk to enable the influence of material heterogeneity to be included in analyses for the range of observed mechanical responses. In modelling the random distribution of material parameters, the time-consuming Monte Carlo simulation method is replaced by a more efficient stochastic modelling technique. The results are given in the form of statistical parameters for the experimental laboratory test responses. The parameters of the distribution law (mean value, range of variation, spatial correlation structure) are fitted to reproduce the range of experimental responses observed at sample scale. Interest is focused on the variability of the chalk bulk modulus, as observed during isotropic compression tests. The proposed methodology provides a satisfactory explanation for the variability of response observed at the sample scale. The potential for extending the proposed approach to reservoir scale is briefly discussed.
Non-parametric simulation of geotechnical variability
Authors: Bourdeau P. L. , Amundaray J. I.
Published: October 2009

The application of bootstrap resampling, a non-parametric simulation technique, to the analysis of uncertainty associated with geotechnical data is investigated. This technique is presented as an alternative to traditional statistical data analysis. An extensive geotechnical database from the Heber Road site, Imperial Valley, Southern California, USA, is utilised as a case example. The database consists of many cone penetration and flat dilatometer tests. Linear correlation between the two types of test results and their vertical autocorrelation properties are analysed using bootstrap resampling, and the outcomes are compared with the same characteristics obtained from direct statistics of the whole database. The convergence and accuracy of bootstrap estimates, as well as their sensitivity to sample size, are studied and discussed. For the type of application investigated through this example, non-parametric bootstrap resampling appears a reliable and robust technique of geotechnical uncertainty analysis. Its main advantage is that statistical estimators of geotechnical properties and associated confidence levels can be computed directly from the empirical data without parametric assumptions on the population distribution.
Cone penetration tests under simulated field conditions
Authors: Huang A.-B. , Hsu H.-H.
Published: October 2009

Cone penetration tests (CPT) performed in a conventional cavity-wall calibration chamber are inevitably affected by boundary conditions. The authors have developed an axisymmetric field simulator whereby CPT calibration tests can be conducted under substantially reduced boundary effects. The simulator consists of a cylindrical, physical sand specimen and a stack of expandable rings to simulate the soil from beyond the physical boundary. A series of CPT calibration tests has been performed in a clean, uniformly graded quartz sand using this simulator system. For a given relative density, the cone tip resistance (qc) was found to be most closely related to mean normal stress. The horizontal stress at the physical boundary increased during CPT under simulated field conditions. This increased horizontal stress had a much more significant and consistent effect on qc than the initial horizontal stress existing in the soil prior to CPT. The paper describes the results of CPT in the simulator, and discusses implications for the performance of CPT in a chamber and their interpretation.
Review of historical buildings' foundations
Authors: Przewłócki I. , Dardzińska I. , Świniański J.
Published: October 2009

The paper presents in brief some aspects of the history of foundation engineering, with particular regard to its development, applied techniques and materials. This study concentrates on medieval and modern foundations and is limited to the Europe area, with special emphasis on Poland. Several types of historical foundation in various soil conditions are considered. Basic methods of preservation of such foundations, their drawbacks and advantages are discussed. Several examples of historical foundations as well as past and contemporary preservation techniques are presented in the paper.
Elastic moduli of soils dependent on pressure: a hyperelastic formulation
Authors: Houlsby G. T. , Amorosi A. , Rojas E.
Published: October 2009

The elastic behaviour of granular materials is non-linear, in that the small-strain tangent stiffness depends on the stress level. The elastic moduli typically vary as power functions of the mean stress. Simple models of this nonlinearity can result in behaviour that violates the laws of thermodynamics. To guarantee that an elasticity model is thermodynamically acceptable it must be possible to derive the elastic behaviour from a free energy potential (or alternatively from a complementary energy potential). In this paper elasticity models are derived that allow for variation of elastic moduli as power functions of mean stress, while guaranteeing thermodynamic acceptability. The important issue of the dependence of secant stiffness on strain amplitude (a phenomenon related to dissipation processes in the soil) is acknowledged but not addressed here.
The ultimate undrained resistance of partially embedded pipelines
Authors: Merifield R. , White D. J. , Randolph M. F.
Published: October 2009

