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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.

One of the major causes of instability in geotechnical structures such as dikes or earth dams is the phenomenon of suffusion including detachment, transport and filtration of fine particles by water flow. Current methods fail to capture all these aspects. This paper suggests a new modeling approach under the framework of the porous continuous medium theory. The detachment and transport of the fine particles are described by a mass exchange model between the solid and the fluid phases. The filtration is incorporated to simulate the filling of the inter-grain voids created by the migration of the fluidized fine particles with the seepage flow, and thus, the self-filtration is coupled with the erosion process. The model is solved numerically using a finite difference method restricted to one-dimensional (1-D) flows normal to the free surface. The applicability of the model to capture the main features of both erosion and filtration during the suffusion process has been validated by simulating 1-D internal erosion tests and by comparing the numerical with the experimental results. Furthermore, the influence of the coupling between erosion and filtration has been highlighted, including the development of material heterogeneity induced by the combination of erosion and filtration.

One of the major causes of instability in geotechnical structures such as dikes or earth dams is internal erosion, an insidious process that occurs over a long period of time. Research on this topic is still fairly new and much more needs to be understood in order to solve the problems posed by this phenomenon. This paper proposes a hydromechanical model based on porous continuous medium theory to assess how internal erosion impacts the safety of earthen structures. The saturated soil is 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 are described by 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 porosity and fines content induced by internal erosion upon the behavior of the soil skeleton. The model has been numerically solved with the finite element method. It has then been applied to the specific case study of a dike foundation subjected to internal erosion induced by the presence of a karstic cavity beneath the alluvium layer. The numerical results show the onset of erosion, the time-space evolution of the eroded zone, and the hydromechanical response of the soil constituting the dike, all of which highlights the effects of the cavity location, the erosion rate, and the fines content.

International audience; The degradation of the effective normal stress in soil surrounding the pile caused by undrained cyclic loading affects the shaft capacity of the pile and can lead to structural instability. In order to investigate this phenomenon, a series of constant volume monotonic and cyclic simple shear tests on Fontainebleau sand has been conducted. Based on the experimental results, the shear stress at phase transformation state is first determined for different initial void ratios and initial normal effective stresses. Then, the number of cycles to liquefaction is estimated as a function of both the cyclic and the average shear stresses normalised by the shear stress at phase transformation. An empirical equation to evaluate the normal stress degradation is formulated and the procedure of parameter identification is presented. The performance of the suggested formulation has been analysed, based on simple shear test results on Fraser River sand and Quiou sand, and triaxial tests on Karlsruhe sand, and it has been validated by a series of additional tests on Fontainebleau sand. All the results indicate that the proposed formulation is able to estimate with good accuracy the effective normal stress degradation in sand subjected to undrained cyclic shearing.

Internal erosion occurs when fines are pulled off by seepage forces and transported throughout the matrix of soil particles, which results in the degradation of soil strength. This phenomenon has been widely studied experimentally and numerically over the last few decades. In fact, soil nature is highly variable in space and then heterogenous in magnitude [10]. At the present time, few studies have been conducted to analyze the influence of variations of soil initial state (i.e. the initial porosity and the initial fraction of fines content) on the internal erosion behaviors. In this paper, a thermodynamically consistent continuum model of internal erosion is combined with the random field theory to investigate the erosion behavior of soil with spatially variable initial state of soil in one-dimensional condition.

The tunnel leakage in sand layer plays an important role in the evolution of the lining stresses and the ground movement. The phenomenon of internal erosion occurs when underground water leaks into the tunnel from the damaged joints or the cracks of the linings. Fines in sand are pulled off by seepage forces and transported throughout the soil matrix into the tunnel. The loss of fines due to seepage flow affects the mechanical behavior of the soil around the tunnel. Conversely, the change of porosity influences the permeability of the soil and, therefore, its hydraulic conductivity. In this study, the effect of tunnel leakage in sand layer is investigated numerically with a coupled hydro-mechanical approach, considering the internal erosion phenomenon induced by the local flows of underground water, using the continuous porous medium theory and a critical-state based constitutive model. Simulation results show that the loss of fines as well as the leakage locations have significant impacts on the tunnel lining and the ground movement.

It is still an open problem to develop a model with a uniform theoretical treatment under various loading conditions. This paper aims to formulate such a model for describing both monotonic...

