bluuebunny/crossref_metadata_2025 huggingface dataset:
Non-linear soil stiffness in routine design
Authors: Atkinson J. H.
Published: October 2009

Soil stress-strain behaviour is highly non-linear and this has an important influence on the selection of design parameters for simple routine geotechnical calculations. Non-linear behaviour can be characterized by rigidity and degree of non-linearity and these can be determined from measurements of very small strain-stiffness, peak strength and failure strain. Very small strain-stiffness can be found from measurements of shear wave velocity in situ or in laboratory tests. Peak strength and failure strain can be measured in routine laboratory tests but are strongly influenced by initiation of shear bands. The important non-linear stiffness parameters for soil are related to its composition and to its current state. Back-analyses of the load settlement behaviour of full-scale and model foundations demonstrate the influence of non-linear soil behaviour. Variations of stiffness with settlement calculated from full-scale and model foundations agree well with non-linear soil stiffnesses based on rigidity and degree of non-linearity. These results suggest a simple method for routine design which takes account of soil non-linear stiffness.
Numerical simulation of one-dimensional behaviour of a kaolinite
Authors: Anandarajah A.
Published: October 2009

The development of physically meaningful mathematical models for describing the geotechnical engineering behaviour of clays requires an understanding at the particle level. In a recent study, it was shown that a numerical simulation technique known as the discrete element method could be used for developing such an understanding for clays. Among the three most important interparticle forces, the mechanical force and the double-layer repulsive force were considered in that study. Owing to the absence of a rational procedure, the van der Waals attractive force was neglected. The attractive force is considered in the present paper. The assembly is assumed to be two-dimensional. With all three types of interparticle forces considered, the numerical behaviour is shown here to compare reasonably well with many aspects of the laboratory behaviour, supporting the validity of the numerical technique. Numerical results reveal certain other aspects of the behaviour of clays that it is difficult to observe by regular laboratory means (e.g. evolution of interparticle contacts, anisotropy and size of particle clusters), thus providing new information on the behaviour of clays.
Micromechanics-based stress–strain behaviour of soils at small strains
Authors: Yimsiri S. , Soga K.
Published: October 2009

A model that describes the small-strain behaviour of soils is derived using micromechanics theory. The model allows examination of the effects of fabric anisotropy, stress conditions and contact characteristics on the small-strain modulus of soils. The closed-form solutions of the small-strain modulus are presented for the case of an isotropic fabric assembly under isotropic stress conditions. A fabric tensor is used to model the fabric anisotropy, and the small-strain modulus for the case of cross-anisotropic fabric assembly under general stress conditions is numerically calculated. The effect of the contact condition between soil particles is examined by incorporating three different contact laws (the linear elastic, Hertz–Mindlin and rough-surface contact models) into the model. It was found that the numerical results using the rough-surface contact model compare well with the published experimental data, and that the model provides microscopic insight into the small-strain behaviour of soils observed in the laboratory.
Soil consolidation associated with grouting during shield tunnelling in soft clayey ground
Authors: Komiya K. , Soga K. , Akagi H. , Jafari M. R. , Bolton M. D.
Published: October 2009

The effectiveness of grouting to reduce surface settlements during underground construction in clayey ground was investigated by a field trial and laboratory tests. The field trial was carried out during shield tunnelling work conducted in alluvial clay deposits in Koto-Ku, Tokyo. Grout was injected at some distance away from the tunnel, and both surface and subsurface settlements above the tunnel were monitored. Although the initial heave was achieved immediately after the grout injection, the ground continued to settle with time, owing to soil consolidation and grout shrinkage. A laboratory investigation was conducted to investigate the parameters that control the long-term behaviour of grouting in clay. It was found that better long-term grout efficiency can be achieved in overconsolidated clay than in normally consolidated clay, and the efficiency increased with increasing injection volume. Finite element analysis of the laboratory experiments confirmed that the amount and extent of excess pore pressures generated during injection govern the long-term grout efficiency. Finite element analysis of the field trial was also performed to simulate the long-term ground deformation after grout injection.
Analysis of progressive liquefaction as a moving-boundary problem
Authors: Sassa S. , Sekiguchi H. , Miyamoto J.
Published: October 2009

