Showing papers in "Geotechnique in 2010"
TL;DR: In this paper, the onset and evolution of localised deformation processes in sand with grain-scale resolution was observed and quantified by combining state-of-the-art X-ray micro tomography imaging with 3D volumetric digital image correlation techniques.
Abstract: The objective of this work was to observe and quantify the onset and evolution of localised deformation processes in sand with grain-scale resolution. The key element of the proposed approach is combining state-of-the-art X-ray micro tomography imaging with three-dimensional volumetric digital image correlation techniques. This allows not only the grain-scale details of a deforming sand specimen to be viewed, but also, and more importantly, the evolving three-dimensional displacement and strain fields throughout loading to be assessed. X-ray imaging and digital image correlation have been in the past applied individually to study sand deformation, but the combination of these two methods to study the kinematics of shear band formation at the grain scale is the first novel aspect of this work. Moreover, the authors have developed a completely original grain-scale volumetric digital image correlation method that permits the characterisation of the full kinematics (i.e. three-dimensional displacements and rotations) of all the individual sand grains in a specimen. The results obtained using the discrete volumetric digital image correlation confirm the importance of grain rotations associated with strain localisation.
503 citations
TL;DR: In this article, a series of laboratory tests were conducted where a stiff pile in drained sand was subjected to between 8000 and 60 000 cycles of combined moment and horizontal loading, and a typical design for an offshore wind turbine monopile was used as a basis for the study, to ensure that pile dimensions and loading ranges were realistic.
Abstract: The driven monopile is currently the preferred foundation type for most offshore wind farms. While the static capacity of the monopile is important, a safe design must also address issues of accumulated rotation and changes in stiffness after long-term cyclic loading. Design guidance on this issue is limited. To address this, a series of laboratory tests were conducted where a stiff pile in drained sand was subjected to between 8000 and 60 000 cycles of combined moment and horizontal loading. A typical design for an offshore wind turbine monopile was used as a basis for the study, to ensure that pile dimensions and loading ranges were realistic. A complete non-dimensional framework for stiff piles in sand is presented, and applied to interpret the test results. The accumulated rotation was found to be dependent on relative density, and was strongly affected by the characteristics of the applied cyclic load. Particular loading characteristics were found to cause a significant increase in the accumulated ro...
424 citations
TL;DR: In this paper, the authors consider the effects of new phenomena and new variables on soil behavior, such as high temperatures, associated with the problem of storage and disposal of high-level radioactive waste; and low temperatures in problems of freezing ground.
Abstract: The range of problems that geotechnical engineers must face is increasing in complexity and scope. Often, complexity arises from the interaction between the soil and the environment – the topic of this lecture. To deal with this type of problem, the classical soil mechanics formulation is progressively generalised in order to incorporate the effects of new phenomena and new variables on soil behaviour. Recent advances in unsaturated soil mechanics are presented first: it is shown that they provide a consistent framework for understanding the engineering behaviour of unsaturated soils, and the effects of suction and moisture changes. Building on those developments, soil behaviour is further explored by considering thermal effects for two opposite cases: high temperatures, associated with the problem of storage and disposal of high-level radioactive waste; and low temperatures in problems of freezing ground. Finally, the lecture examines some issues related to chemical effects on soils and rocks, focusing i...
340 citations
TL;DR: In this paper, the influence of the mechanical and geometrical properties of the constituent grains on the overall material response of cohesionless granular materials was examined using glass ballotini as an analogue soil.
Abstract: This paper describes an experimental study examining the influence of the mechanical and geometrical properties of the constituent grains on the overall material response of cohesionless granular materials. Glass ballotini were used as an analogue soil; their relatively simple geometry allowed the influence of particle shape and inter-particle friction to be examined independently. Techniques were developed to control the surface roughness of the ballotini to facilitate a parametric study. The particle shape was also varied by crushing the ballotini. At the micro-scale, the particle characterisation included accurate measurements of inter-particle friction, contact stiffness, particle surface roughness and particle shape. At the macro-scale the sensitivity of overall material response to changes in surface roughness and geometry was characterised using triaxial tests and oedometer tests on smooth spherical ballotini, roughened ballotini and crushed angular ballotini. Compression tests indicated that the i...
328 citations
TL;DR: In this paper, the authors compared several alternative stress state variables in unsaturated soils, with a special focus on the use of an effective stress, and suggested that the proportion of suction contributing to the effective stress is often much smaller than predicted by the term "suction times degree of saturation" generally used in effective stress expressions of the Bishop type.