Accumulation of heavy metals can occur on clay barriers or slurry-trench cutoff wall, resulting in their high concentrations at which the retention reaction takes place. Or high concentration conditions are often adopted to accelerate the reaction process at a laboratory scale for investigating their impacts on engineering behaviors (hydraulic or mechanical) of clays. So far, there is lack of data regarding heavy metal retention behavior on clays under those extreme concentration conditions. This paper aims to mitigate this gap with emphasis on pH effects. To do so, several series of sorption batch tests were conducted to characterize the retention capacity of three ubiquitous heavy metals (Pb, Zn and Cu) on two reference clays (Speswhite kaolin and Wyoming bentonite). A wide range of individual metal concentration varying from 0.1 to 100 mM and different pH conditions were tested. Results suggest that the acidifying the metal–clay system can result in a significant decrease in the heavy metal retention, which can be attributed to an increase in competition of H+ with metal ions to the surface sites and a decrease in metal ion precipitation as hydroxides at high pH levels. In addition, the heavy metal retention isotherm over a wide range of concentrations for the bentonite follows Temkin and Langmuir adsorption models while Freundlich and Langmuir models for the kaolin. Interestingly, at a high concentration of 10 mM, the Pb precipitation unlikely occurs in the presence of both clays tested over a wide range pH level (

Adding refinement and accuracy to constitutive models of soil results in the introduction of complexities along with more model parameters. These parameters (such as hardening-/softening-, dilatancy-/contractancy-related parameters and critical state parameters) are usually not easily obtained in a straightforward way. How to identify these key parameters and estimate their correlations of advanced soil models is a particular issue for geotechnical engineering. This paper was aimed to investigate multi-objective genetic algorithms for identifying parameters of advanced sand models based on standard laboratory tests, followed by the correlation analysis of parameters. A critical-state-based sand model has been developed to simulate three triaxial compression tests performed on loose and dense Hostun sand. Two widely used genetic algorithms with two initialisation methods are examined. The performance of the two genetic algorithms is assessed by comparing their simulation performance using optimal parameters, the convergence speed and the distribution of solutions on the Pareto front. The optimal parameters can then be classified into two factors by their correlation relationship.

This study focuses on the quantitative description of the evolution of creep coefficient (C αe) with both soil density and soil structure under 1D compression. Firstly, conventional consolidation test results on various reconstituted clays are selected in order to investigate the evolution of C αe with void ratio of soils, which can be described by a simple nonlinear creep formulation. Secondly, the contributions of the inter-particle bonding and debonding for soft structured clays to C αe are analyzed based on test results on intact and reconstituted samples of the same clay. A material constant ρ, function of the bonding ratio χ, is introduced in order to quantify the contribution of the soil structure to C αe, and a nonlinear creep formulation accounting for both soil density and soil structure is finally proposed. Furthermore, the parameters used in the formulation are correlated with Atterberg limits, allowing us to suggest a relationship between C αe, Atterberg limits and inter-particle bonding for a given soil. Finally, the validity of the proposed formulation is examined by comparing experimental and predicted C αe values for both reconstituted and intact samples of natural soft clays. The proposed formulation is also validated by comparing the computed and measured void ratio with time on two intact clays.

Finite element analysis was conducted to study the ground and tunnel response to partial tunnel leakage coupled with anisotropic soil permeability, a novel and little-known approach that was developed from in-situ inspections of shield tunnel leakage in the Shanghai Metro system. The ground and tunnel settlements, which were caused by water leaking into the tunnel, significantly degraded both the serviceability and safety of the tunnel and its surroundings. In a typical analysis, such leakage is commonly assumed to be uniform along the tunnel circumference with a soil permeability that is isotropic. Numerical simulation results obtained in this paper indicated that partial tunnel leakage in only one side of the shield tunnel caused a greater pore pressure reduction at the tunnel spring line and a larger ground surface settlement than otherwise caused by uniform tunnel leakage with the same rate of water inflow. Indeed, observations showed that partial leakage in only one side of the shield tunnel caused remarkable lateral movements of the tunnel. Our analysis showed that coupling leakage boundary conditions with favorable soil permeability anisotropy could significantly decrease the maximum ground settlements induced by the leakage, which leads to an increase in the width of the surface settlement trough. Furthermore, the effect of soil permeability anisotropy on ground settlement was found more profound for cases involving partial tunnel leakage boundary conditions.

International audience; In geotechnical and particularly in dam engineering, observations in the field and, in some cases, dramatic failures of structures have often incited research and increased knowledge. For instance, the first rockfill dams were elevated without any prior knowledge of the mechanical capacities of the rockfill mass. As natural and local resources are usually preferred for economic and environmental issues, their non-optimal properties can lead to potential severe damage to the hydraulic structure. Similarly, a basic principle of construction of rockfill dams or embankments consists in sluicing rockfill after deposition and before compaction: this recommendation has been adopted for a long time. However, the beneficial effects of such a practice have been understood only in light of recent studies on unsaturated soil and multi-scale analyses.

Micromechanics-based model have proved to be efficient in describing the mechanical behaviour of granular soils with few physical meaning parameters. Among these models, a family of micromechanical models was constructed based on the static approach that is a bridge connecting micro forces and macro stress. This paper presents the application of a static hypothesis based CH micromechanical model to solve boundary value problems (BVPs) using an implicit integration method. The model was first calibrated by results of elementary tests of Hostun sand and then was implemented into a finite element code. Two typical boundary value problems: a biaxial test and a square footing were numerically analysed to demonstrate how the micromechanical model can be applied to multiscale modelling of geotechnical structures.