The propagation of liquefied zones in sand beds under fluid-wave loading is theoretically analysed in the present study. The completely liquefied state of sand is modelled as an inviscid fluid of a particular density, and the underlying sub-liquefied soil is modelled as a poro-elastoplastic material obeying a simple law of cyclic plasticity. It is shown that the proposed theoretical model is capable of consistently predicting the prograssive nature of liquefaction as observed in centrifuge wave tank tests on loose deposits of fine-grained sand. The theoretical model can also predict the final depth of the liquefaction front under a given wave loading.
A random method for simulating loose packs of angular particles using tetrahedra
Authors: Latham J.-P. , Lu Y. , Munjiza A.
Published: October 2009

The problems posed by the need to consider angular shapes in order to achieve more realistic micro-mechanical models of rock particulates are introduced. A relatively simple and fast particle deposition algorithm for packing simulations is developed. The details of the algorithmic procedures for deposition of tetrahedron-shaped particles of different size and aspect ratio are outlined. Numerical results including predictions of porosity for spheres, tetrahedra, different particle size distributions, binary mixtures and tetrahedron shapes are presented. For the loose packing of spheres extrapolated to zero wall effect, the porosity obtained was 0·414. Mono-sized tetrahedron packs with wall effects for a 3·288 cm edged equilateral tetrahedron packing in a 30 × 30 × 30 cm box produced an average porosity of 0·627 with a value of 0·584 when extrapolated to zero wall effect. Binary mixtures of tetrahedra show the characteristic minimum in porosity for a size mixture. The minimum is less well defined than for spheres. Continuous size distributions of tetrahedra based on a truncated Schuhmann distribution from 0·28 to 2·8 cm edge length in a 10 × 10 × 10 cm box do not indicate that distributions with a lower uniformity will necessarily produce a minimum in porosity. For tetrahedra of constant volume the porosity increases with decreasing sphericity in tetrahedra with different aspect ratios.
Natural slopes and cuts: movement and failure mechanisms
Authors: Leroueil S.
Published: October 2009

Movements and failure of cuts and natural slopes constitute an important geotechnical problem that involves a variety of geomaterials in a variety of geological and climatic contexts, and which has a major socio-economic impact in many countries. The paper reviews the state of knowledge in this domain, examining the basic aspects of soil mechanics that are relevant to the context of slopes, the importance of water on slope behaviour, and then the different stages of slope movements: pre-failure, failure, post-failure and reactivation. Finally it is shown how the geotechnical characterisation of slope movements can be used, in particular for assessing risk associated with such movements. Emphasis is put on for the brittleness of soils and its practical implications for the progressive failure developing at the pre-failure stage and on the characteristics of post-failure movements. The influence of other factors such as creep, fatigue, destructuration, partial saturation and infiltration is also considered.
Active and passive critical slip fields for cohesionless soils and calculation of lateral earth pressures
Authors: Zhu D.-Y. , Qian Q.-H. , Lee C. F.
Published: October 2009

A new method of solving earth pressure problems is proposed in this paper within the framework of the limit equilibrium approach. The concept of the critical slip field (CSF) is postulated: the active critical slip field (ACSF) in the active case, and the passive critical slip field (PCSF) in the passive case. Based on the principle of extremum thrust force (which is theoretically consistent with the principle of optimality) in conjunction with the method of slices, a numerical procedure is presented for the determination of such fields and consequently the distribution of earth pressures on retaining walls. For simplicity at this stage, the backfill material is assumed to be a homogeneous cohesionless soil with sloping ground surface carrying uniform and vertical surcharge, while the retaining wall is allowed to be battered, and the strength of the soil may be either fully or partially mobilised. Compared with the rigorous method (i.e. the method of characteristics), the proposed method is simple in principle and easily implemented in a computer program. However, it is more accurate than other approximate techniques and agrees well with available closed-form solutions. A number of examples of ACSF and PCSF are given in this paper, and the validity and efficiency of the proposed method are demonstrated. In addition, it is easy to extend this numerical procedure to obtain more general ACSF and PCSF accommodating non-homogeneous c, ϕ soils subject to complicated loading conditions.
Winkler modulus for axially loaded piles
Authors: Mylonakis G.
Published: October 2009

