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Abstract In traditional finite element failure analyses of geotechnical structures, the micro grain rotations cannot be modelled and numerical solutions are mesh dependent. In this study, a user element including rotational degree of freedom has been developed based on micropolar theory (Cosserat theory), then an enhanced non-associated sand model is calibrated with laboratory data and used to model the plane strain tests. The simulated results demonstrate the polarized model is able to model reasonably the sand behavior as well as the grain rotations in the localized region. What’s more, with this enhanced model, the mesh independent numerical solutions in terms of mechanical responses, shear bands thickness and orientations have been obtained. Article highlights In failure analysis of geostructures, significant rotations of soil grains have been observed to occur in the strain localized regions, but the current commercial Finite Element tools cannot model the micro rotations. Therefore, a user defined element must be developed to include the rotational degree of freedom. The micropolar approach is proven to be effective to model the grain rations in present paper. More suitable than other classical soil or sand constitutive models, the selected non-associated sand model in present paper is capable of describing well the contraction and shear dilatancy behaviors of sand. Then the model has been enhanced by means of micropolar technique, in this way, the reasonable strain localization phenomena in laboratory tests could be predicted well. There are always the mesh dependent problems for traditional simulations of the strain localization phenomena in finite element analysis. It can be found in present paper that the mesh independent numerical solutions are obtained by means of micropolar technique. Furthermore, the micropolar approach can obviously improve convergence difficulties in finite element analyses.

Soft soil subjected to cyclic loading typically exhibits an increase in excess pore pressure under undrained condition which brings the soil to an overconsolidated state. Then, under a subsequent large number of cycles (e.g. more than one million) which also takes time, the creep at overconsolidated state influences the cyclic effect and thus results in a complicated long-term undrained behavior. This paper aims to clarify this long-term undrained behavior of overconsolidated clay. The reconstituted samples are prepared from natural samples retrieved in the north of France. First, the shear strength characteristics along monotonic triaxial stress paths are identified. Then load control cyclic tests on overconsolidated specimens are conducted in fully saturated and undrained conditions. Small cyclic deviatoric stresses are applied in order to investigate more particularly the behavior under a very large number of cycles, during which an unusual pore pressure evolution is observed. To explain this, undrained triaxial creep tests are performed on reconstituted specimens with different values of OCRs under some specified stress states. The evolutions of axial strain, excess pore pressure, stress ratio, stress path, plastic strain rates and stress dilatancy during undrained creep are discussed. The additional undrained creep tests also show that two processes are simultaneously acting in a competitive manner: increase in the pore pressure due to the cyclic loading and decrease in the pore pressure because of creep.

It is well known that the compressibility of crushable granular materials increases with the moisture content, due to the decrease of particle strength in a humid environment. An existing approach to take into account the effect of grain breakage in constitutive modeling consists in linking the evolution of the grain size distribution to the plastic work. But how the material humidity can affect this relationship is not clear, and experimental evidence is quite scarce. Based on compression tests on dry and saturated crushable sand recently reported by the present authors, a new non-linear relationship is proposed between the amount of particle breakage and the plastic work. The expression contains two parameters: (1) a material constant dependent on the grain characteristics and (2) a constant depending on the wetting condition (in this study, dry or saturated). A key finding is that the relationship does not depend on the stress path and, for a given wetting condition, only one set of parameters is necessary to reproduce the results of isotropic, oedometric, and triaxial compression tests. The relationship has been introduced into an elastoplastic constitutive model based on the critical state concept with a double yield surface for plastic sliding and compression. The breakage ratio is introduced into the expression of the elastic stiffness, the critical state line and the hardening compression pressure. Incremental stress-strain computations with the model allow the plastic work to be calculated and, therefore, the evolution of particle crushing can be predicted through the proposed non-linear relationship and reintroduced into the constitutive equations. Accurate predictions of the experimental results in terms of both stress-strain relationships and breakage ratio were obtained. Keywords: Grain crushing, Wetting, Constitutive modeling

International audience; We examine stationary regimes in granular materials from a dynamical systems theory (DST) perspective. The aim is to enrich the classical view of the critical state regime in granular materials, and more broadly, improve fundamental understanding of the underlying mesoscale mechanisms responsible for macroscopic stationary states in complex systems. This study is based on a series of discrete element method (DEM) simulations, in which 2D assemblies of non-uniformly sized circular particles are subjected to biaxial compression under constant lateral confining pressure. The lifespan and life expectancy of specific cluster conformations, comprising particles in force chains and grain loops, are tracked and quantified. Results suggest that these conformational clusters reorganize at similar rates in the critical state regime, depending on strain magnitudes and confining pressure levels. We quantified this rate of reorganization and found that the material memory rapidly fades, with an entirely new generation of force chains and grain loops replacing the old within a few percent strain.