A microstructural model to consider the mechanical behavior of granular materials has been enhanced to incorporate the evolution of the critical state when significantly high grain breakage occurs during loading. The influence of grain size on grain strength is highlighted and its impact on the properties of granular assemblies is shown through the results of oedometer test simulations.

Under a microstructural approach for stress-strain modeling, a set of microsystems (e.g. inter-particle contacts) is used to represent the material. Material properties are defined for each microsystem and the overall stress-strain relationship for the material is obtained from averaging the behaviors of this set of microsystems.

Ferroic domain walls could play an important role in microelectronics, given their nanometric size and often distinct functional properties. Until now, devices and device concepts were mostly based on mobile domain walls in ferromagnetic and ferroelectric materials. A less explored path is to make use of polar domain walls in nonpolar ferroelastic materials. Indeed, while the polar character of ferroelastic domain walls has been demonstrated, polarization control has been elusive. Here, we report evidence for the electrostatic signature of the domain-wall polarization in nonpolar calcium titanate (CaTiO3). Macroscopic mechanical resonances excited by an ac electric field are observed as a signature of a piezoelectric response caused by polar walls. On the microscopic scale, the polarization in domain walls modifies the local surface potential of the sample. Through imaging of surface potential variations, we show that the potential at the domain wall can be controlled by electron injection. This could enable devices based on nondestructive information readout of surface potential., Comment: 30 pages, 12 figures

We present results of ring-shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone. Little rate dependency of shear resistance was observed in sand-sized granules in any wet or dry test, while saturated gravel-sized granules exhibited significant and abrupt reversible rate-weakening (from μ = 0.6 to 0.05) at about 2 mm/s. Repeating resistance variations occurred also under constant shear displacement rate. Mudstone granules generate mud as they are crushed and softened. Shear-thinning and thixotropic behavior of the mud can explain the observed behavior: with the viscosity decreasing, the mud can flow through the coarser soil pores and migrate out from the shear zone. This brings new granules into contact which produces new mud. Thus, the process can start over. Similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behaviour may play a role in some landslide kinematics.

International audience; This paper presents a numerical model of internal erosion based on the approach of continuous porous media. The soil skeleton saturated by a pore fluid is treated as the superposition of four constituents in interaction: solid skeleton, erodible fines, fluidized particles, and fluid. The detachment and transport of fine particles are modeled by the mass exchange between solid and fluid phases. In order to take into account the influence of the change of void ratio induced by internal erosion on the soil skeleton, a critical state based constitutive model is used to calculate the effective stress-strain response of the soil skeleton. This coupled hydro-mechanical analysis is applied to study the impact of erosion on a dike. The numerical simulations show the progressive development of internal erosion within the foundation of the dyke. The effect of the mechanical degradation due to internal erosion is demonstrated by evaluating the factor of safety of the dyke slope by means of the shear strength reduction method.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; This paper presents an experimental study of mechanical response of natural Camargue silty sand. The analysis of test results used the equivalent intergranular void ratio instead of the global void ratio. The calculation of equivalent intergranular void ratio requires the determination of parameter b which represents, physically, the fraction of active fines participating on the chain forces network, hence the strength of the soil. A new formula for determining the parameter b by using an approach based on the coordination number distribution and probability calculation is proposed. The validation of the developed relationship was done through back-analysis of published datasets in literature on the effect of fines content on silty sand behaviour. It is shown that the equivalent intergranular void ratio calculated with the b value obtained by the new formula is able to provide strong correlation to not only the critical state of but also the onset of instability of various silty sands, in different terms as peak deviator stress, peak stress ratio or cyclic resistance. Therefore, it is suggested that the use of the equivalent void ratio concept and the new b calculating formula is highly desirable in predicting of the silty sand behaviour.

International audience; Grain crushing is in part responsible for soil plasticity and dissipates most of the input energy at high stresses. This energy dissipation, as well as the energy dissipated by friction, occurs through heat exchanges. Real-time observations of temperature fields on the surface of a breaking specimen are performed by the infrared thermography technique to determine the heat generated at the crack tip when a crack is propagating. This research was motivated by recent trends in soil mechanics where constitutive models have been developed based on thermodynamic potentials including energy dissipation. A high-speed infrared camera was used to monitor the cracking of multiple specimens of rock and mortar in a diametral compression configuration. In the case of rock specimens, cracking was accompanied by a sudden rise in temperature. No temperature changes were observed in the case of mortar specimens. The maximum temperature reached and the heat dissipation profile attained depend on the nature of the rock specimen.

A semi-analytical method has been developed for computing the dynamic response of railways composed of viscoelastic layers under moving loads. One of the aims of this new approach is to be able to simulate railways incorporating asphalt materials. These materials are increasingly used or highly considered for construction projects. We have modeled the mechanical behavior of bituminous mixes with the Huet–Sayegh thermo-sensitive viscoelastic law that is specifically intended for such materials. This method, implemented in a numerical program, is based on a quasi-stationary approach, so that time is not a specific factor in the solution. The required calculation time is therefore small, for the convenience of parametric studies. The capability of this method is proved with a numerical example that provides several validation points and illustrates at the same time the temperature effect on the response of a railway including an asphalt layer.