Abstract: Current alternative choices of stress state variables in unsaturated soils are described and compared, with a special focus on the use of an effective stress. Experimental data on stiffness and shear strength evolution with suction suggest that the proportion of suction contributing to the effective stress is often much smaller than predicted by the term ‘suction times degree of saturation' generally used in effective stress expressions of the Bishop type. It is suggested that effective stress in unsaturated soils should be related to soil microstructure. An effective degree of saturation is defined as describing the volume of water partially filling the soil macroporosity. This effective degree of saturation defines the proportion of the prevailing suction that actually contributes to the effective stress. Two alternative expressions (piecewise linear and non-linear) are proposed for the effective degree of saturation. They offer a similar performance. Available data on stiffness and shear strength varia...
320 citations
TL;DR: In this paper, the e−log σ v compression curves show an inverse "S" shape due to suction pressure resisting deformation, similar to that of soft natural clays caused by consolidation yield stress.
Abstract: Oedometer tests starting from a very small effective vertical stress of 0·5 kPa were performed on three reconstituted clays with different liquid limits The soils were prepared at various initial water contents, ranging from 0·7 to 2·0 times their corresponding liquid limits It is observed that the e–log σ ′ v compression curves show an inverse ‘S' shape due to suction pressure resisting deformation, similar to that of soft natural clays caused by consolidation yield stress The suction pressure σ ′ s of the reconstituted clays can be correlated with the ratio of initial void ratio to void ratio at liquid limit e0/eL The suction pressure curve defined by a unique relationship between suction pressure σ ′ s and the normalised void ratio at suction pressure es/eL is also proposed to distinguish between the pre-suction and the post-suction states In addition, Burland's concept of the intrinsic compression line is adopted for correlating the compression curves of various reconstituted clays at high initia
193 citations
TL;DR: In this article, the effects of initial soil fabric on the shear behavior of granular materials are investigated by employing distinct element method (DEM) numerical analysis, where soil specimens are represented by an assembly of non-uniform sized spherical particles.
Abstract: The effects of initial soil fabric on the shear behaviour of granular materials are investigated by employing distinct element method (DEM) numerical analysis. Soil specimens are represented by an assembly of non-uniform sized spherical particles. DEM specimens that have different initial contact normal distributions were prepared at two different densities (loose and dense) and then isotropically consolidated triaxial tests were simulated with the following conditions: (a) different drainage conditions (undrained/drained), (b) different modes of shearing (compression/extension), and (c) different directions of shearing (vertical/horizontal). The numerical analysis results are compared qualitatively with the observed experimental data and the effects of initial soil fabric on resulting soil behaviours are discussed. The discussions include the effects of specimen reconstitution methods, effects of large preshearing, and anisotropic characteristics in drained and undrained conditions. The effects of initia...
189 citations
TL;DR: The influence of fabric on mechanical behavior is explicitly considered in some of the current constitutive models for unsaturated soils as discussed by the authors, which are based on assumptions regarding the interaction between fabric and mechanical behavior.
Abstract: The influence of fabric on mechanical behaviour is explicitly considered in some of the current constitutive models for unsaturated soils. These are based on assumptions regarding the interaction b...
183 citations
TL;DR: In this paper, particle crushing, shear banding, interface abrasion and migration of crushing products all have the potential to influence the behavior of displacement piles in sands, reporting experiments with model displacement piles installed in uniform pressurised sand and parallel interface ring shear tests.
Abstract: Particle crushing, shear banding, interface abrasion and migration of crushing products all have the potential to influence the behaviour of displacement piles in sands. This paper considers these particulate processes, reporting experiments with model displacement piles installed in uniform pressurised sand and parallel interface ring shear tests. The findings offer new insights into the mechanics of displacement piles in sands.
175 citations
TL;DR: In this paper, a formulation of a multi-physics model of methane hydrate flow coupled to soil deformation is developed, and a two-phase flow formulation of water and methane gas is suggested according to Darcy's law and capillary pressure law.
Abstract: Methane hydrate is estimated to be present in substantial amounts below deep sea floors. Particular scientific and engineering interests that encourage studies of mechanical behaviour of methane hydrate soils include submarine geohazards, such as the initiation of marine landslides through hydrate dissociation, wellbore stability and estimation of future gas production from wells. To study these problems, a formulation of a multi-physics model of methane hydrate flow coupled to soil deformation is developed. By assuming deformable porous media (soil matrix) that accommodate non-movable but dissociable hydrate, a two-phase flow formulation of water and methane gas is suggested according to Darcy's law and capillary pressure law. A single-phase elastic–perfectly plastic constitutive model for hydrate soil sediments, based on the concept of effective stress, is developed to account for the effect of hydrate saturation on mechanical strength and stiffness. The formulation is incorporated into the explicit sch...