International audience; We revisit the Critical State (CS) in a dynamic perspective at a mesoscopic scale to enrich the classical view of the critical state as a steady state. The dynamics of meso-structures has been characterized by recording generating and vanishing processes in discrete element simulations. Our results suggest that mesostructures have a short but regular lifespan at critical state along a biaxial compression loading path.

International audience; The aim of this paper is to offer a fresh perspective on the classic concept of critical state (CS) in granular materials by suggesting that CS can be defined through the use of a single proportional strain test. In classic conventional testing, CS manifests itself under constant lateral stress and controlled strain in one given direction whenever continuous shearing is applied without change being induced to material volume. However, a comparison between proportional strain tests and biaxial tests simulated with DEM has clearly shown that the CS line (CSL) characterized by stresses, void ratio and fabric indexes can act as an attractor. The mechanical responses and fabric metrics evolve along dilatant proportional strain loading paths according to similar values after the strain level has become large enough to wipe out the material memory in the homogeneous domains considered in this analysis, i.e., the shear band area in dense samples and the whole area in loose samples. This suggests that the micro-structure of a granular material subjected to any dilatant proportional strain loading paths evolves while preserving its ability to withstand shearing without volume change at any time. Therefore, the CS concept can be generalized to a wide class of loading paths which shows that CS acts as a general attractor irrespective of the loading path considered.

Investigating the mechanical properties of rockfills requires laborious, time-consuming, and expensive large-scale testing. Therefore, practitioners frequently adopt design parameters based...

The effect of water on the hydromechanical behavior of unsaturated granular materials has been studied with a micromechanical model based on thermodynamic principles. A general framework based on the theory of thermodynamics with internal variables for constructing thermodynamically consistent multiscale constitutive relations for unsaturated granular materials has been developed. Within this framework, the microscopic total Helmholtz free energy has been separated between a mechanical and a hydraulic part, each of which is a function of either the elastic displacement or the capillary bridge volume and the distance between particles at the microscale. The inter-particle dissipation of energy, assumed to be frictional in origin, is a function of the incremental plastic displacements at the microscale. Both the microscale Helmholtz free energy and the dissipative energy have been volumetrically averaged to obtain the homogenized energy functions at the macroscale. In accordance with the suggested multiscale thermomechanical framework, a micromechanical model has been constructed to describe the behavior of partially saturated granular soils. This model has considered the deformation of soil skeleton by applying a Coulomb-type criterion at the inter-particle contacts. The hydraulic potential is made to be dependent on the size of the particles and is derived through use of the expression for the water retention curve by assuming that liquid bridges are isotropically distributed within the specimen. The performance of the suggested model has been demonstrated through numerical simulations of the behavior of sand under various degrees of saturation and a wide range of mechanical loadings.

International audience; Grain breakage during compression and shearing is one important mechanism responsible for irrecoverable changes of the mechanical properties of granular materials. Here we present results of triaxial tests on limestone fragments under some monotonic and cyclic stress paths and we investigate the relationships between the progression of grain breakage, the plastic work and the evolution of the critical state line. Using the plastic work concept, we propose a method for determining grain shape and grain breakage indices, and we show how grain breakage influences the critical stress state, and hence the mechanical behaviour. The validity of the relationships is then verified on different granular assemblies (granite fragments and quartz sands), although further tests remain necessary to validate them under more generic stress paths.

Fluidised landslides are one of the most dangerous types of mass movements as they can run over long distances at high velocity. A flow-like landslide can occur in both artificially designed and natural slopes, resulting in extensive property damage and significant loss of life. Although various studies have examined the initiation mechanism of this type of landslide, the understanding of the phenomenon of instability which can occur in loose granular slopes deserves further attention. Specific influential factors, such as the changes in the soil microstructure due to water infiltration and seepage forces, still need to be further examined. In this study, the initiation of a fluidised landslide is investigated through flume tests. The tested material was collected from a coseismic landslide deposit in the 2008 Wenchuan earthquake area. These loose, granular deposits can fail due to intense rainfall and exhibit flow-like movement, potentially evolving into destructive debris flows. Two flume apparatuses have been used, which were equipped with multiple sensors to evaluate the pore pressures and the internal displacements. A high-sensitivity seismic accelerometer was installed at the bottom of the flume to record the vibrations produced by the movement of the soil particles during the tests. The soil mass was progressively wetted either by a uniform inflow from the ground surface at the top of the slope, to simulate the infiltration of run-off water arriving from upslope, or by a gradual rise of the water level at the upper boundary of the slope, to simulate a gradual rise of the groundwater table. Internal erosion of the finer soil fraction driven by the seepage forces is thought to have played a significant role in the slope collapse. Possible changes in the particle arrangement were recorded by the accelerometers in the form of an increasing high-frequency vibration signal before any significant rise of pore pressure and any significant displacement of the slope could be observed. Although further parametric investigation is still necessary, and tests at real scale should be devised, the observed behaviour suggests that seismic monitoring of loose, granular deposits could be a promising method for detecting an incipient slope collapse.

International audience; This paper studies the dilative behavior of clay during drained creep along triaxial tests under constant mean effective stress, where the volumetric strain is caused by the sole deviatoric stress variation. Tests were conducted on a saturated reconstituted clayey material by the use of a GDS stress path control apparatus. The development of the axial and volumetric strains was studied with respect to time, stress level (SL) and overconsolidation ratio (OCR). Analysis of the results ascertained that both dilatancy and contractancy phenomena could occur during creep. The magnitude of the dilatancy/contractancy during creep was guided by the test conditions (SL and OCR), which specifically governed the direction of the volumetric strain variations. The position of the stress level vis-à-vis the different volumetric domains defined by monotonic triaxial tests in the (p′–q) plane controls the evolution of the volumetric creep strain. The failure of an overconsolidated specimen could be observed for a stress level located under the maximum strength envelope, but above the critical state line in the p′–q plane, accompanied by a significant dilative behavior during creep.

The aim of this paper is to answer the question: how to construct a thermodynamically consistent micromechanical model and to analyze the existing micromechanical models from an energy perspective. For this purpose, we extended the framework of thermodynamics to micromechanical models by considering the energy stored and dissipated at the inter-particle contacts. In the suggested framework, the Helmholtz free energy and the dissipation energy at the macro scale are equaled to the volumetric average of the Helmholtz free energy and the dissipation energy at the micro scale. Consequently, the elasto-plastic formulation at inter-particle contacts can be obtained from the expressions of the micro free energy and the micro dissipation potentials. A thermodynamically consistent micromechanical model has been constructed on the basis of the static hypothesis. An isotropic compression and several triaxial tests were simulated with the model to analyze the energy conservation and dissipation under loading. Free energy and dissipation energy were computed at both micro and macro scales, and the orientation of the failure plane was explained by the evolution of the local dissipation energy. The maximum micro dissipation energy agreed with the static hypothesis for which, when one direction reaches the limit state, the rupture at the macro scale is obtained. The model has been implemented into a finite element code, and with this code a biaxial compression test was performed. Strain localization appeared, and the orientations of the shear band displayed by either the accumulated deviatoric plastic shear strain or the dissipative energy were consistent.

Landslides of the flow type are among the most destructive mass movements due to their potential high mobility, long runout distance and capacity of entraining further material during movement. Generation of positive pore pressure excess is considered to be a fundamental triggering mechanism which, in some cases, can produce stress state instability and fluidization of both fine-grained and granular deposits, resulting in extremely fast and catastrophic events. Several experimental works and theoretical studies have been carried out so far to investigate the initiation mechanism and runout of flowslides. However, efforts are still needed to improve the understanding, for instance, of the role of the soil grading on the initiation and evolution of the movement. To this aim, a series of flume tests has been carried out using granular mixtures of coarse and small particles. High temporal resolution data acquisition has been used to investigate the onset of failure in terms of pore pressures, surface and internal displacements and vibration signals. By means of shear-controlled triaxial tests and permeability tests, the hydraulic conductivity has been found to be a controlling factor in the landslide initiation, whereas the uniqueness of the instability line – the lower bound of potentially unstable stress states – regardless of the soil grading, suggests that pore pressure measurement in the field can serve as a reliable indicator in early-warning systems.