The problem of elastic soil–pile interaction and its modelling using the concept of a Winkler support are revisited. It is shown that dividing the vertical soil shear tractions and the corresponding settlements along the pile generates depth-dependent Winkler springs that accurately describe the pile–soil interaction, contrary to the widespread belief that the Winkler representation is always approximate. A simplified theoretical model is then derived for analysing the response of an end-bearing cylindrical pile in a homogeneous soil stratum. Explicit solutions are obtained for: (a) pile settlement; (b) depth-dependent Winkler modulus; (c) average (depth-independent) Winkler modulus to match pile head settlement. Both infinitely long piles and piles of finite length are examined. The approximate analytical solution compares favourably with finite-element and boundary-element solutions. A simple regression formula for the average Winkler modulus is developed.
Combined loading of spudcan foundations on clay: numerical modelling
Authors: Martin C. M. , Houlsby G. T.
Published: October 2009

The load–displacement response of a spudcan foundation on clay is described by means of an incremental plasticity model with three degrees of freedom (vertical, rotational, horizontal). The model is termed ‘Model B’ and employs a yield surface and flow rule that are derived from a programme of carefully controlled small-scale laboratory tests. Behaviour inside the yield surface is defined by a set of elastic stiffness factors for embedded conical footings, determined from three-dimensional finite element analysis. The hardening law, which defines the vertical bearing capacity as a function of plastic spudcan penetration, is based on a set of theoretical lower bound bearing capacity factors for embedded conical footings. Care is needed to ensure consistent treatment of partially and fully penetrated spudcans under combined loading. Full details of the elastic and elasto-plastic stiffness matrices needed to predict incremental load–displacement behaviour are provided, and the performance of the model is demonstrated by simulating some of the laboratory calibration tests numerically. While Model B succeeds in capturing many important features of the observed spudcan behaviour, its prediction of sudden rather than gradual yielding is a noticeable weakness, particularly in the simulation of cyclic loading events. Practical applications of the foundation model are discussed with reference to the soil–structure interaction analysis of independent leg jack-up units.
Behaviour of reinforced embankments on soft rate-sensitive soils
Authors: Rowe R. K. , Li A. L.
Published: October 2009

The behaviour of reinforced embankments constructed over rate-sensitive soft foundation soils is studied. Factors such as the viscoplastic properties and hydraulic conductivity of the soil, reinforcement stiffness and construction rate are examined. The time-dependent responses of excess pore pressures, reinforcement strains and foundation deformations are investigated. The short-term embankment stability is of particular interest. The construction of reinforced embankments to the height determined based on a limit equilibrium design is simulated to examine the assumptions made in the conventional undrained analysis. It is shown that creep and stress-relaxation of viscoplastic soils after the end of embankment construction may be significant for rate-sensitive soils. The embankment stability is shown to be critical during creep and stress-relaxation of foundation soils after construction. The undrained shear strength measured in laboratory triaxial tests using currently recommended strain rates without an appropriate correction may lead to unsafe design for rate-sensitive soils. For such soils, the increase in reinforcement strain shortly after the completion of construction can be higher than that developed during the construction. Excess pore pressures increase after construction owing to the viscoplastic behaviour of the foundation soil. Reinforcement is shown to have the potential to both increase stability and decrease long-term creep deformations.
An elastoplastic model for sand–structure interface behaviour
Authors: Ghionna V. N. , Mortara G.
Published: October 2009

Soil–structure interaction problems are often involved in geotechnical applications. Sand–structure interface behaviour depends on several factors, but principally on the tendency of the interface soil to dilate or contract in a restraining medium. This feature explains the differences observed between interface friction angles derived from model pile tests and interface direct shear tests performed at constant normal load (CNL). In the laboratory the basic device that can reproduce this behaviour is the constant normal stiffness (CNS) direct shear apparatus. Through these tests, both the characteristics of the discontinuity surface and the role of dilatancy can be examined. The aim of this paper is to formulate an elastoplastic model for interfaces between sand and rigid inclusions. Based on some experimental tests performed with the CNS direct shear apparatus, a plastic potential, a yield surface and a hardening law are derived, which require only CNL test results in the parameter derivation. Finally, the predictions of the model have been compared with experimental results obtained from CNL and CNS tests on different interfaces.
On the applicability of cross-anisotropic elasticity to granular materials at very small strains
Authors: Kuwano R. , Jardine R. J.
Published: October 2009

