Categorification and applications in topology and representation theory

International audienc

Modeling the effect of wetting on the mechanical behavior of crushable granular materials

ABSTRACT: 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

Multiscale approaches to describe mechanical responses induced by particle removal in granular materials

International audienceA 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

Etude des propriétés de transfert des sols injectés

A l'origine, procédé de traitement destiné à l'industrie pétrolière, l'injection des sols granulaires, et notamment l'injection au coulis de ciment, voit sa technologie se développer durant ces vingt dernières années. Ainsi pour optimiser cette technique, une bonne connaissance des conditions initiales du sol en place s'avère indispensable. L'amélioration consécutive à l'injection ainsi réalisée peut être évaluée par une série de tests. Des mesures de densité permettent de mieux suivre l'‘efficacité de l'injection et en particulier le phénomène de rétention de particules indissociable de ce procédé de traitement. Des expériences unidimensionnelles en laboratoire ont été conduites pour étudier l'injection de coulis de ciment ultrafin dans des colonnes de sable de Loire préalablement saturées à l'eau. Ces essais permettent de mettre en évidence l'effet de quelques facteurs clefs : le rapport ciment/eau, la densité initiale du squelette granulaire sur la filtration. La gammamétrie montre ainsi comment l'accumulation de particules de ciment s'opère au sein du massif de sable. Les résultats démontrent une dépendance spatiale de la rétention de particule du sable injecté

Micromechanical analysis of kinematic hardening in natural clay

This paper presents a micromechanical analysis of the macroscopic behaviour of natural clay. A microstructural stress-strain model for clayey material has been developed which considers clay as a collection of clusters. The deformation of a representative volume of the material is generated by mobilizing and compressing all the clusters along their contact planes. Numerical simulations of multistage drained triaxial stress paths on Otaniemi clay have been performed and compared the numerical results to the experimental ones in order to validate the modelling approach. Then, the numerical results obtained at the microscopic level were analysed in order to explain the induced anisotropy observed in the clay behaviour at the macroscopic level. The evolution of the state variables at each contact plane during loading can explain the changes in shape and position in the stress space of the yield surface at the macroscopic level, as well as the rotation of the axes of anisotropy of the material

Modelling the impact of particle removal on granular material behaviour

International audiencePresented 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

Cyclic behavior of an over-consolidated remoulded clay

International audienc

Comportement mécanique des sols injectés

Ce travail de recherche porte sur la détermination de l'amélioration des propriétés mécaniques des sols injectés par des coulis de ciment ultra-fin par rapport à celles des sols vierges de tout traitement. On s'intéresse plus particulièrement au comportement de ces matériaux dans le domaine des très petites et petites déformations. Un dispositif expérimental par propagation d'ondes de cisaillement, les "bender elements", a été installé dans une cellule triaxiale conventionnelle. Il a permis de suivre l'évolution du module de cisaillement Gmax d'échantillons de sols vierges ou injectés en laboratoire sur des chemins de compression isotrope ou de cisaillements drainés.This study aims at determining the improvement of the mechanical properties of soils injected with microfine cements grouts in comparison with those of the uncemented natural soils. We are more especially interested in the behavior of such grouted soils in the very small and small strain domain. An experimental device based on shear wave propagation, the "bender elements" transducers, was set up in a conventional triaxial cell.NANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocSudocFranceF