A fully coupled damage-plasticity model for unsaturated geomaterials accounting for the ductile-brittle transition in drying clayey soils

International audienceThis paper presents a hydro-mechanical constitutive model for clayey soils accounting for damage-plasticity couplings. Specific features of unsaturated clays such as confining pressure and suction effects on elastic domain and plastic strains are accounted for. A double effective stress incorporating both the effect of suction and damage is defined based on thermodynamical considerations, which results in a unique stress variable being thermodynamically conjugated to elastic strain. Coupling between damage and plasticity phenomena is achieved by following the principle of strain equivalence and incorporating the double effective stress into plasticity equations. Two distinct criteria are defined for damage and plasticity, which can be activated either independently or simultaneously. Their formulation in terms of effective stress and suction allows them to evolve in the total stress space with suction and damage changes. This leads to a direct coupling between damage and plasticity and allows the model to capture the ductile/brittle behaviour transition occurring when clays are drying. Model predictions are compared with experimental data on Boom Clay, and the flexibility of the model is illustrated by presenting results of simulations in which either damage or plasticity dominates the coupled behaviour

Influence of soil properties and geometrical characteristics of sediment-filled valleys on earthquake response spectra.

International audienceIt is well-known that the response of a site to seismic excitation depends on both its local topography and its soil properties. Although the recent building codes (i.e. Eurocode8) incorporate these site effects, they only take into account unidimensional ones and ignore complex cases due to two-dimensional irregular configurations. Recent work has been mainly focusing either on the development of numerical methods allowing always more precise results or on the elaboration of simplified approaches usable for engineering purposes. The aim of this work is to propose a simple criterion, combining soil properties and geometrical characteristics of valleys to estimate the amplification of earthquake response spectra in sediment-filled valleys. We will first study the response of unidimensional soil layers to obtain a criterion that can be extended to bidimensional configurations. The seismic input is a synthetic SV Ricker wave with vertical incidence. Horizontal displacements at surface points are computed by using the HYBRID code which combines finite elements in the near field and boundary elements in the far field (FEM/BEM)

A multi-scale seismic response of two-dimensional sedimentary valleys due to the combined effects of topography and geology.

International audienceIt is well known that the response of a site to seismic excitation depends on the local topographical and geological conditions. The current building codes already take into account unidimensional site effects but ignore complex site effects due to two-dimensional irregular configurations. The aim of this work is to contribute to the establishment of a simple predictive method to estimate site effects. The horizontal ground movements at the surface of sedimentary valleys subjected to SV waves with vertical incidence are calculated by using the HYBRID program, combining finite elements in the near field and boundary elements in the far field (FEM/BEM). A parametric study is conducted to examine the combined effects of topography and geology on the amplification of the response spectrum at various points across the valley. The influence of different parameters is considered, such as filling ratios (from empty to full valleys), impedance contrasts between bedrock and sediments, and dimensions

Mean stress dependent nonlinear hyperelasticity coupled with damage stiffness degradation. A thermodynamical approach

International audiencePorous materials, such as geomaterials, exhibit a behaviour dependent on the confining pressure. The aim of this paper is to study the degradation of the elastic stiffness of mean stress dependent materials, due to the deterioration of the microstructure during loading. Continuum damage mechanics offers a framework to model this rigidity deterioration. In addition to the concept of effective stress, a choice has to be made between two widely used hypotheses, the principle of strain equivalence and the principle of equivalent elastic energy, in order to build a complete modelling framework. A mean stress dependent hyperelastic formulation is used to ensure a conservative behaviour, and associated to the two previous damage modelling assumptions, whose effects are compared. This allows for mean stress dependent elasticity to be reproduced, with elastic moduli increasing with mean stress while decreasing with damage

Modélisation hydro-mécanique du couplage endommagement-plasticité dans les géomatériaux non saturés