The paper examines the validity of assuming that granular material behaviour can be considered as cross-anisotropic, linear elastic, within a kinematic ‘kernel’ yield surface that is dragged through stress space with the current effective stress point. Data from more than 300 high-resolution triaxial probing and bender element tests performed on 15 samples are reported in which both small stress cycles and shear wave measurements in both the vertical and horizontal directions were conducted on sand and glass ballotini specimens under various effective stress states. It was found that behaviour is not fully elastic. However, provided the effective stress ratio, R, was kept below around 2·2, most features of the very small-strain behaviour could be described in terms of strongly anisotropic compliance terms that varied with void ratio and current effective stress state, apparently independently of the specimen's stress history. The anisotropy depended on the effective stress states. Specimens loaded to R values exceeding 2·2 that experienced shear-induced dilation followed different patterns of behaviour. The concept of a kinematic closed elastic yield surface, or ‘kernel’, proved to be an appropriate way of describing the pronounced effects of recent stress history on the apparent limits to the linear range observed in a wide range of stress probing tests.
A plasticity model for the behaviour of footings on sand under combined loading
Authors: Houlsby G. T. , Cassidy M. J.
Published: October 2009

A complete theoretical model is described for the behaviour of rigid circular footings on sand, when subjected to combined vertical, horizontal and moment loading. The model, which is expressed in terms of work-hardening plasticity theory, is based on a series of tests specifically designed to allow evaluation of the various components of the theory. The model makes use of the force resultants and the corresponding displacements of the footing, and allows predictions of response to be made for any load or displacement combination. It is verified by comparison with the database of tests. The use of the model is then illustrated by some demonstration calculations for the response of a jack-up unit on sand. This example illustrates the principal purpose of the development, which is to allow a realistic modelling of foundation behaviour to be included as an integral part of a structural analysis.
Seismic bearing capacity factors for spread foundations
Authors: Kumar J. , Mohan Rao V. B. K.
Published: October 2009

The effect of horizontal earthquake body forces on the bearing capacity of foundations has been examined computationally in a rigorous manner by employing the method of stress characteristics. The bearing capacity factors Nc, Nq and Nγ, due to the components of soil cohesion, ground surcharge pressure and soil unit weight respectively, have been plotted as a function of earthquake acceleration coefficient (αh) for different values of soil friction angle (ϕ). The inclusion of earthquake body forces causes a considerable reduction in the bearing capacity factors. The bearing capacity factors Nc and Nq are seen to be approximately of the same magnitude as those reported in the literature on the basis of different solution methods. However, the obtained values of Nγ are found to be significantly smaller than the available results. The nature of the pressure distribution along the footing base and the geometry of the observed failure patterns vary with the consideration of earthquake body forces.
Dynamic thermo-poro-mechanical analysis of catastrophic landslides
Authors: Vardoulakis I.
Published: October 2009

In this paper a dynamic analysis is presented of the early stages of an earth slide, considering two mechanically coupled substructures: (a) the rapidly deforming shear band at the base of the slide, and (b) the accelerating (rotating) rigid body. This is done by exploiting the concepts of frictional rate softening and thermo-poro-mechanical softening of the soil, and by modifying the friction-circle method of Taylor (1948) . First the equations are summarised that govern the phenomenon of heat-generated pore pressures inside a rapidly deforming shear band, and then the estimates for the various material parameters that enter the governing equations are critically discussed. In particular an attempt is made to adjust these material parameters to existing experimental data and to the properties of the clay soil responsible for the Vaiont slide of October 1963. The numerical analysis of the governing equations for the two mechanically coupled substructures leads to the following conclusions. The accelerated motion is caused by material strain-softening, which in turn feeds further material strain-rate softening inside the shear band. Owing to the speed of the initiated process, heat is trapped inside the shear band, which leads rapidly to a pore-pressure explosion in it owing to thermoplastic collapse of the clayey gouge, leading rapidly to total loss of strength and to uninhibited sliding motion on a frictionless base.
Analysis of laterally loaded pile groups using a variational approach
Authors: Shen W. Y. , Teh C. I.
Published: October 2009

This paper presents a variational approach for the analysis of laterally loaded pile groups. The displacements and reaction pressures of piles in a group are represented by finite series, and no discretisation of the pile shaft is required. The principle of minimum potential energy is applied to determine the response of a group of piles. The validity of the present approach is verified through comparison with other theoretical solutions. Case studies are also presented to show the application of the present method to pile groups in the field.
Influence of particle shape on engineering properties of assemblies of two-dimensional polygon-shaped particles
Authors: Mirghasemi A. A. , Rothenburg L. , Matyas E. L.
Published: October 2009