Straining depicts the geometrical retention of fine particles within materials'pore network. Whether or not fines can be retained depends on (1) the structure of materials'pore network as well as (2) fines'size distribution, manner of deposition, and retained amount. In this paper, the main results of straining tests are presented for randomly packed beads filters. The experimental program included a series of tests performed in both dry and water-saturated filters. The tests provided information on the dynamics of fines in the filters'pore network and changes of filters'efficiency. For instance, data supported the existence of four distinct stages: the filling of accessible cavities, the formation then clogging of the preferential pathways, the transfer of fines toward the lateral unobstructed capillaries, and eventual formation of a surface cake. The postulated mechanisms inherent to every single stage are discussed in terms of retention rate, straining length, lateral transfer, andprogressive obstruction of constrictions. Additionally, the straining mechanisms were also probed in hydrodynamic conditions at increasingwater flows.

International audience; Presented here is a numerical method able to predict the mechanical behaviour of granular materials subjected to particle removal. The purpose of this study is to improve understanding of the mechanical behaviour of soils subjected to internal erosion in hydraulic works. The approach is based on a homogenisation technique for deriving the stress–strain relationship of a granular assembly from forces and displacements at the particle level. The soil's local behaviour is assumed to follow a Hertz–Mindlin elastic law and a Mohr–Coulomb plastic law. Sliding resistance on each plane is made to depend on the actual void ratio of the granular assembly. The solid fraction removal is modelled by progressively increasing the granular assembly void ratio, which provokes a decrease of the sliding resistance of each interparticle contact, leading to macroscopic deformations of the soil specimen. At elevated stress levels, large deformations can develop and lead to soil failure. Numerical simulations also demonstrate that a type of failure called diffuse failure can occur in eroded soil masses whenever an increase in pore pressure is generated within the soil. These numerical results appear to be coherent with observations made on embankment dams that have suffered internal erosion and, in particular, with the description of their modes of failure.

International audience; A novel approach has been developed to predict the mechanical behavior of very coarse granular materials with a constitutive model, which considers both grain breakage and size effect. The behavior of granular assemblies is significantly influenced by particle breakage. A critical-state double-yield-surface model incorporating the change in the critical-state line and in elastic stiffness caused by grain breakage during loading has been adopted. The amount of grain breakage was estimated by extending the size effect theory on individual grains to granular assemblies. The results from earlier studies on granular materials with parallel gradations have been usefully exploited to calibrate and to validate the model. Comparisons between experiments and simulations suggest that this approach can predict the mechanical behavior of very coarse granular materials from test results performed on a finer fraction with a homothetic gradation.

This paper discusses the reliability and the efficiency of a time homogenization method employed to reduce the computational time during cyclic loading for two common geotechnical tests and two elastoplastic models for clays. The method of homogenization is based upon splitting time into two separate scales. The first time scale relates to the period of cyclic loading and the second to the characteristic time of the fatigue phenomenon. The time homogenization method is applied to simulate an undrained triaxial test (homogeneous stress state) and a pressuremeter test (nonhomogeneous stress state) under one-way cyclic loading on normally consolidated clay. This method is coupled with two elastoplastic models dedicated to cyclic behavior of clay (a bounding surface plasticity model and a bubble model). Both linear and nonlinear elasticities are considered. The difficulty encountered when applying this method to models introducing nonlinear elasticity and kinematic hardening is pointed out. The performance of time homogenization related to the main parameters is numerically investigated by comparison with conventional finite element simulations.

SUMMARY Internal erosion (IE) affects the stability of natural and reinforced materials by causing instability within their granular structure. The dislodgement and transport of eroded particles affect both the particulate concentration of eroding fluid and the pore network of eroded material. In this study, we examined these modifications using a transport model with a finite element code. First, IE tests on chemically reinforced sand columns were performed to obtain information about eroded material loss of mass, particulate concentration of effluent, porosity and permeability modifications, and existing IE stages. Second, based on experimental results, a mathematical one-dimensional model has been formulated to monitor the evolution and spatial distribution of erodible solids, fluidized particles, porosity, permeability, and seepage stresses. The model consists of a set of coupled nonlinear differential equations solved in sequence. It provides valuable information about the extent and the dynamics of structural changes, which can be used to estimate an IE time for the hydraulic work to reach failure. Copyright © 2011 John Wiley & Sons, Ltd.

Cementations formed in geological timescale are observed in various stiff clays. A micromechanical stress strain model is developed for modeling the effect of cementation on the deformation behavior of stiff clay. The proposed approach considers explicitly cementations at intercluster contacts, which is different from conventional model. The concept of inter-cluster bonding is introduced to account for an additional cohesion in shear sliding and a higher yield stress in normal compression. A damage law for inter-cluster bonding is proposed at cluster contacts for the debonding process during mechanical loading. The model is used to simulate numerous stress-path tests on Vallericca stiff clay. The applicability of the present model is evaluated through comparisons between the predicted and the measured results. In order to explain the stress-induced anisotropy arising from externally applied load, the evolution of local stresses and local strains at inter-cluster planes are discussed.