Summary A comparative study of optimization techniques for identifying soil parameters in geotechnical engineering was first presented. The identification methodology with its 3 main parts, error function, search strategy, and identification procedure, was introduced and summarized. Then, current optimization methods were reviewed and classified into 3 categories with an introduction to their basic principles and applications in geotechnical engineering. A comparative study on the identification of model parameters from a synthetic pressuremeter and an excavation tests was then performed by using 5 among the mostly common optimization methods, including genetic algorithms, particle swarm optimization, simulated annealing, the differential evolution algorithm and the artificial bee colony algorithm. The results demonstrated that the differential evolution had the strongest search ability but the slowest convergence speed. All the selected methods could reach approximate solutions with very small objective errors, but these solutions were different from the preset parameters. To improve the identification performance, an enhanced algorithm was developed by implementing the Nelder-Mead simplex method in a differential algorithm to accelerate the convergence speed with strong reliable search ability. The performance of the enhanced optimization algorithm was finally highlighted by identifying the Mohr-Coulomb parameters from the 2 same synthetic cases and from 2 real pressuremeter tests in sand, and ANICREEP parameters from 2 real pressuremeter tests in soft clay.

International audience; In soil mechanics, the material behaviour is significantly affected by grain crushing. This energy dissipating mechanism, along with the frictional rearrangement of grains, is responsible for the energy loss associated with plasticity. However, the micromechanical factors at the grain scale have not been studied by constitutive models which tend to focus on macroscopic parameters. In this study, the influence of the coordination number on the fragmentation of a single cylindrical grain specimen has been examined with a new device by which a series of multipoint crushing tests has been conducted. Experiments conducted with this device have shown the importance of the contacts number, position, type and force in the fragmentation of the individual grain. The results, analysed by imaging techniques, demonstrate that the existing models treating grain rupture are indeed incapable of reproducing the observed fragmentation. Two types of cracks were distinguished, each corresponding to a different crack mode, depending on the contact arrangement. The results of this study could be integrated into fragmentation models for predicting the occurrence of cracks, and the shape of the resulting fragments.

According to field feedbacks from high-speed lines (HSL), the increase of train operating speeds is responsible for unusual fast evolving geometrical disorders in ballasted tracks. This paper deals with the search of solutions applicable at the design stage to mitigate these disorders. The starting point of the present work relies on the assumption, comforted by the literature, of a strong correlation between disorders and vertical accelerations in the ballast layer induced by the train passages. This led us focus herein on the calculation and the analysis of accelerations in the railway structure. The vertical accelerations (γ z) are computed using the in-house developed numerical program ViscoRail and on the basis of a reference HSL. These are shown to increase strongly with the train speed attesting to the link between the train speed and the geometrical disorders in ballast. Then, other simulations are run varying some structural parameters to evaluate their impact on the acceleration field γ z. In that way, we show that decreasing the stiffness of the mechanical connection between the rails and the ballast, increasing the moment of inertia of the rails or the Young modulus of the sub-ballast layer, leads to a decrease of γ z and could provide solutions for the design of future HSL. The solution consisting in the incorporation of an asphalt sub-ballast layer, as already experimented on sites, is finally examined in more details.

A mechanical approach for unsaturated expansive soils considering double porosity has been developed based on the porous media theory. In this approach, the adsorbed and the capillary water, as well as the micropores and macropores, are two distinct phases. An interaggregate stress considered as the work-conjugate of the macrostructural strain increment has been defined. Both physicochemical and capillary effects of the pore water have been introduced at the macroscopic level. Other work-conjugate variables relevant for the constitutive modeling of double-porosity unsaturated media have also been identified, consisting of the modified suction as conjugate of the increment of the macrostructural degree of saturation and the microstructural effective stress as conjugate of the microstructural volumetric strain increment. A hydromechanical model for unsaturated expansive clays taking into account the interaction between the micro- and the macrostructures in expansive clays can thus be built. Based on the bounding surface concept, an anisotropic loading – collapse yield surface has been introduced to reproduce the three-dimensional mechanical behavior. To analyze the model capabilities, two series of laboratory tests consisting of multiple wetting and drying cycle tests on Boom clay and triaxial tests on Zaoyang (ZY) expansive clay were simulated. The comparisons between numerical and experimental results show that the model can reproduce with reasonable accuracy the mechanical behavior and the water retention characteristic of unsaturated expansive clays.

Most constitutive models of soils are developed under the framework of elastoplasticity. This chapter presents the step-by-step modeling methodology from Mohr-Coulomb to SIMSAND. First, the experimental phenomena of shear behavior for normally consolidated or loose soils and for overconsolidated or dense soils are presented. Capturing the shear characteristics of soils, the Mohr-Coulomb model is presented to discuss the significance of the model parameters. By implementing nonlinear elasticity, nonlinear plastic hardening, and nonlinear stress–dilatancy, the development of a nonlinear Mohr-Coulomb model is presented. Further considering the critical state feature of soils, the model is extended to the SIMSAND model. For practical purposes, the development of the MATLAB code for simulating triaxial tests using SIMSAND is presented. Finally, some elastoplastic models in the EROSLAB platform are presented for practical applications.

Due to the complexity of the hydro-mechanical behaviour of soils subjected to internal erosion, a high number of parameters are usually required for the erosion models and the constitutive models. This aspect makes it difficult to determine by trial-error their relevant values from laboratory tests. To address this issue, an efficient optimisation-based procedure for identifying the parameters of a recently developed hydro-mechanical model for internal erosion using an enhanced backtracking search algorithm (so-called MBSA-LS) has been proposed. The MBSA-LS incorporates two points: (1) modifying the mutation of the original Backtracking Search Algorithm (BSA) and (2) incorporating an efficient differential evolution (DE) as a local search to improve the optimisation performance. A mono-objective framework with six different criteria has been proposed to identify the parameters related to the interlocking effect and the erosion process. The proposed procedure was successfully applied to identify the parameters from the erosion tests of Hong Kong-Completely Decomposed Granite mixture (HK-CDG). All results demonstrated that coupling the MBSA-LS and the hydro-mechanical erosion model could efficiently solve the issue of parameter identification accounting for both the mechanical behaviour and internal erosion.

Currently, numerical studies at the real scale of an entire engineering structure considering internal erosion are still rare. This paper presents a three-dimensional (3D) numerical simulation of the effects of internal erosion within a linear dike located on a foundation. A two-dimensional (2D) finite element code has been extended to 3D in order to analyze the impact of internal erosion under more realistic hydromechanical conditions. The saturated soil has been considered as a mixture of four interacting constituents: soil skeleton, erodible fines, fluidized fine particles, and fluid. The detachment and transport of the fine particles have been modeled with a mass exchange model between the solid and the fluid phases. An elastoplastic constitutive model for sand-silt mixtures has been developed to monitor the effect of the evolution of both the porosity and the fines content induced by internal erosion upon the behavior of the soil skeleton. An unsaturated flow condition has been implemented into this coupled hydromechanical model to describe more accurately the seepage within the dike and the foundation. A stabilized finite element method was used to eliminate spurious numerical oscillations in solving the convection-dominated transport of fluidized particles. This numerical tool was then applied to a specific dike-on-foundation case subjected to internal erosion induced by a leakage located at the bottom of the foundation. Different failure modes were observed and analyzed for different boundary conditions, including the significant influence of the leakage cavity size and the elevation of the water level at the upstream and downstream sides of the dike.

This chapter presents a step-by-step modeling methodology for clayey soils from the modified Cam-clay model (MCC) to the anisotropic structured clay model (ASCM). First, the experimental behavior of clayey soils under compression is presented. Then, a typical MCC model capturing both the shear and compression characteristics of clayey soils is presented, and the significance of the model parameters is discussed. By implementing the nonlinear stress–strain relationship through the bounding surface method, the effect of structures on plasticity (i.e., anisotropic yield surface, cementation/bonding, and adhesive cohesion) and the effect of structures on elasticity, the development of the ASCM model is presented. For practical purposes, the development of the MATLAB code for simulating triaxial tests using ASCM is presented. Finally, the MCC and ASCM models in the EROSLAB platform are presented for practice.

In order to study the impact of internal erosion at the scale of an engineering structure, a hydro-mechanical continuous modelling approach considering suffusion is needed. It requires a relevant mechanical model for granular soils considering the-dependency (fines content) and a hydraulic model for suffusion to control the changes in the fines content. To this purpose, the mechanical models for granular soil, the unified modelling approaches for-dependency of granular soils and the hydraulic modelling of suffusion are first reviewed. Then, a hydro-mechanical model considering both suffusion and mechanical loading is developed by combining the three components. For each component of the model, alternative choices are provided. Simulations of laboratory tests as well as an example of a dike-on-foundation problem demonstrated the reliability and applicability of this coupled numerical approach for internal erosion problems.

In this chapter, a newly constructed constitutive model is presented for modeling the time-dependent behavior of granular materials. The model consists of dividing the stress tensor into an inviscid stress using a hypoplastic formulation and a viscous stress with a high-order term representing the strain acceleration. Then, different laboratory tests on granular soils are simulated to evaluate the model performance, i.e., a series of constant strain rate triaxial tests and a stepwise strain rate triaxial test on Toyoura sand, a stepwise strain rate triaxial test on Chiba gravelly soil, and creep tests under different stress levels on Toyoura sand. Comparisons between the experimental and numerical results show the capability of the model in describing the strain rate and acceleration effects, as well as the creep behavior of granular materials. Moreover, the coupling between the inviscid stress and viscous stress enables the evolution of the strain rate increase in the creep tests. Consequently, the three stages of creep, namely, primary, secondary, and tertiary creep, can be well described. Finally, the model, named HYPOSAND, is presented in the ErosLab platform as a practical exercise.

In this chapter, some of the typical mechanical behaviors of saturated soils are introduced and summarized, starting with the most basic materials known as “remolded soils”: compression behavior, shear behavior, influence of intermediate principal stress, induced anisotropy, noncoaxiality, small-strain stiffness, cyclic behavior, time dependency, temperature effects, and water chemistry effects. Then, additional mechanical properties are gradually introduced with constitutive modeling methods for intact natural soils, namely, inherent anisotropy, soil structure, and destructuration. Finally, special problematic soils (i.e., organic soft soil, quick clay) are introduced with their special mechanical properties and constitutive modeling approaches. Each basic characteristic is presented with its definition or basic description, and then, the testing methods with the interpretation of test results, as well as commonly used constitutive modeling methods, are introduced.

This chapter presents the multiscale modeling methodology for soils from micro to macro. First, the multiscale feature of soils within the general framework of the multiscale approach to granular materials is presented. Then, the fundamentals of micromechanics are presented in more detail, including interparticle contact laws, definitions of the strain tensor, effective stress tensor and fabric tensor, averaging and localization operators, and homogenization integration. As an example, the micromechanics-based Chang–Hicher (CH) model is presented with experimental validation. Some possible developments of models based on the CH model are presented considering the capillary force, the chemical force in grouted sand, the surface energy force, and the mechanical force in clayey materials. Finally, the MicroSoil model in the ErosLab platform is presented for a practical exercise.

This chapter presents the classic constitutive modeling method known as the conventional elastoplastic theory with different components, such as the elastic stress–strain relationship, the elastoplastic relationship, which includes the yield surface, flow rule, and hardening rule, and Drucker’s stability hypothesis, convexity, and orthogonality. Some typical numerical methods for solving plastic problems are also presented, such as the general explicit solution, the cutting plane method-based implicit solution, and the closest point projection method-based implicit solution.

International audience; This experimental work aimed to identify local mechanisms in remoulded clays under different triaxial loading conditions. The behaviour at a given stress level reached by two different stress paths, i.e. the conventional constant σ′3 stress path and the purely deviatoric stress path, and the behaviour at two different stress levels along a given stress path were examined. The investigation concerned both normally consolidated and overconsolidated saturated specimens of remoulded kaolin clay. The microstructural states of the clay specimens induced by different triaxial loading paths were examined at two different scales using the scanning electron microscopy and the X-ray microtomography. The results highlighted five particle orientation modes that could be activated. These modes seem to be highly dependent on the stress level, the overconsolidation ratio and the stress path. Mesocracks were found to develop within strongly overconsolidated specimens accompanied by the occurrence of dilatancy. The proposed conceptual modes provide an interesting approach to understand the mechanical behaviour of remoulded clays, especially the macroscale dilatancy phenomenon, through the possible activation of different mechanisms at micro- and mesoscales.

In this chapter, the ErosLab platform is first briefly introduced. Then, the numerical scheme of stress–strain mix control is presented, followed by the operations of the ErosLab platform. Subsequently, three examples of simulations, the oedometer test, triaxial test, and simple shear test, are conducted using the ErosLab platform following the above operations. These examples demonstrate the powerful ability of the ErosLab platform in constitutive modeling of laboratory tests. Finally, an example of UMAT allowing users to develop and test their own constitutive models is described, which demonstrates how the platform is open to users.

This chapter summarizes six common laboratory tests, including the oedometer test, triaxial test, simple shear test, biaxial test, true triaxial test, and hollow cylinder torsional shear test, and their associated stress and strain paths for developing constitutive models and calibration are also introduced. Additionally, the operation for simulating each test in the ErosLab platform is introduced for in-depth understanding and practice.