A code based on the discrete element method (DEM) was developed to simulate the behaviour of angular granular materials by loading assemblies of two-dimensional polygon-shaped particles. Two series of test simulations—isotropic loading–unloading and biaxial shear—were conducted on assemblies with different particle angularity. To avoid the probable effect of particle eccentricity and size distribution on assembly behaviour, equal-sized, equal-sided particles were employed. Six assemblies of particles with different angularities (one assembly for each type) were used in this study. The test results are presented in terms of macroscopic and microscopic behaviour. It was found that particle angularity had a significant influence on the behaviour of simulated granular material.
Movements in the Piccadilly Line tunnels due to the Heathrow Express construction
Authors: Cooper M. L. , Chapman D. N. , Rogers C. D. F. , Chan A. H. C.
Published: October 2009

This paper presents the results of analyses of monitoring records taken from inside one of the existing Piccadilly Line tunnels during the construction of three station tunnels at the Heathrow Express Central Terminal Area. The monitoring data allow a detailed picture of the way in which existing segmental concrete-lined tunnels behave as a result of tunnelling works below. Settlement curves at critical construction stages are presented. It is shown that the computed ‘volume loss’ varied between 1·3% and 2·5%. Long-term settlement records indicate that the maximum settlement increased by 27% for a period three years after tunnel completion. This result is compared with earlier findings from the Heathrow trial tunnel. Asymmetry of the settlement troughs caused by the outer tunnels, constructed after the central concourse tunnel, is discussed, and asymmetry parameters are defined from a number of case histories. The relationship of asymmetry parameters to pillar width between tunnels is presented and discussed. This suggests that interaction between the adjoining tunnels becomes negligible when the separation is six to seven diameters. Rotation in the existing tunnels at various construction stages is described, and the progressive rotation of the existing tunnel immediately above the concourse is shown. A tunnel longitudinal section showing ground movement contours predicted using a semi-empirical method illustrates the development of rotation. Distortion patterns along the existing tunnel, obtained from tape extensometer readings taken after completion of new tunnelling, are illustrated. These results together represent the first comprehensive study of tunnel and lining response to adjacent tunnelling published in the literature. The patterns of settlement, rotation and distortion, when considered together, provide a graphic description of a complex soil–structure interaction response to stress changes in the ground. The data can be used to predict responses in other soils and for other geometries by extrapolation.
Predicting soil—water characteristic curves of compacted plastic soils from measured pore-size distributions
Authors: Simms P. H. , Yanful E. K.
Published: October 2009

The unsaturated hydraulic conductivity function, k, is often predicted from the soil–water characteristic curve (SWCC). Most methods implicitly or explicitly derive a pore-size distribution (PSD) from the SWCC, which is then used to calculate k at any suction. Two important factors ignored by most methods are the change in the PSD during the SWCC test, and the influence of pore geometry on the SWCC. In this paper the SWCCs and evolution of the PSDs of four fine-grained compacted soils are measured. The SWCCs are measured from 0 to 3000 kPa by the axis-translation technique, and the PSDs are obtained by mercury intrusion porosimetry. A method is developed to predict the SWCCs from the measured PSD distribution data. Processes modelled include the effect of pore geometry on shrinkage during the SWCC tests, the isolation of water by the invading non-wetting phase (air), and differences in the relative accessibility of pores from the surface of the mercury intrusion and SWCC test samples. Different methods to model the above phenomena are developed and compared. The best predictions of gravimetric water content lie between one and two standard deviations of the measured SWCC data.
Cementation of porous materials using calcite
Authors: Ismail M. A. , Joer H. A. , Randolph M. F. , Meritt A.
Published: October 2009

A technique of cementing porous materials artificially using calcite is presented. The technique depends on flushing a mixture of chemical solutions through a porous medium, leading to precipitation of calcite due to reaction of the solution ingredients. The experimental procedures adopted to achieve the cementation are described. The response of different soils to the cementation process is analysed. The bond mechanism created by the calcite is illustrated, using optical and electron micrographs. The paper discusses the factors that appear to determine the response of geomaterials to calcite treatment. It has been found that the strength of a calcite-treated material increases with: (a) intrinsic strength of the individual grains, (b) density, (c) decreasing particle size of the host grains, (d) pre-coating of grains with calcite, and (e) roundness and non-angularity of soil grains. The cementation technique described in this paper has proved successful in reproducing the mechanical behaviour of natural calcarenite from the North West Shelf of Australia.
A numerical study of the effect of groundwater movement on long-term tunnel behaviour
Authors: Shin J. H. , Addenbrooke T. I. , Potts D. M.
Published: October 2009

