The goal of this research is to develop a robust, continuum-based approach for a three-dimensional, Finite Element Method (FEM) simulation of layered geomaterials. There are two main approaches to the numerical modeling of layered geomaterials; discrete or discontinuous techniques and an equivalent continuum concept. In the discontinuous methodology, joints are explicitly simulated. Naturally, discrete techniques provide a more accurate description of discontinuous materials. However, they are complex and necessitate care in modeling of the interface. Also, in many applications, the definition of the input model becomes impractical as the number of joints becomes large. In order to overcome the difficulties associated with discrete techniques, a continuum-based approach has become popular in some application areas. When using a continuum model, a discrete material is replaced by a homogenized continuous material, also known as an 'equivalent continuum'. This leads to a discretization that is independent of both the orientation and spacing of layer boundaries. However, if the layer thickness (i.e., internal length scale of the problem) is large, the classical continuum approach which neglects the effect of internal characteristic length can introduce large errors into the solution. In this research, a full 3D FEM formulation for the elasto-plastic modeling of layered geomaterials is proposed within the framework of Cosserat theory. The effect of the bending stiffness of the layers is incorporated in the matrix of elastic properties. Also, a multi-surface plasticity model, which allows for plastic deformation of both the interfaces between the layers and intact material, is introduced. The model is verified against analytical solutions, discrete numerical models, and experimental data. It is shown that the FEM Cosserat formulation can achieve the same level of accuracy as discontinuous models in predicting the displacements of a layered material with a periodic microstructure. Furthermore, the method is capable of reproducing the strength behaviour of materials with one or more sets of joints. Finally, due to the incorporation of layer thickness into the constitutive model, the FEM Cosserat formulation is capable of capturing complicated failure mechanisms such as the buckling of individual layers of material which occur in stratified media.

The goal of this research is to develop a robust, continuum-based approach for a three-dimensional, Finite Element Method (FEM) simulation of layered geomaterials. There are two main approaches to the numerical modeling of layered geomaterials; discrete or discontinuous techniques and an equivalent continuum concept. In the discontinuous methodology, joints are explicitly simulated. Naturally, discrete techniques provide a more accurate description of discontinuous materials. However, they are complex and necessitate care in modeling of the interface. Also, in many applications, the definition of the input model becomes impractical as the number of joints becomes large. In order to overcome the difficulties associated with discrete techniques, a continuum-based approach has become popular in some application areas. When using a continuum model, a discrete material is replaced by a homogenized continuous material, also known as an 'equivalent continuum'. This leads to a discretization that is independent of both the orientation and spacing of layer boundaries. However, if the layer thickness (i.e., internal length scale of the problem) is large, the classical continuum approach which neglects the effect of internal characteristic length can introduce large errors into the solution. In this research, a full 3D FEM formulation for the elasto-plastic modeling of layered geomaterials is proposed within the framework of Cosserat theory. The effect of the bending stiffness of the layers is incorporated in the matrix of elastic properties. Also, a multi-surface plasticity model, which allows for plastic deformation of both the interfaces between the layers and intact material, is introduced. The model is verified against analytical solutions, discrete numerical models, and experimental data. It is shown that the FEM Cosserat formulation can achieve the same level of accuracy as discontinuous models in predicting the displacements of a layered material with a periodic microstructure. Furthermore, the method is capable of reproducing the strength behaviour of materials with one or more sets of joints. Finally, due to the incorporation of layer thickness into the constitutive model, the FEM Cosserat formulation is capable of capturing complicated failure mechanisms such as the buckling of individual layers of material which occur in stratified media.

The design of interface elements in geotechnical engineering traditionally involves empiricism and lacks a solid fundamental underpinning based on the controlling mechanisms. These design shortcomings exist due to deficiencies in the fundamental understanding of geotechnical interface behaviors and the lack of test methods and devices available to directly measure interface properties in situ. The current work strives to improve the state of geotechnical knowledge and design with regard to interface behavior through fundamental laboratory studies and the development and use of a new in situ testing device. The current investigations are focused across a range of scales from micromechanical interactions to full scale field implementation. A series of laboratory investigations at the micromechanical level have been performed, specifically aimed at investigating the mechanisms controlling granular interactions against conventional and textured friction sleeves, and hook and loop type interactions present within textured geomembrane - geotextile systems. Additionally, a new in situ testing device has been designed and developed, the Multi Piezo Friction Attachment (MPFA), to allow for the characterization of geotechnical interface properties in situ within the context of an effective stress framework. The MPFA simultaneously provides four independent measures of interface friction (f

Ce travail de recherche porte sur le comportement des milieux poreux (triphasiques), plus particulièrement les sols non saturés sous sollicitations hydro-mécaniques. Un modèle constitutif élastoplastique couplé est développé. Ce modèle original est formulé selon les principes suivants: une loi constitutive est développée pour décrire le comportement de chaque phase (squelette solide, liquide, et gaz). Ensuite, des relations de couplage sont ajoutées entre chacune des phases. Pour le comportement du squelette solide, une loi élastoplastique non associée est adoptée, avec deux surfaces de charges, en cisaillement et en compression. La partie hydrique est décrite par une formulation qui permet de prendre en compte l’effet d’hystérésis. Ce modèle a été enrichi par une relation de couplage hydromécanique qui permet d’exprimer la pression d’entrée d’air en fonction de la porosité. Ensuite, le couplage complet se fait avec la contrainte effective de Bishop en utilisant une nouvelle définition du paramètre de succion χ grâce à laquelle, les différents phénomènes présents dans la réponse des milieux poreux sous différentes sollicitations peuvent être reproduits. Ce modèle est validé par une confrontation à des données expérimentales issues de la littérature sur différents types de sol (sable, limon,…). Le modèle est implanté dans le code aux éléments finis Cast3M. L’analyse de problèmes particuliers, tels que la mise en œuvre d’un cas test d’un sol d’assise soumis à un cycle pluvial, ainsi que l’étude de la stabilité d’une pente, permette de montrer la capacité du modèle à reproduire le comportement des milieux poreux non saturés. / This work focuses on the behaviour of porous triphasic media, particularly on unsaturated soils subjected to hydromechanical loading. A coupled elastoplastic constitutive model has been developed. This original model is formulated according to the following principles: (1) a constitutive law describing the behaviour of different phases (solid skeleton, liquid and gas). (2) coupling relationships between each phase. For the behaviour of the solid skeleton, a non associated elastoplastic constitutive law is adopted, with two loading surfaces: shear surface and compression cap surface. The hydric part is discribed using a formulation which allows to take into account the hysteresis effect. This model has been extended using a hydromechanical coupling relation between the air entry value and the porosity. Then the coupling is completed with the Bishop effective stress, using a new definition for the suction parameter χ. Using this formulation, the various phenomena present in the porous media behaviour under different loading can be reproduced. The developed model has been validated through a comparison with experimental data on different types of soil (sand, silt,…). This model is implemented in the french finite element code Cast3M. The analysis of specific problems, such as (1) the study of shallow foundation subjected to cyclic rain event, as well as (2) the study of slope stability, show the model capacity to reproduce the behaviour of unsaturated porous media.