First, a one-dimensional elastic viscoplastic model is developed based on the experimental results of strain rate dependency in oedometer tests. Using this model, the coherence of strain rate dependency, creep, and stress relaxation is established. Then, the model is extended for structured clays by accounting for the effect of interparticle bonds. A three-dimensional viscoplastic model called ANICREEP is presented under the framework of Perzyna’s overstress theory, and the choice of the scaling function is discussed. The developed ANICREEP model also accounts for anisotropy and destructuration. To simulate the different tests, a general explicit algorithm is presented. The development of a MATLAB code for simulating the triaxial tests is also presented. Finally, the ANICREEP model in the ErosLab Platform is presented for practical exercises.

This paper attempts to formulate a coupled practical model in the framework of continuum mechanics to evaluate the phenomenon of internal erosion and its consequences on the mechanical behavior of soils. For this purpose, a four-constituent numerical approach has been developed to describe the internal erosion process. The detachment and transport of the fine particles have been described by a mass exchange formulation between the solid and fluid phases. The stress–strain relationship of the soil is represented by a nonlinear incremental model. Based on experimental data, this constitutive model has been enhanced by the introduction of a fines content–dependent critical state, which allows accounting for the influence of fines on soil deformation and strength. The applicability of the practical approach to capture the main features of the internal erosion process and its impact on the mechanical behavior of the eroded soil have been validated by comparing numerical and experimental results of internal erosion tests on Hong Kong completely decomposed granite (HK-CDG) mixtures, which demonstrated that the practical model was able to reproduce, with reasonable success, the experimental data. Furthermore, the influence of the stress state, the initial soil density, and the initial fraction of fines have been analyzed through numerical simulations using the proposed model.

Up to now, the effect of strain rate on the adhesive bond shearing resistance of stiff clay, which plays a fundamental role in constitutive modeling of such kind of soils, remains unclear. This paper aims to experimentally investigate the strain rate dependency of the adhesive bond shearing resistance of stiff clay. Both undisturbed and reconstituted overconsolidated samples of Merville stiff clay from the Flanders region in the north of France are prepared with different overconsolidation ratios (OCR, up to 56). Oedometer tests and undrained triaxial tests under different confining stresses on samples with different OCRs are conducted. The effect of inter-particle bond is gradually investigated by comparing test results of undisturbed and reconstituted samples with the same OCR. All results suggest a uniqueness of the strain rate dependency in one-dimensional and triaxial conditions with different OCRs for the reconstituted clay, whereas the strain rate dependency of shear strength of overconsolidated clay is found to be higher in undisturbed samples than in reconstituted samples due to inter-particle bonding. This new finding is formulated as the adhesive bond shearing resistance against the strain rate, which should be useful for improving the time-dependent modeling of highly overconsolidated stiff clays.

Stress relaxation is an important phenomenon of viscous behaviour of clay. Current studies only concern the stress relaxation during loading and lack of that under complex loading conditions. This paper focuses on constitutive modelling of stress relaxation of soft clay during complex loading conditions. For this, a newly developed anisotropic elastic viscoplastic model of soft clays is adopted as a basis. The model is extended taking into account the swelling deformation at constant stress or the increasing of stress at constant strain for over-consolidated state of clay, which is inverse to the conventional viscoplastic behaviour for normally or lightly over-consolidation state. The extended model is verified by simulating a series of newly conducted stress relaxation tests during an undrained loading-unloading-reloading shear test on soft clay.

The paper aims to construct an efficient optimization method for identifying creep and destructuration related governing parameters of soft structured clay. An elastic viscoplastic model has been developed and adopted. Different optimization processes, by genetic algorithm or particle swarm optimization with uniform or random samplings initialization methods, are carried out to obtain the material parameters from conventional undrained triaxial tests performed on a K 0-concolidated natural soft clay. All comparisons demonstrate that the uniqueness of the solution is better guaranteed with the genetic algorithm rather than with the particle swarm optimization method. Furthermore, the efficiency of genetic algorithm has been verified by simulating other tests.

Experimental observations have shown that the mechanical behavior of sand–silt mixtures is highly dependent on the proportions of sand and silt. Our present paper proposes a simple elastoplastic model for the stress–strain behavior of sand–silt mixtures. The inter-grain contact index is firstly reviewed. A formulation which links the inter-grain contact index with the void ratio is then proposed and validated by measuring the index void ratios of various sand–silt mixtures. The formulation is applied to determine the position of the critical state line of sand–silt mixtures from sand to silt. Based on this formulation, a simple elastoplastic model is then developed. The model parameters can be easily determined from conventional triaxial tests and measurements of the index void ratios. The predictive capability of the model was examined by comparing the simulations and the experimental results of undrained triaxial tests on Foundry and Ottawa sand–silt mixtures from sand to silt.