Soft ground tunnelling beneath the water table induces pore water pressure changes and alters the hydraulic regime in the ground. There are various factors that control the tunnel/soil interaction behaviour during the equilibration period after tunnel construction through to the long-term steady state. Through the presentation of results from coupled non-linear finite element analyses, this paper considers the differences in predicted ground and lining responses in the long term for different post-tunnelling flow conditions. A 4 m diameter circular tunnel constructed 20 m below ground level is adopted for the geometry, with the soil conditions based on those found in London, UK, and Seoul, South Korea. The lining permeability is varied from fully permeable to impermeable, and an approach to modelling finite lining permeability is presented and assessed. For the permeable lining analyses two flow regimes are modelled, one in which the water table is maintained at its original elevation and one in which the water table is drawn down towards the tunnel. The importance of giving careful consideration to lining permeability and hydraulic boundary conditions is highlighted. The effects of groundwater movements on numerical predictions of tunnel behaviour are also identified.
Microstructure and pore characteristics of synthetic filters under confinement
Authors: Gardoni M. G. , Palmeira E. M.
Published: October 2009

This paper presents a study on the microstructure and pore characteristics of virgin and partially clogged non-woven geotextile filters under confinement using different experimental techniques. Geotextiles commonly found in practice were tested under normal stresses up to 1 MPa and pore dimensions and constriction sizes evaluated by the bubble point test, hydrodynamic sieving test and image analyser. The evaluation of the accuracy of theoretical methods for the prediction of filtration opening sizes of geotextiles under pressure was also assessed. The results obtained show a marked influence of stress level and partial clogging on the filter pore sizes and retention capability, and extend current guidance regarding current filter criteria for geotextiles.
Stereological relationship between trace length and size distribution of elliptical discontinuities
Authors: Zhang L. , Einstein H. H. , Dershowitz W. S.
Published: October 2009

This paper presents an approach for estimating the size distributions of elliptical discontinuities. This is an extension of approaches for estimating size distributions of circular discontinuities. By assuming an elliptical shape and changing the aspect ratio k of the ellipses (i.e. the ratio of the major to minor axes), many real discontinuities can be reasonably represented. For elliptical discontinuities, a general stereological relationship between distributions of trace length and distributions of discontinuity size (expressed by the major axis length, a, of the ellipse) is derived for area (or window) sampling. From this relationship, expressions are derived between the mean (μa) and standard deviation (σa) of the discontinuity size, a, and the mean (μl) and standard deviation (σl) of the trace length, l, respectively for log-normal, negative exponential and gamma distributions of discontinuity sizes. Using the derived expressions, the effect of sampling plane orientation on trace lengths is investigated. The results show that the sampling plane orientation has an important effect on both the mean and the standard deviation of trace lengths. Finally, the derived stereological relationship between the trace length distribution and the discontinuity size distribution was used in a simulation with the FracMan discrete fracture code. The results show that the approach for inferring the major axis orientation, aspect ratio and size distribution of discontinuities from observations on outcrops is satisfactory.
Deformation and failure modes of drystone retaining walls
Authors: Powrie W. , Harkness R. M. , Zhang X. , Bush D. I.
Published: October 2009

In this paper, the factors controlling the deformation of drystone retaining walls are investigated by means of discrete element analyses. It is shown that toppling failure of unweathered drystone retaining walls is likely to occur in a brittle manner, with wall crest deflections not exceeding 1% of the backfill height until the factor of safety (based on soil strength) falls below 1·05. A compressible sub-base and weathering of the blocks will both tend to reduce the backfill height at failure to below that indicated by a limit equilibrium analysis. Bulging failure is more likely to be associated with a deterioration in block joint stiffness due to weathering than a compressible sub-base, although the latter will decrease the reduction in joint stiffness needed to cause bulging failure. Bulging is much less brittle than toppling, and the proximity to failure of bulging walls could in some circumstances be assessed on the basis of the size of the bulge.
Shear wave propagation in jointed rock: state of stress
Authors: Fratta D. , Santamarina J. C.
Published: October 2009

The mechanical characteristics and orientation of joints determine the behaviour of rock masses, including the strength, the stiffness, and all forms of conduction and diffusion. Furthermore, the preferential orientation of joints renders the medium anisotropic. In turn, these properties affect the velocity and damping of propagating elastic waves. The purpose of this study is to investigate the propagation of long-wavelength shear waves through a rock mass with open joints filled with softer gouge, and subjected to low confinement. These conditions are selected in view of near-surface geophysical characterisation studies for geotechnical engineering applications such as foundations, tunnels and slopes. Experimental data are gathered with a slender column made of discrete elements with gouge material in the joints. The column is excited in the first torsional vibration mode to gather accurate measurements of velocity and damping for wave propagation direction normal to the joints. Data are analysed with mixture models. The potential use of wave propagation methods to assess jointed rock masses is discussed.
A thermomechanical analysis of a family of soil models
Authors: Collins I. F. , Kelly P. A.
Published: October 2009

Techniques of thermomechanics, based upon the use of internal variables, are used to develop a systematic procedure for deriving elastic/plastic models of soils and granular materials. Fundamental thermodynamic state variables are defined and used to formulate free energy and dissipation potentials. These are used to determine yield conditions and flow rules. It is demonstrated that it is necessary to distinguish between plastic work and plastic energy dissipation. It is suggested that the stored energy associated with the plastic deformations is due to the fact that only a proportion of the intergrain contacts are actually plastic in a plastically deforming continuum element. The stored plastic energy of the continuum model arises from locked-in elastic energy on the micro scale. Some well-known existing critical-state models are re-examined and some of their shortcomings are highlighted. New models are proposed that overcome some of these objections. These models are able to predict non-associated flow rules, contractive behaviour and pre-peak failure for ‘loose’ soils, and aspects of static liquefaction, and can predict the position of the failure, phase change, instability and ultimate state lines. In some extreme cases the yield surfaces are found to contain concave segments, and dilatant behaviour can occur below the critical (characteristic) state line.
Evaluation of typical conventional and unconventional plasticity models for prediction of softening behaviour of soils
Authors: Hashiguchi K. , Saitoh K. , Okayasu T. , Tsutsumi S.
Published: October 2009

Conventional elastoplasticity is based on the idealisation that the interior of the yield surface is a purely elastic domain. This idealisation would not lead to unrealistic prediction of hardening behaviour, but would cause unrealistic prediction of softening behaviour, which is observed typically in overconsolidated soils, whereas metals and normally consolidated soils exhibit hardening behaviour. By contrast, unconventional elastoplasticity that does not use this idealisation makes it possible to describe the plastic deformation due to the change of stress inside the yield surface exhibiting a smooth elastic–plastic transition, and is thus expected to describe softening behaviour more accurately. In this paper the capabilities of conventional and unconventional elastoplasticity for describing softening behaviour are examined, comparing the Drucker–Prager model as the conventional one and the subloading surface model as the unconventional one for predicting the deformation behaviour of overconsolidated soils. It is concluded that unconventional elastoplasticity has to be adopted for predicting softening behaviour.
The influence of partial soil saturation on pile behaviour
Authors: Georgiadis K. , Potts D. M. , Zdravkovic L.
Published: October 2009

Finite element analyses of geotechnical problems that involve partially saturated soils are commonly carried out using conventional fully saturated soil models. Such models, however, cannot successfully reproduce partially saturated soil behaviour. This paper investigates the influence of partial soil saturation on the behaviour of piles. A total stress constitutive model for partially saturated soils has been developed and implemented into the Imperial College finite element Program (ICFEP). This model, which is presented here, was used to perform a series of single pile analyses. The results of these analyses show the effect of partial saturation and in particular of fluctuations of the groundwater table on the behaviour of single piles. The effects of a rising groundwater table on pile behaviour are also investigated in the case of a pile foundation of a high-rise building in London. The paper indicates where prediction of pile capacity and displacement under different load and groundwater table fluctuations may be very different when using fully or partially saturated constitutive models.
An elasto-plastic model for unsaturated soil incorporating the effects of suction and degree of saturation on mechanical behaviour
Authors: Gallipoli D. , Gens A. , Sharma R. , Vaunat J.
Published: October 2009