On-bottom pipelines for transporting oil and gas in deep water undergo significant changes in temperature and pressure during operating cycles, which cause a tendency for lateral buckling. Prediction and control of this phenomenon are required for the safe design and operation of these pipelines. However, the soil response under combined vertical and lateral loading is a significant area of uncertainty, and current practice relies on empirical expressions for the estimation of lateral pipe–soil resistance. This paper reports the results of finite element (FE) analyses of shallowly embedded pipelines under vertical and horizontal load. These analyses have been compared with collapse loads calculated using the upper-bound theorem of plasticity, and are used to construct yield envelopes defining the limiting combinations of vertical and horizontal load. The FE limiting loads were found to compare well with upper-bound plasticity solutions, and the internal soil displacements calculated in the FE analyses match the upper-bound and experimentally observed deformation patterns. The yield envelopes generated by the FE and upper-bound analyses have been fitted by simple solutions, which aid assessment of the ultimate resistance of shallowly embedded pipelines.
Finite element verification of an enhanced limit equilibrium method for slope analysis
Authors: Tan D. , Sarma S. K.
Published: October 2009

To determine the critical slip surface in slope analysis, a new procedure has been proposed, based on limit equilibrium method and stress acceptability criteria. This paper uses finite element analysis as an independent tool to validate the new procedure for both homogeneous and non-homogeneous slopes. Good agreement is found between the new procedure and finite element analysis, in terms of the critical acceleration, the critical slip surface and normal and shear stress distribution on the slip surface, in both homogeneous slopes and non-homogeneous slopes. However, when non-associated flow rules are adopted in finite element analysis and the corresponding degree of non-normality is high, some differences are noticed. The reason for this difference is explained.
Hydraulic conductivity to Jet-A1 of GCLs after up to 100 freeze–thaw cycles
Authors: Rowe R. K. , Mukunoki T. , Bathurst R. J.
Published: October 2009

Needle-punched geosynthetic clay liner (GCL) specimens subjected to up to 100 freeze–thaw cycles in the laboratory, and GCL samples recovered from the field after 3 years, are examined to assess their permeability with respect to both water and Jet A-1 fuel using flexible wall permeameters. The mean GCL hydraulic conductivity with respect to water is shown to be 3·3 × 10–11 m/s before freeze–thaw. The mean hydraulic conductivities after up to 100 freeze–thaw cycles did not change significantly, and were in the range 2·2–5·3 × 10–11 m/s. Freeze–thaw cycles did reduce the entry pressure required for Jet A-1 to begin to permeate through the GCL from about 27 to 55 kPa with no freeze–thaw cycles to 13·8–20·7 kPa for 3 to 50 freeze–thaw cycles, and to 0–13·8 kPa after 100 freeze–thaw cycles. For the GCL specimens subjected to 5, 12 and 50 freeze–thaw cycles in the laboratory, the hydraulic conductivity with respect to Jet A-1 is less than 3 × 10–11 m/s at a pressure just above the entry pressure, compared with less than 3 × 10–12 m/s for GCL samples recovered from the field. The combined effect of many freeze–thaw cycles and permeation with Jet A-1 did result in an increase in hydraulic conductivity; however, the effect was small, and the GCL performed well with a maximum hydraulic conductivity of 1 × 10–10 m/s after 100 freeze–thaw cycles.
Parametric Gmax sounding of granular soils by vibration methods
Authors: Pak R. Y. S. , Ashlock J. C. , Kurahashi S. , Abedzadeh F.
Published: October 2009

To solicit a fundamental understanding of past difficulties of vibration sounding methods for granular soils, a collaborative experimental and analytical study was developed to examine the performance of dynamic surface plate load tests by scaled modelling. Focused on the benchmark small-strain regime and Gmax condition, multiple small-amplitude vibration tests on a rigid square plate resting on a uniform dry sand were conducted on a large geotechnical centrifuge. A set of basic experimental results on the vertical and rocking dynamic characteristics of the soil–plate system were generated by means of a new eccentric test format, as well as by conventional symmetrical vertical vibration. Through a comparison of the measured data with canonical continuum solutions of a half-space with constant as well as common depth-dependent shear modulus profiles, some fundamental issues in regard to the relevance of such commonly accepted idealisations of the intended practical application were revealed. As an experimental way to confirm and circumvent a key aspect of the problem, the novel test strategy of using a lightweight foundation in an embedded configuration was proposed and evaluated.