142 citations
Abstract: Difficulties in obtaining high-quality soil samples from deepwater sites have necessitated increasing reliance on piezocone, T-bar and ball penetration tests to determine soil properties for design purposes. This paper reports the results of an international collaborative project in which a worldwide high-quality database of lightly overconsolidated clays was assembled and used to evaluate resistance factors for the estimation of intact and remoulded undrained shear strength from the penetration resistance of each device. The derived factors were then compared with existing theoretical solutions to evaluate the influence of particular soil characteristics. The overall statistics showed similar levels of variability of the resistance factors, with low coefficients of variation, for all three types of penetrometer. However, correlations of the resistance factors with specific soil characteristics indicated that the resistance factors for the piezocone were more influenced by soil stiffness, or rigidity inde...
TL;DR: In this article, a triaxial sample of railway ballast is modelled using clumps of spheres and the interlocking and fracture of larger asperities is modeled by bonding eight small balls to the clump.
Abstract: Triaxial samples of railway ballast have been modelled using clumps of spheres. The bulk of the clump is formed from ten balls in a tetrahedral shape. The interlocking and breaking of very small asperities which find their way into the voids is modelled using weak parallel bonds between clumps. The interlocking and fracture of larger asperities has been modelled by bonding eight small balls to the clump. Monotonic tests have been performed on triaxial samples under a range of confining pressures from 15 kPa to 240 kPa and the results compared with existing experimental data. Tests were also simulated using uncrushable clumps to highlight the important role of asperity abrasion. Cyclic triaxial tests were then simulated on the same aggregates using the same microparameters as the monotonic tests under a range of stress conditions and the results were compared to existing experimental data for the same simulated ballast. The results show that the clumps are able to capture the behaviour of ballast under bot...
TL;DR: In this article, the authors investigated the time-dependent behavior of Murro test embankment in Finland, which exhibits large strain anisotropy, destructuration and viscosity.
Abstract: This paper investigates the time-dependent behaviour of Murro test embankment in Finland. The embankment was built in 1993 on a soft natural clay deposit, which exhibits large strain anisotropy, destructuration and viscosity. The study is based on extensive experimental data from triaxial and oedometer tests on intact and reconstituted soil samples which shed light on the influence of time on mechanical properties, including testing designed for studying soil anisotropy and destructuration. The interpretation of the results is done in the framework of a recently developed elasto-viscoplastic model EVP-SCLAY1S, which is used to simulate the soft soil deposit coupled with Biot's consolidation theory. The determination of model parameters from the test results demonstrates that the model can be relatively easily used for practical applications. Using these parameters, two-dimensional finite-element analyses have been made as large deformation analysis. The comparisons between calculations and measurements de...
TL;DR: In this paper, a novel technique of distributed strain sensing is introduced using Brillouin optical time domain reflectometry to examine the performance of an old masonry tunnel during the construction of a new tunnel beneath it in London.
Abstract: A novel technique of distributed strain sensing is introduced using Brillouin optical time-domain reflectometry to examine the performance of an old masonry tunnel during the construction of a new tunnel beneath it in London. Optical fibres attached along the intrados of the tunnel lining at five circumferential sections recorded the relative strains of tunnel deformation between the fixing points as well as detecting localised strain such as cracking in the masonry. The results indicated where cracking might have occurred and were confirmed with visual inspection. Measurements were compared with the data obtained by surveying using total station units and the results show close agreement between the two systems. Flexural behaviour along the longitudinal section of the tunnel was also examined by measuring strain along the springlines and crown.
TL;DR: In this article, the authors present results of periodic cell simulations on a polydisperse system of 27, 000 elastic spheres and show that the relationship between the Lode angle for stress and the Lodes angle for strain can be characterized by the curvature of a circular arc and, for radial devi...