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 skeleton stress, is a unique function of the bonding effect due to water menisci at the inter-particle contacts. The same result is obtained when examining critical states at different suctions. Based on these observations, an elasto-plastic constitutive model is developed using a single yield surface the size of which is controlled by volumetric hardening. In spite of this simplicity, it is shown that the model reproduces correctly many important features of unsaturated soil behaviour. It is especially remarkable that, although only one yield surface is used in the formulation of the model, the irreversible behaviour in wetting–drying cycles is well captured. Because of the behaviour normalisation achieved by the model, the resulting constitutive law is economical in terms of the number of tests required for parameter determination.
Mechanical behaviour of heavily compacted bentonite under high suction changes
Authors: Lloret A. , Villar M. V. , Sánchez M. , Gens A. , Pintado X. , Alonso E. E.
Published: October 2009

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.
Retention and transport of a hydrocarbon in a silt
Authors: Cui Y. J. , Delage P. , Alzoghbi P.
Published: October 2009

The paper presents an experimental investigation of the retention and transport properties of a light hydrocarbon pollutant (Soltrol) in unsaturated compacted samples of a low-plasticity Jossigny silt. Based on the work of Busby et al. and Steffy et al., a special multiphase cell was developed that allowed for the independent control of oil, water and air pressures. Oil retention curves of samples compacted at various densities and water contents were determined, together with oil permeability calculated using the methods of Gardner and Kunze & Kirkham. Microstructure investigations based on mercury intrusion porosimetry were conducted to help interpret the results. It was observed that the initial water degree of saturation of the soil conditioned the extent of oil infiltration. In the samples studied, the intra-aggregate pores were saturated with water, and oil infiltration affected only the inter-aggregate pores. For very loose samples, although some physico-chemical interactions were suspected between oil and soil, oil/air capillary action appeared to be the dominant phenomenon influencing oil retention and transport. In most samples, a 25 kPa oil suction allowed for the draining of a significant quantity of oil. The experimental results from this study help in better understanding the fundamentals of oil pollutant transport and decontamination mechanisms in oil-polluted soil.
Mobilised strength components in brittle failure of rock
Authors: Hajiabdolmajid V. , Kaiser P. , Martin C. D.
Published: October 2009

In deep underground excavations in hard rocks where stresses easily exceed the micro-crack initiation stress level inside the rock mass, proper consideration of the behaviour of rockmass during the brittle fracturing process in constitutive modelling is of paramount importance. Current empirical and conventional experimental methods for obtaining the deformational behaviours of hard rocks under loading do not lead to results that can be matched with in situ failure observations. This paper demonstrates that this problem is not necessarily a matter of the general notion of size effect but rather can be related to the different circumstances under which the cohesive and frictional strength components are mobilized in laboratory compression tests and around underground openings. It is also demonstrated that the propagation of the failed or breakout zone (depth and extent) is a function of the strain-dependent brittleness index IBε introduced in this paper, which explicitly considers the relative delay in friction mobilization relative to the rate of cohesion loss as functions of plastic strain. This new brittleness index characterizes the entire stress–strain curve (pre- to post-peak stages) and represents the involved micro-mechanisms during the brittle failure process: that is, initiation, propagation, and coalescence of cracks. This study shows that brittleness of rock is the most dominant factor, in controlling breakout shape, which explains the failure of stress-based criteria adopted by many researchers in predicting the stress-induced breakout depth around openings.
Seismic bearing capacity of foundations on slopes
Authors: Kumar J. , Mohan Rao V. B. K.
Published: October 2009

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 Nγ based on the both-sides failure mechanism, for smaller values of earthquake acceleration coefficient (αh), has been found to be significantly smaller than that obtained using the single-side mechanism; however, in the presence of αh the both-sides mechanism becomes kinematically inadmissible in many cases for higher values of ϕ. Only the single-side mechanism was found statically admissible for computing the bearing capacity factors Nc and Nq on sloping ground. All the bearing capacity factors reduce considerably with increase in αh for various ground inclinations.
Generalised framework of limit equilibrium methods for slope stability analysis
Authors: Zhu D. Y. , Lee C. F. , Jiang H. D.
Published: October 2009

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 Janbu method. The present framework can be readily implemented in a computer program, giving solutions of slope stability associated with a number of conventional methods of slices.
Numerical analysis: a virtual dream or practical reality?
Authors: Potts D. M.
Published: October 2009

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.