This article presents two image-processing techniques which allow studying grout propagation in granular soils. The first one deals with the evaluation of the cement grout concentration using UV spectrophotometry. The second is based on image segmentation processing: active contour model. We propose to develop the active contour model image analysis technique. This process, currently applied in the field of medicine, can be transposed to grout flow detection during soil injection. They are powerful image segmentation techniques that combine geometry, physics and approximation theory. Two distinct formulations exist to employ these techniques, parametric or geometric curves. These models have proven to be effective in segmenting and tracking non-rigid structures. They exploit features derived from the image data together with a prior knowledge about the location, size and shape of these structures. We focus on a parametric approach for deformable models. They support highly intuitive interaction mechanisms. Those mechanisms allow, when necessary, researchers to bring their expertise to bear on the model-based image interpretation task. In this paper, we show the development of the two different image analysis methods employed in our experiments for grout detection and grout concentration measurement.

Instability of granular material may lead to catastrophic events such as the gross collapse of earth structures, and thus it is an important topic in geotechnical engineering. In this paper, we adopt the micromechanics approach for constitutive modeling, in which the soil is considered an assembly of particles, and the stress-strain relationship for the assembly is determined by integrating the behavior of the interparticle contacts in all orientations. Although analyses regarding material instability have been extensively studied for a soil element at the constitutive level, it has not been considered at the interparticle contact level. Through an eigenvalue analysis, two modes of instability are identified at the local contact level: the singularity of tangential stiffness matrix and the loss of positiveness of second-order work. The constitutive model is applied to simulate drained and undrained triaxial tests on Toyoura sand of various densities under various confining pressures. The predictions are compared with experimentally measured instability at the assembly level. The modes of stability at the interparticle contact level and their relations to the overall instability of the assembly are also analyzed.

This paper discusses the quality of the procedure employed in identifying soil parameters by inverse analysis. This procedure includes a FEM-simulation for which two constitutive modelsa linear elastic perfectly plastic MohrCoulomb model and a strain-hardening elasto-plastic modelare successively considered. Two kinds of optimization algorithms have been used: a deterministic simplex method and a stochastic genetic method. The soil data come from the results of two pressuremeter tests, complemented by triaxial and resonant column testing. First, the inverse analysis has been performed separately on each pressuremeter test. The genetic method presents the advantage of providing a collection of satisfactory solutions, among which a geotechnical engineer has to choose the optimal one based on his scientific background and/or additional analyses based on further experimental test results. This advantage is enhanced when all the constitutive parameters sensitive to the considered problem have to be identified without restrictions in the search space. Second, the experimental values of the two pressuremeter tests have been processed simultaneously, so that the inverse analysis becomes a multi-objective optimization problem. The genetic method allows the user to choose the most suitable parameter set according to the Pareto frontier and to guarantee the coherence between the tests. The sets of optimized parameters obtained from inverse analyses are then used to calculate the response of a spread footing, which is part of a predictive benchmark. The numerical results with respect to both the constitutive models and the inverse analysis procedure are discussed.

International audience; Over the past twenty years, growing technology allowed to treat a wide range of subsurface conditions for construction. When a properly designed cement grout is injected into sand soil, homogeneous grout bulbs are formed which make dense and thus strengthen the surrounding soil. For successful growing, the knowledge of subsurface conditions is important. Ground improvement can be assessed by permeability testing and mercury porosimetry testing. Laboratory experiments were conducted to examine grout injection through Loire sand columns under saturated conditions. These tests were performed in order to highlight the effect of some key factors: cement-to-water ratio, relative density of the granular skeleton on the properties of the grouted sand. Subsequently Mercury Intrusion Porosimetry tests showed that both porosity and its distribution are modified. Besides, the intrinsic permeability was measured by a gas permeability device. Then, an evaluation of intrinsic permeability using the Katz-Thompson equation on these tests is proposed and compared to gas permeability which is still commonly used in civil engineering to measure this intrinsic parameter.; Pendant les vingt dernières années, la technologie de l’injection des sols a permis de traiter un grand nombre de terrains pour une bonne préparation à la construction. Quand un coulis de ciment correctement conçu est injecté dans un sable, des bulbes d’injection sont formés ce qui densifie et renforce ainsi le sol environnant. Pour réussir l’opération d’injection, la connaissance des conditions du sous-sol est importante. L'amélioration des sols injectés peut être évaluée par des tests de perméabilité et de porosimétrie par intrusion de mercure. Des expériences de laboratoire ont été conduites pour examiner l'injection de coulis dans des colonnes de sable de Loire dans des conditions saturées. Ces essais ont été exécutés pour mettre en évidence l'effet de quelques facteurs clefs : proportion de ciment-eau, densité relative du squelette granulaire sur les propriétés du sable injecté. Par la suite, les essais de porosimétrie mercure ont montré que tant la porosité que sa distribution sont modifiées. En plus, la perméabilité intrinsèque a été mesurée par un dispositif de perméabilité au gaz. On propose ensuite une évaluation de la perméabilité intrinsèque utilisant l'équation Katz-Thompson sur ces essais qui est comparée à la perméabilité au gaz qui est généralement utilisée en génie civil pour mesurer ce paramètre intrinsèque.