Abstract: The paper presents results of periodic cell simulations on a polydisperse system of 27 000 elastic spheres. In order to explore general three-dimensional stress space an initially isotropically compressed system is subjected to radial deviatoric strain paths and the corresponding stress and fabric responses are illustrated. It is shown that two parameters can be identified that characterise the stress and fabric respectively during general three-dimensional straining. Each parameter, when plotted against deviatoric strain, provides a unique evolution curve, irrespective of the loading direction. It is demonstrated that, for general states of stress, the magnitude of the deviatoric stress is entirely due to the strong force chains that constitute the sub-network of contacts transmitting greater than average contact forces. It is also demonstrated that the relationship between the Lode angle for stress and the Lode angle for strain can be characterised by the curvature of a circular arc and, for radial devi...
TL;DR: In this paper, a simple model is presented for quantification of the contribution of suction to the effective stress of the solid skeleton including the hysteresis effects due to wetting and drying.
Abstract: In this note, results from a careful programme of experimental research are presented examining the variation of the effective stress parameter along main drying and wetting paths and in transition from drying to wetting. Both dimensional and non-dimensional representations of the results are discussed and the general trends observed are highlighted. A simple model is presented for the quantification of the contribution of suction to the effective stress of the solid skeleton including the hysteresis effects due to wetting and drying. In addition, it is shown that the degree of saturation may not be a suitable candidate as the effective stress parameter for such quantification.
TL;DR: In this article, a small-scale physical modeling method was developed to study the movement of clay during pile installation, and the results of this study could be used to assess the impact of disturbance due to pile installation on, for example, buried services or archaeology.
Abstract: A small-scale physical modelling method was developed to study the movement of clay during pile installation The clay was simulated using a mixture of amorphous silica and mineral oil, which becomes almost transparent when the refractive indices of the oil and the silica are well matched After adding reflective particles and consolidating the mixture in a transparent container, cylindrical model piles were driven vertically at the centre A vertical section aligned with the pile centreline was illuminated by a laser light sheet, and a sequence of digital images was recorded These were analysed using particle image velocimetry, and the complete displacement distribution during the pile installation was obtained Notwithstanding some discrepancies at shallow depths, the observed displacements generally showed fairly good agreement with the theoretical predictions of the shallow strain path method (SSPM) once the effect of some soil trapped beneath the flat pile tip was taken into account Normalisation of the horizontal and vertical components of movement employing both the pile length and radius, based on SSPM theory, was shown to be valid The normalised vertical displacement contours were similar to those published by previous researchers The results of this study could be used to assess the impact of disturbance due to pile installation on, for example, buried services or archaeology
TL;DR: In this article, the meso-scale kinematics are quantified from digital-image-correlation-based, grain-scale displacement analyses performed on digital images of specimens undergoing plane strain compression.
Abstract: Precise knowledge of the post-peak constitutive response occurring within shear bands in sands is of keen interest in geomechanics, particularly for accurate modelling of progressive failure phenomena. There is mounting evidence that the displacement field within shear bands in sands is non-uniform and distinguished by distinct meso-scale features: namely, particle force chains. Experimental validation of such features will help elucidate the precise nature of the deformation field within shear bands. This paper presents experimental evidence of the kinematic signatures of force chain activity within shear bands in sands. The meso-scale kinematics are quantified from digital-image-correlation-based, grain-scale displacement analyses performed on digital images of specimens undergoing plane strain compression. As in previous work, the data reveal distinct, systematic patterns in the kinematics along the length of the shear band, which serve as indirect evidence of force chain build-up and collapse. Herein,...
TL;DR: In this article, a study of the micromechanics of granular materials as affected by the direct shear test scale using the discrete element method (DEM) is presented.
Abstract: This paper presents a study of the micromechanics of granular materials as affected by the direct shear test scale using the discrete element method (DEM). Parametric studies were conducted to investigate the effects of specimen length and height scales (in relation to the particle size) on the bulk material shear strength and shear banding behaviour in the direct shear test. A mesh-free strain calculation method previously developed by the authors was used to capture and visualise the evolution of strain localisation inside the direct shear box. Simulation results show that the maximum shear strength measured at the model boundaries increases with decreasing specimen length scale and increasing specimen height scale. Micromechanics-based analysis indicates that the local and global aspects of fabric change and failure are the major mechanisms responsible for the specimen scale effect. Global failure along the primary shear band prevails when the specimen length scale and length to height aspect ratio are...
TL;DR: In this paper, the authors present a quantitative assessment of particle orientation, contacts and spatial variation of void ratio in a sand specimen loaded under plane-strain conditions, and use a synchrotron X-ray microtomography technique to acquire high-resolution three-dimensional images of the cores.
Abstract: The paper presents a quantitative assessment of particle orientation, contacts and spatial variation of void ratio in a sand specimen loaded under plane-strain conditions The specimen has a prismatic shape, and measures 57 mm wide × 121 mm long × 180 mm high It was prepared using a dry natural silica sand known as F-75 Ottawa sand at a dense packing, and was sheared until failure The specimen failed along a well-defined single shear band It was then stabilised by slowly percolating a low-viscosity epoxy Cylindrical subsamples 8 mm in diameter and 50 mm high were cored from the stabilised specimen at regions of interest for further investigation A synchrotron X-ray microtomography technique was used to acquire high-resolution three-dimensional images of the cores The scans have a voxel size of 15·06 μm, which is high enough to visualise individual sand particles Post-scanning analyses included mapping the spatial distribution of void ratio and particle contacts inside and outside the shear band A
TL;DR: In this article, the authors presented data from laboratory triaxial tests on quartzitic sand reinforced with polypropylene fibres, and developed a framework of behaviour for the sand-fibre material, which provides a solid base for future research on fibre-reinforced soils.
Abstract: Fibres can be an effective means of reinforcing soils. This paper presents data from laboratory triaxial tests on quartzitic sand reinforced with polypropylene fibres. By keeping the studied composite consistent throughout the study (host sand and fibre characteristics kept constant), it has been possible to develop a framework of behaviour for the sand–fibre material, which provides a solid base for future research on fibre-reinforced soils. Data from previous work and from new tests have been analysed within the Critical State framework, that is in terms of normal compression line, critical state line and state boundary surface.
TL;DR: In this article, a general classification for scale in geotechnical engineering is used to explore the modelling of large geographically distributed systems and their response to geohazards, both component and net.
Abstract: A general classification for scale in geotechnical engineering is used to explore the modelling of large, geographically distributed systems and their response to geohazards. Both component and net...
TL;DR: In this paper, a simple method to reduce the error in the principal moments of inertia of overlapping mass is proposed, and the effect on a falling heap of ballast particles is shown; the method improves not only behaviour but also computational time.
Abstract: The use of clumps of overlapping spheres in DEM can be used to represent realistic particle shapes. This Technical Note discusses a method that is simple and fast, and highlights the key parameters influencing particle resolution. However, the problem with overlapping mass is that the density is non-uniform, which leads to incorrect moments of inertia and therefore rotational resistance. This Technical Note addresses this issue, and proposes a simple method to reduce the error in the principal moments of inertia. The effect on a falling heap of ballast particles is shown; the method improves not only behaviour, but also computational time.
TL;DR: In this paper, the authors developed a parallel continuum model for the mechanical consequences of erosion, based on two-dimensional discrete element analyses of assemblies of circular discs of various gradings, where the process of internal erosion of fine particles from a soil progressively narrows the grading of the soil.
Abstract: The process of internal erosion of fine particles from a soil progressively narrows the grading of the soil. Development of a continuum model for the mechanical consequences of erosion has taken inspiration from two-dimensional discrete element analyses of assemblies of circular discs of various gradings. The process of erosion has been described in these analyses by progressively removing the finer particles while maintaining the sample under constant stresses. The removal of particles produces an increase in specific volume because the volume of solid decreases and the volume of void increases, but the looser structure compresses under the external stresses. Parallel continuum modelling takes an existing distortional hardening model, in which the critical state line plays a central role, and adds a volumetric deformation mechanism to describe the observed compression. The discrete element modelling analyses and other results remind us that the first-order effect of narrowing grading is the raising of th...
TL;DR: In this article, a model for predicting the accumulated rotation of stiff piles under random two-way loading is presented, based on a strain superposition rule similar to Miner's rule and uses rainflow-counting to decompose a random time-series of varying loads into a set of simple load reversals.
Abstract: A model for predicting the accumulated rotation of stiff piles under random two-way loading is presented. The model is based on a strain superposition rule similar to Miner's rule and uses rainflow-counting to decompose a random time-series of varying loads into a set of simple load reversals. The method is consistent with the work of LeBlanc et al. (2010) and is supported by 1g laboratory tests. An example is given for an offshore wind turbine indicating that accumulated pile rotation during the life of the turbine is dominated by the worst expected load.
TL;DR: In this article, a series of resonant column tests were carried out on a range of sand-sized geomaterials to explore the effects of different hydrate cement morphologies on the very small strain stiffness of the materials.