This thesis work objective is the development of a hydro-mechanical constitutive model which accounts for damage-plasticity couplings in unsaturated geomaterials. The coupling of this model with hydraulic transfers is made possible by its implementation into the Finite Element code Θ-Stock. In order to achieve this implementation, a specific stress-point algorithm has been developed. Fully coupled hydro-mechanical problems have been simulated, such as the creation of the Excavation Damaged Zone around a tunnel and the initiation of damage due to desiccation and humidification. A double effective stress incorporating both the effect of suction and damage (assumed isotropic) is defined based on thermodynamical considerations. The advantage of this approach is that it results in a unique stress variable being thermodynamically conjugated to elastic strains. A pressure-dependent hyperelastic formulation is used to describe the behaviour inside the elastic domain. The evolution of elastic rigidities with damage is then studied. Two hypotheses are compared, the principle of strain equivalence and the principle of equivalent energy. Coupling between damage and plasticity phenomena is achieved by following the principle of strain equivalence and incorporating the effective stress into plasticity equations. The plasticity framework is based on the Barcelona Basic Model (Alonso extit{et al.}, 1990), itself based on the Cam-Clay model. Two distinct criteria are defined for damage and plasticity, which can be activated either independently or simultaneously. Their formulation in terms of effective stress and suction allows them to evolve in the total stress space with suction and damage changes. This leads to a direct coupling between damage and plasticity and allows the model to capture the ductile/brittle behaviour transition occurring when clays are drying. A specific explicit algorithm has been developed to handle the association of the two dissipative phenomena. The implementation of the constitutive model into the Finite Element code Θ-Stock allowed for the simulation of fully coupled hydro-mechanical problems. The hydraulic transfer laws also consider the saturation state. This fully coupled model is first applied to simulate the development of micro-cracks during desiccation of a soil sample. Damage initiation is explained mainly by the important pressure gradient appearing at the boundary when applying a high suction change. Finally, a full-scale problem is simulated. The excavation of a tunnel, the desaturation of the surrounding soil and the creation of the excavation damaged zone are studiedCette thèse a pour but le développement d'un modèle constitutif hydromécanique prenant en compte le couplage de l'endommagement et de la plasticité dans les géomatériaux non-saturés. Le couplage de ce modèle avec les transferts hydrauliques est rendu possible par son implémentation dans le code aux éléments finis Θ-Stock. Un algorithme local a été développé spécifiquement dans ce but. Des problèmes hydromécaniques complètement couplés, tels que la création de la zone endommagée par excavation autour d'un tunnel ou la création d'un endommagement de dessiccation et d'humidification ont été simulés. Une contrainte doublement effective incorporant les effets de la succion et de l'endommagement (considéré isotrope) a été définie en s'appuyant sur des bases thermodynamiques. Cette approche a l'avantage de considérer une unique variable de contrainte étant thermodynamiquement conjuguée aux déformations élastiques. Une formulation hyperélastique dépendante de la pression de confinement est utilisée pour décrire le comportement dans le domaine élastique. L'évolution des rigidités élastiques avec l'endommagement est comparée en considérant deux hypothèses : le principe des déformations équivalentes et le principe de l'énergie élastique équivalente. L'hypothèse d'équivalence des déformations permet d'introduire la contrainte doublement effective dans les équations de plasticité et ainsi de coupler plasticité et endommagement. Les équations de plasticité sont dérivées du modèle de Barcelone (Alonso extit{et al.}, 1990), lui même basé sur le modèle de Cam-Clay. Deux critères distincts d'endommagement et de plasticité sont définis qui peuvent être activés aussi bien indépendamment que simultanément. Les surfaces de charges étant exprimées en fonction de la contrainte effective et de la succion, elles évoluent automatiquement en fonction de la succion et de l'endommagement dans l'espace des contraintes totales. Cela permet de représenter la transition d'un comportement ductile vers un comportement fragile lorsque le matériau est asséché. Un algorithme local explicite a été développé pour gérer le couplage des deux phénomènes dissipatifs. L'implémentation du modèle constitutif dans le code aux éléments finis Θ-Stock permet l'étude de problèmes hydromécaniques complètement couplés, les lois de transfert dépendant elles aussi de l'état de saturation du matériau. Ce modèle a d'abord été appliqué à la simulation du développement de micro-fissures lors du séchage d'une éprouvette. L'apparition de l'endommagement est expliquée principalement par le gradient de pression très important créé à la surface de l'échantillon lorsqu'on applique une forte variation de succion. Enfin, un problème à échelle réelle est simulé. L'excavation d'un tunnel, la désaturation du sol environnant dû à la ventilation, ainsi que la création de la zone endommagée par excavation sont étudiés. L'étendue des zones l'endommagement et de déformations plastiques autour du tunnel est étudié

Hydro-mechanical modelling of damage-plasticity couplings in unsaturated geomaterials