This paper aims at showing that the reversible behavior of track ballast can be reasonably modeled by a homogeneous elastic layer in the computation of the reversible dynamic response of railway structures subjected to moving train loads. To accomplish this, 2D simulations by means of the discrete element method (DEM) and triaxial tests were carried out. Importantly, these two approaches were performed placing a soft support under the granular specimen. A small number of loading cycles has been considered in both approaches since the long-term settlement of the ballast layer is not the focus of this study, unlike most of the existing research on this topic. In the DEM part, the transfer of the mechanical loads applied to the sample through force chains in the granular medium has been illustrated and discussed based on the numerical responses obtained. In addition, the analysis of the overall response of the grain samples using a homogenized constitutive law shows that the reversible behavior of a ballast layer can be modeled by a continuum approach. However, investigating the effect of the subbase stiffness on the mechanical response of the ballast layer indicates that the homogenized behavior is not intrinsic but depends on the sublayer. These results were confirmed by triaxial tests performed in the laboratory on samples of crushed stones resting on an elastomer mat with different values of the stiffness. Finally, the results from the DEM simulations and the triaxial tests attest that the reversible response of railways can be computed using an elastic layer for the ballast bed provided that an appropriate choice for its apparent modulus is made. The approach that consists of deducing a reversible homogenized behavior of track ballast based on DEM simulations and/or triaxial tests taking into account the support layer looks promising. In future research, more realistic DEM simulations could be envisaged to develop more accurate constitutive laws that could integrate nonlinear and/or anisotropic aspects.

International audience; The aim of this study is to understand the local mechanisms related to creep behavior of a typical clay under triaxial loading. The investigation concerns both normally consolidated and overconsolidated specimens of remolded kaolin clay. The macroscopic results showed that both dilatancy and contractancy could occur during creep depending mainly on the clay behavior prior to the creep test. The magnitude of the dilatancy/contractancy was controlled by the stress level, on one hand, and by the overconsolidation ratio which governed the sign of the volumetric strain variation during triaxial loading, on the other hand. At the microscopic scale, the dilatancy/contractancy phenomena were analyzed using the scanning electron microscopy (SEM). The results indicated that the microstructural evolution of the clay along mechanical loading depended on the stress history. The structural evolution during the creep phases followed the structural pattern developed under monotonic loading. The creep dilatancy phenomenon appeared strongly related to the expansion of micro pores and micro cracks within the overconsolidated clay specimens.

Volumetric strain of a soil element surrounding the pile accumulated under the drained cyclic loading condition is one of major concerns in geotechnical engineering design. In this paper, twenty stress-controlled drained simple shear tests, with 5000 cycles on Fontainebleau sand have been carried out, which aims to investigate four factors influencing cyclic accumulation behaviour: initial void ratio, normal stress, cyclic shear stress ratio and average shear stress ratio. Based on results, the cyclic volumetric strain accumulation for each influencing factor is summarized, which is found highly sensitive to the cyclic shear stress ratio that a bigger magnitude (more than 0.2) leads to a larger degree of densification. In addition, a simple analytical model is then proposed accounting for these factors with seven parameters that can be directly calibrated from laboratory tests. Finally, the applicability of the model is examined by simulating the additional tests through an adoption of the parameters calibrated by the training tests.

Suffusion occurs when fines are plucked off by seepage forces and transported throughout the pores of the matrix constituted by coarser soil particles. Natural or human made soils are seldom homogeneous, which makes suffusion more complex. Suffusion is usually combined with the self-filtration of the fine particles and the transport of these fines may cause the soil structure to become looser. At the same time a clogging may occur which could reduce the permeability leading to an increase of excess pore pressure. The combination of these two phenomena will result in strength degradation. Currently, most suffusion analyses are performed without taking into account the soil's spatial variability. In this paper, a four-constituent continuum finite difference model for suffusion has been extended through the self-filtration process. A random field theory was at this point introduced into the finite difference code to investigate soil suffusion with a randomly distributed initial porosity and fines content. A probabilistic study using the Monte Carlo method was conducted to analyze the effect of the variance, the spatial correlation length, and the cross correlation of the randomly distributed initial porosity and fines content on the eroded mass and on the evolution of the hydraulic conductivity under 1D and 2D conditions. Based on all the simulations, it was possible to quantify the probability of particle blockage during erosion.