This paper examines the mechanical behavior of a sensitive marine clay. Various laboratory tests on intact and reconstituted samples of Guinea Gulf marine clay were performed under isotropic compression and drained triaxial compression at constant confining stresses. Microstructure analysis on intact and reconstituted samples was also carried out under different loading conditions. The effect of inter-aggregates bonding on mechanical properties is discussed. Based on experimental analysis, a new modeling method is proposed. In this approach, the clay is regarded as an assembly of aggregates of clay particles. An inter-aggregate contact law is introduced relating contact forces to aggregates relative displacements. The deformation of the assembly can be obtained by integrating the movement of the inter-aggregate contacts in all orientations. Thus, the effect of inter-aggregates bonds and debonding is considered in a direct way. The model is evaluated through comparisons between the predicted and measured results on Guinea Gulf marine clay. The evolutions of local stresses, strains, and bonds in inter-aggregates planes are discussed to explain the anisotropy induced by the applied loading. Copyright © 2010 John Wiley & Sons, Ltd.

This paper presents a synthesis of the works performed by various teams from France, Italy and Canada around the question of second‐order work criterion. Because of the non‐associative character of geomaterials plastic strains, it is now recognized that a whole bifurcation domain exists in the stress space with various possible modes of failure. In a first part these failure modes are observed in lab experimental tests and in discrete element modelling. Then a theoretical study of second‐order work allows to establish a link with the kinetic energy, giving a basis to explain the transition from a prefailure (quasi)static regime to a postfailure dynamic regime. Eventually the main features of geomaterials failure are obtained by applying second‐order work criterion to five different constitutive rate‐independent models—three being phenomenological and two micromechanical. As a whole this paper tries to gather together all the elements for a proper understanding and use of second‐order work criterion in geomechanics.

International audience; A multi-scale approach is proposed as an attempt to assess internal erosion induced effects on the mechanical properties of a granular medium. When submitted to internal flow, some particles tend to be removed from the initial granular material, leading to drastic changes in the microstructure. The medium is changing during its lifetime and this cannot be ignored in attempts to model its mechanical behavior. In this first analysis, the degradation of a granular assembly is simulated through the progressive removal of its finest particles for both isotropic and anisotropic stress states. A discrete element model as well as an analytical micromechanical model are used to compare induced deformations and properties changes along the erosion process. The shear strength and flow properties appear to be strongly modified by this extraction phase: both models highlight a change from a dilatant to a contractant behavior with the degradation as well as strong dependency of the overall stability on the mobilized friction level. In particular, results show how failure is triggered when particle removal occurs for mobilized frictions greater than the one reached at the critical state.; Une approche multi-échelle est proposée dans le but d'étudier les effets de l'érosion interne sur les propriétés mécaniques d'un milieu granulaire. Dans cette première analyse, la dégradation d'un assemblage granulaire est simulée par la suppression progressive de ses particules les plus fines pour différents états de contrainte, isotropes et anisotropes. Un modèle aux élements discrets ainsi qu'un modèle micromécanique analytique sont utilisés dans le but de comparer les déformations et les modifications induites sur le milieu. La resistance au cisaillement et les propriétés d'écoulement du matériau semblent être significativement modifiées par un tel processus d'extraction : les deux modèles sont unanimes pour mettre en évidence le passage d'un comportement dilatant à un comportement contractant du matériau avec sa dégradation, ainsi q'une dépendance critique de sa stabilité globale selon le niveau du frottement mobilisé auquel l'extraction de ses particules solides est réalisée. En particulier, les resultats montrent une tendance à l'effondrement lorsque l'extraction intervient pour des frottements mobilisés supérieurs à celui atteint à l'état critique.

The inherent anisotropy more or less exists in sand when preparing samples in laboratory or taking from field. The purpose of this paper is to model cyclic behaviour of sand by means of a micromechanical approach considering inherent anisotropy. The micromechanical stress–strain model developed in an earlier study by Chang and Hicher (2005) is enhanced to account for the stress reversal on a contact plane and the density state-dependent dilatancy. The enhanced model is first examined by simulating typical drained and undrained cyclic tests in conventional triaxial conditions. The model is then used to simulate drained cyclic triaxial tests under constant p ′ on Toyoura sand with different initial void ratios and different levels of p ′, and undrained triaxial tests on dense and loose Nevada sand. The applicability of the present model is evaluated through comparisons between the predicted and the measured results. The evolution of local stresses and local strains at inter-particle planes due to externally applied load are discussed. All simulations have demonstrated that the proposed micromechanical approach is capable of modelling the cyclic behaviour of sand with inherent and induced anisotropy.