Cette thèse a pour but le développement d'un modèle constitutif hydromécanique prenant en compte le couplage de l'endommagement et de la plasticité dans les géomatériaux non-saturés. Le couplage de ce modèle avec les transferts hydrauliques est rendu possible par son implémentation dans le code aux éléments finis Θ-Stock. Un algorithme local a été développé spécifiquement dans ce but. Des problèmes hydromécaniques complètement couplés, tels que la création de la zone endommagée par excavation autour d'un tunnel ou la création d'un endommagement de dessiccation et d'humidification ont été simulés. Une contrainte doublement effective incorporant les effets de la succion et de l'endommagement (considéré isotrope) a été définie en s'appuyant sur des bases thermodynamiques. Cette approche a l'avantage de considérer une unique variable de contrainte étant thermodynamiquement conjuguée aux déformations élastiques. Une formulation hyperélastique dépendante de la pression de confinement est utilisée pour décrire le comportement dans le domaine élastique. L'évolution des rigidités élastiques avec l'endommagement est comparée en considérant deux hypothèses : le principe des déformations équivalentes et le principe de l'énergie élastique équivalente. L'hypothèse d'équivalence des déformations permet d'introduire la contrainte doublement effective dans les équations de plasticité et ainsi de coupler plasticité et endommagement. Les équations de plasticité sont dérivées du modèle de Barcelone (Alonso extit{et al.}, 1990), lui même basé sur le modèle de Cam-Clay. Deux critères distincts d'endommagement et de plasticité sont définis qui peuvent être activés aussi bien indépendamment que simultanément. Les surfaces de charges étant exprimées en fonction de la contrainte effective et de la succion, elles évoluent automatiquement en fonction de la succion et de l'endommagement dans l'espace des contraintes totales. Cela permet de représenter la transition d'un comportement ductile vers un comportement fragile lorsque le matériau est asséché. Un algorithme local explicite a été développé pour gérer le couplage des deux phénomènes dissipatifs. L'implémentation du modèle constitutif dans le code aux éléments finis Θ-Stock permet l'étude de problèmes hydromécaniques complètement couplés, les lois de transfert dépendant elles aussi de l'état de saturation du matériau. Ce modèle a d'abord été appliqué à la simulation du développement de micro-fissures lors du séchage d'une éprouvette. L'apparition de l'endommagement est expliquée principalement par le gradient de pression très important créé à la surface de l'échantillon lorsqu'on applique une forte variation de succion. Enfin, un problème à échelle réelle est simulé. L'excavation d'un tunnel, la désaturation du sol environnant dû à la ventilation, ainsi que la création de la zone endommagée par excavation sont étudiés. L'étendue des zones l'endommagement et de déformations plastiques autour du tunnel est étudiéeThis thesis work objective is the development of a hydro-mechanical constitutive model which accounts for damage-plasticity couplings in unsaturated geomaterials. The coupling of this model with hydraulic transfers is made possible by its implementation into the Finite Element code Θ-Stock. In order to achieve this implementation, a specific stress-point algorithm has been developed. Fully coupled hydro-mechanical problems have been simulated, such as the creation of the Excavation Damaged Zone around a tunnel and the initiation of damage due to desiccation and humidification. A double effective stress incorporating both the effect of suction and damage (assumed isotropic) is defined based on thermodynamical considerations. The advantage of this approach is that it results in a unique stress variable being thermodynamically conjugated to elastic strains. A pressure-dependent hyperelastic formulation is used to describe the behaviour inside the elastic domain. The evolution of elastic rigidities with damage is then studied. Two hypotheses are compared, the principle of strain equivalence and the principle of equivalent energy. Coupling between damage and plasticity phenomena is achieved by following the principle of strain equivalence and incorporating the effective stress into plasticity equations. The plasticity framework is based on the Barcelona Basic Model (Alonso extit{et al.}, 1990), itself based on the Cam-Clay model. Two distinct criteria are defined for damage and plasticity, which can be activated either independently or simultaneously. Their formulation in terms of effective stress and suction allows them to evolve in the total stress space with suction and damage changes. This leads to a direct coupling between damage and plasticity and allows the model to capture the ductile/brittle behaviour transition occurring when clays are drying. A specific explicit algorithm has been developed to handle the association of the two dissipative phenomena. The implementation of the constitutive model into the Finite Element code Θ-Stock allowed for the simulation of fully coupled hydro-mechanical problems. The hydraulic transfer laws also consider the saturation state. This fully coupled model is first applied to simulate the development of micro-cracks during desiccation of a soil sample. Damage initiation is explained mainly by the important pressure gradient appearing at the boundary when applying a high suction change. Finally, a full-scale problem is simulated. The excavation of a tunnel, the desaturation of the surrounding soil and the creation of the excavation damaged zone are studie