Abstract: Cementing of sediment occurs naturally in many soils and weak rocks, during both the early and late stages of diagenesis. This paper reports the results of a series of resonant column tests carried out on a range of sand-sized geomaterials to explore the effects of different hydrate cement morphologies on the very small strain stiffness of the materials. It is shown that the proportion of void space filled by hydrate cement, cement location, sand size and grain shape all have a significant effect on shear modulus. The change in stiffness between different host materials is affected by the density, specific surface and grading of the geomaterial.
TL;DR: In this article, ground deformations were measured using geophones at set distances from the centreline of the track within each borehole, and the results showed vertical displacements reducing with depth, from a maximum at the sleeper.
Abstract: This paper presents the results of a detailed investigation into the ground deformations that occur under a railway line during the passage of a train. Four horizontal boreholes were installed at different depths below a ballasted railway track. Ground deformations were measured using geophones at set distances from the centreline of the track within each borehole. The results show vertical displacements reducing with depth, from a maximum at the sleeper. Sleeper displacements are dominated by pairs of bogies at the ends of adjacent wagons (which have a frequency of loading 1 Hz), although the effects of individual bogies (2 Hz) and axles (6 Hz) are also apparent. Higher loading frequencies attenuate with depth so that at a depth of 0·780 m below the sleeper soffit no axles are visible within the displacement data and by a depth of 1·98 m only the combined effect of pairs of adjacent bogies is apparent. In contrast, longitudinal horizontal motion is greatest at a depth of 0·78 m below the sleeper soffit, ...
TL;DR: In this paper, the pore size distribution of clays during compression has been investigated and compared with fine-grained clays, and the analysis is applied to six other sensitive clays.
Abstract: Investigation of micro–macro relationships in soils most often concerns granular soils in which both the elemental unit (the grain) and the physical laws governing inter-grain interactions appear to be better known. The situation is different for clays because the elementary unit and the inter-unit interactions at the micro-scale are more difficult to characterise. In fine-grained soils, it has been shown that an intermediate level, corresponding to the way clay and fine-grained particles are arranged together, had to be considered so as to link microscopic features to macroscopic behaviour. An investigation of the change in microstructure during compression carried out some time ago on a sensitive clay from Canada demonstrated that studying the changes in pore size distribution provides a satisfactory description of microstructure changes during compression. The analysis is applied here to six other sensitive clays. First, a careful examination of the intact and remoulded microstructure is conducted in o...
TL;DR: In this article, a series of centrifuge model tests has been conducted to investigate the bearing resistance of spudcan foundations of offshore jack-up rigs in sand overlying normally consolidated clay.
Abstract: A series of centrifuge model tests has been conducted to investigate the bearing resistance of spudcan foundations of offshore jack-up rigs in sand overlying normally consolidated clay. The spudcan bearing-resistance profiles measured in most of the tests recorded a peak resistance at a shallow depth within the upper sand layer, followed by an abrupt post-peak reduction in resistance causing the spudcan to plunge into the underlying soft clay. This phenomenon is commonly termed as 'spudcan punch-through hazard', which potentially leads to severe structural damage of jack-up rigs in the field. The centrifuge test results revealed that the ratio of upper sand layer thickness over spudcan diameter and the ratio of bearing resistance between the upper sand and underlying clay affect the development of spudcan bearing resistance. The limitations of existing design methods, which were derived for the ultimate bearing capacity of pre-embedded shallow foundation to assess the spudcan bearing resistance-depth profile, are also identified. It is proposed that the profile may be assessed by focusing on three key characteristic bearing resistances.
TL;DR: In this article, the authors used discrete element modeling (DEM) to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory.
Abstract: Discrete element modelling (DEM) has been used to simulate creep in assemblies of spherical grains possessing an interfacial coefficient of friction that varies with sliding velocity according to rate process theory. Soil stiffness is represented by a pair of values of linear spring stiffness normal and tangential to each inter-granular contact, and the limiting coefficient of contact friction is described as varying linearly with the logarithm of sliding velocity. DEM simulations of an assembly of 3451 spheres reproduce a number of significant phenomena including: creep rate as a function of the mobilisation of deviatoric stress; initially linear decay of creep strain rate with time plotted on log-log axes and with a slope m in the range −0·8 to −1; and ultimate creep failure in triaxial simulations at high deviatoric stress ratios. Creep-induced failure is shown to occur at a unique axial strain for a given state of initial packing, and to be linked with dilatancy. The numerical results are compared qua...