In this paper, we study the influence of the scaling functions in Perzyna's type elasto-viscoplastic models for predicting time-dependent behaviour of natural clays. The constitutive models were developed based on the overstress theory with different scaling functions and on the elastoplastic model S-CLAY1S which accounts for induced anisotropy and gradual degradation of apparent soil bonds. Laboratory tests were simulated on natural clays under one-dimensional and triaxial conditions to evaluate the scaling function influence in the modelling of the strain-rate effect on soil strength and creep effect. The influence of scaling functions on modelling the time-dependent behaviour under pressuremeter condition was also numerically examined. This test is treated as an example of a boundary value problem, which also allows us to see if this in situ testing is capable of deriving the parameters controlling the time-dependent behaviour.

Experimental evidences have shown deficiencies of the existing overstress and creep models for viscous behaviour of natural soft clay. The purpose of this paper is to develop a modelling method for viscous behaviour of soft clays without these deficiencies. A new anisotropic elastic–viscoplastic model is extended from overstress theory of Perzyna. A scaling function based on the experimental results of constant strain-rate oedometer tests is adopted, which allows viscoplastic strain-rate occurring whether the stress state is inside or outside of the yielding surface. The inherent and induced anisotropy is modelled using the formulations of yield surface with kinematic hardening and rotation (S-CLAY1). The parameter determination is straightforward and no additional experimental test is needed, compared to the Modified Cam Clay model. Parameters determined from two types of tests (i.e., the constant strain-rate oedometer test and the 24h standard oedometer test) are examined. Experimental verifications are carried out using the constant strain-rate and creep tests on St. Herblain clay. All comparisons between predicted and measured results demonstrate that the proposed model can successfully reproduce the anisotropic and viscous behaviours of natural soft clays under different loading conditions.

International audience; Clay material can be considered as a collection of clusters, which interact with each other mainly through mechanical forces. From this point of view, clay is modeled by analogy to granular material in this paper. An elastoplastic stress-strain relationship for clay is derived by using the granular mechanics approach developed in previous studies for sand. However, unlike sand, clay deformation is generated not only by the mobilizing but also by compressing clusters. Thus, in addition to the Mohr-Coulomb's plastic shear sliding and a dilatancy type flow rule, a plastic normal deformation has been modeled for two clusters in compression. The overall stress-strain relationship can then be obtained from the mobilization and compressing of clusters through a static hypothesis of the macro-micro relations. The predictions are compared with the experimental results for clay under both drained and undrained triaxial loading conditions. Three different types of clay, including remolded and natural clay, have been selected to evaluate the model's performance. The comparisons verify that this model is capable of accurately reproducing the overall behavior of clay, which accounts for the influence of key parameters such as void ratio and mean stress. A section of this paper is devoted to show the model's capability of considering the influence of inherent anisotropy on the stress-strain response under undrained triaxial loading conditions.

This paper presents a micromechanical study of unsaturated granular media in the pendular regime, based upon numerical experiments using the discrete element method, compared to a microstructural elastoplastic model. Water effects are taken into account by adding capillary menisci at contacts and their consequences in terms of force and water volume are studied. Simulations of triaxial compression tests are used to investigate both macro and micro-effects of a partial saturation. The results provided by the two methods appear to be in good agreement, reproducing the major trends of a partially saturated granular assembly, such as the increase in the shear strength and the hardening with suction. Moreover, a capillary stress tensor is exhibited from capillary forces by using homogenisation techniques. Both macroscopic and microscopic considerations emphasize an induced anisotropy of the capillary stress tensor in relation with the pore fluid distribution inside the material. In so far as the tensorial nature of this fluid fabric implies shear effects on the solid phase associated with suction, a comparison has been made with the standard equivalent pore pressure assumption. It is shown that water effects induce microstrural phenomena that cannot be considered at the macro level, particularly when dealing with material history. Thus, the study points out that unsaturated soil stress definitions should include, besides the macroscopic stresses such as the total stress, the microscopic interparticle stresses such as the ones resulting from capillary forces, in order to interpret more precisely the implications of the pore fluid on the mechanical behaviour of granular materials., 39 pages

International audience; This article presents a model of flow and transport with filtration in porous media which is used to analyze large-scale grouting tests. A program based on the finite element method is developed to solve the model equations; a particular attention is paid to inherent issues of transport problems. The analysis of these tests aims at providing insights on the propagation patterns associated to the injection of cement-based grouts in sand. To apprehend particular features that characterize field injections, the experiments are performed by using a tube-a-manchette and a patented grout. Finally, the role of filtration during the tests is discussed.

Studies in the past have tried to reproduce the mechanical behaviour of granular materials by proposing constitutive relations based on a common assumption that model parameters and parameters describing the properties, including gradation of individual grains are inevitably linked. However successful these models have proved to be, they cannot account for the changes in granular assembly behaviour if the grains start to break during mechanical loading. This paper proposes to analyse the relation between grading change and the mechanical behaviour of granular assembly. A way to model the influence of grain breakage is to use a critical state-based model. The influence of the amount of grain breakage during loading, depending on the individual grain strength and size distribution, can be introduced into constitutive relations by means of a new parameter that controls the evolution of critical state with changes in grain size distribution. Experimental data from a calcareous sand, a quartz sand, and a rockfill material were compared with numerical results and good-quality simulations were obtained. The main consequences of grain breakage are increased compressibility and a gradual dilatancy disappearance in the granular material. The critical state concept is also enriched by considering its overall relation to the evolution of the granular material. Copyright © 2009 John Wiley & Sons, Ltd.

The aim of this paper is to present a methodology for identifying the soil parameters controlling the delayed behaviour from laboratory and in situ pressuremeter tests by using an elasto-viscoplastic model (EVP-MCC) based on Perzyna's overstress theory and on the elasto-plastic Modified Cam Clay model. The influence of both the model parameters and the soil permeability was studied under the loading condition of pressuremeter tests by coupling the proposed model equations with Biot's consolidation theory. On the basis of the parametric study, a methodology for identifying model parameters and soil permeability by inverse analysis from three levels of constant strain rate pressuremeter tests was then proposed and applied on tests performed on natural Saint-Herblain clay. The methodology was validated by comparing the optimized values of soil parameters and the values of the same parameters obtained from laboratory test results, and also by using the identified parameters to simulate other tests on the same samples. The analysis of the drainage condition and the strain rate effect during a pressuremeter test demonstrated the coupled influence of consolidation and viscous effects on the test results. The numerical results also showed that the inverse analysis procedure could successfully determine the parameters controlling the time-dependent soil behaviour. Copyright © 2008 John Wiley & Sons, Ltd.

International audience; The mechanical properties of sand: stiffness, cohesion and, to a less extent, friction angle can be increased through the process of grouting. A constitutive model adapted for cohesive-frictional materials from a homogenization technique which allowed us to integrate constitutive relations at the grain level has been developed to obtain constitutive equations for the equivalent continuous granular medium. A representative volume was obtained by mobilizing particle contacts in all orientations. Thus, the stress-strain relationship could be derived as an average of the behavior of these local contact planes. The local behavior was assumed to obey a stress-dependent elastic law and Mohr-Coulomb's plastic law. The influence of the cement grout was modeled by means of adhesive forces between grains in contact, which were added to the contact forces created by an external load. The intensity of these adhesive forces is a function of nature and amount of grout present inside the material and can be reduced due to a damage mechanism at the grain contact during loading. In this paper, we present several examples of simulation which show that the model can reproduce with sufficient accuracy the mechanical improvement induced by grouting as well as the damage of the grain cementation during loading..

International audience; The homogenization technique is used to obtain an elastoplastic stress-strain relationship for dry, saturated and unsaturated granular materials. Deformation of a representative volume of material is generated by mobilizing particle contacts in all orientations. In this way, the stress-strain relationship can be derived as an average of the mobilization behavior of these local contact planes. The local behavior is assumed to follow a Hertz-Mindlin's elastic law and a Mohr-Coulomb's plastic law. For the non-saturated state, capillary forces at the grain contacts are added to the contact forces created by an external load. They are calculated as a function of the degree of saturation, depending on the grain size distribution and on the void ratio of the granular assembly. Numerical simulations show that the model is capable of reproducing the major trends of a partially saturated granular assembly under various stress and water content conditions. The model predictions are compared to experimental results on saturated and unsaturated samples of silty sands under undrained triaxial loading condition. This comparison shows that the model is able to account for the influence of capillary forces on the stress-strain response of the granular materials and therefore, to reproduce the overall mechanical behavior of unsaturated granular materials.

This paper presents a prospective study for identifying selected parameters of the modified Cam-Clay model representing the behaviour of heavy oil reservoirs. The first part shows that the plastic compressibility, which controls the main recovery mechanism of such reservoirs, can be accurately determined, simultaneously with other parameters, by an inverse analysis of pressure–strain curves. The conditions of the identification procedure mainly involve two tests conducted under different drainage conditions or at different strain rates. The numerical study also establishes the sequence of an original in situ experimental program, in which three dilatometer tests at a relatively great depth (several hundreds of metres) were carried out. The comparison of the experimental data with the numerical computations reveals a significant over-consolidation ratio which does not allow the plastic compressibility to be determined but supports the findings regarding the geological erosion of the site. Copyright © 2006 John Wiley & Sons, Ltd.

The main objective of this study is to understand the effects of the coordination number on the behavior of a single grain, before and during fracture. A new apparatus is designed, with the purpose of conducting multipoint crushing tests, and to investigate the effects of the contacts number, type and position on the failure of a particle. Some of the tests were monitored using digital image correlation (DIC), in order to get an in-depth view into the mechanics of fracture. The fragmentation of the grains was studied, and the cracks were shown to follow specific paths. The failure forces of multiple contact configurations were compared to show the effect of the number and position of contact forces on the primary and secondary cracks. It was shown that the position and magnitude of the contact forces plays a significant role in the fragmentation of the grain.