Résumé : Ces travaux de thèse s'intéressent à l'étude du comportement d'une discontinuité dans un géomatériau sous sollicitations chemo-mécaniques à l'échelle du laboratoire. Des essais de traction-compression cyclique étudient la refermeture d'une fissure. Ils indiquent que les déformations inélastiques seraient gouvernées en partie par les frottements générés lors du ré-emboîtement des lèvres de la discontinuité, non correspondantes du fait des contraintes internes. Des joints rocheux altérés chimiquement sous sollicitation tangentielle sont étudiés au travers d'essais de cisaillement direct : le comportement des joints dégradés est profondément modifié du fait de la diminution des propriétés mécaniques du matériau de part et d'autre de la discontinuité et de l'accentuation de la non-correspondance des profils rugueux. Le comportement d'une discontinuité est modélisée par le couplage d'un modèle élasto-plastique endommageable continu avec une résolution discrète du problème de contact/frottement (code calcul aux éléments finis Cast3M). Les résultats numériques confirment les phénomènes constatés expérimentalement.//Abstract: To probate the technology of CO[indice inférieur 2] geological storage, the integrity of the site must be assure over time. This industruial problematic involves the study of the mechanical properties alteration of geomaterials in the presence of CO[indice inférieur 2]. The scenario at the origin of this thesis illustrates the possibility of a CO[indice inférieur 2] leakage on a fault located in the caprock. This geological problem is complicated by the many parameters to consider: in situ temperature and pressure, scale effect, heterogeneities of the geomaterial, geofluide composition, chemical reactions ... These works focus on the behavior of a discontinuity in a geomaterial solicited chemomechanically at the laboratory scale. They were realised in cotutelle between SIAME laboratory at the University of Pau and Pays de l'Adour (France) and the laboratory of rock mechanics and engineering geology from the University of Sherbrooke (Quebec, Canada). The first part of the experimental program was defined to characterize the reclosing of a crack under cyclic uniaxial stress. The second experimental campaign has studied the shear behavior of a rock joint chemically degraded. The data obtained were used to model the behavior of a discontinuity by the finite element method. The mechanical behavior of a crack under normal stress is assessed with cyclic tension-compression tests. Stress curve showed hysteresis during opening and closing cycles of a discontinuity in concrete, it indicated inelastic deformations The analysis of displacement field by image correlation indicated that theses deformations were partially governed by the friction generated during the closing of the discontinuity lips. Frictional phenomena are due to asperities mismatching induced by the internal stresses in the concrete. The shear behavior of a rock joint chemically damaged was studied through direct shear tests. Rough surfaces were immersed in acid solution during 6 hours at constant pH. Digitalization of these surfaces befor and after immesion, with a lase profilometer, indicates little modifications of the geometry induced by dissolution of material. Results of tests pointed out significant modifications for altered joints illustred by a of the peak shear strength and an increased of contractancy. They are induced by: 1) the mismatch enhancement of the rough profiles of the discontinuity and, 2) the degradation of the mechanical properties of the material on both sides of the discontinuity due to the chemical attack. Numerical contribution of the thesis lies in modeling the behavior of a discontinuity by the coupling of an continuous elastic-plastic damaged model with a discrete resolution of the contact/friction problem. The model is developed with the finite element code Cast3M. Geometries lips discontinuities are modeled directly from the roughness profiles from experimental scans. The numerical results correctly represent the friction phenomena observed experimentally. Finally, a model of the shear test altered joints is performed by coupling the mechanical model with chemical damage model.

Ces travaux de thèse s'intéressent à l'étude du comportement d'une discontinuité dans un géomatériau sous sollicitations chemo-mécaniques à l'échelle du laboratoire. Des essais de traction-compression cyclique étudient la refermeture d'une fissure. Ils indiquent que les déformations inélastiques seraient gouvernées en partie par les frottements générés lors du ré-emboîtement des lèvres de la discontinuité, non correspondantes du fait des contraintes internes. Des joints rocheux altérés chimiquement sous sollicitation tangentielle sont étudiés au travers d'essais de cisaillement direct : le comportement des joints dégradés est profondément modifié du fait de la diminution des propriétés mécaniques du matériau de part et d'autre de la discontinuité et de l'accentuation de la non-correspondance des profils rugueux. Le comportement d'une discontinuité est modélisée par le couplage d'un modèle élasto-plastique endommageable continu avec une résolution discrète du problème de contact/frottement (code calcul aux éléments finis Cast3M). Les résultats numériques confirment les phénomènes constatés expérimentalement. / This PhD dissertation presents a study aiming at a better understanding of cracks behavior in a geomaterial. The study focuses on the behavior of discontinuities under chemo-mechanical solicitations at the laboratory scale. The mechanical behavior under normal stress is assessed with cyclic tension-compression tests. Experimental data indicate that the inelastic deformations could be partially governed by the friction generated during the closing of the discontinuity lips, and the asperities mismatch is related to the internal stresses. The shear behavior of a rock joint chemically degraded was studied through direct shear tests. Results pointed out significant modifications for altered joints induced by: 1) the mismatch enhancement of the rough profiles of the discontinuity and, 2) the degradation of the mechanical properties of the material on both sides of the discontinuity due to the chemical attack. These experimental results have been used as input data to model the behavior of a discontinuity by the coupling of a continuous elastic-plastic damaged model with a discrete solving of the contact/friction problem. The simulations performed under Cast3M correctly represent the phenomena observed during the experimental testing program.

R??sum?? : Ces travaux de th??se s'int??ressent ?? l'??tude du comportement d'une discontinuit?? dans un g??omat??riau sous sollicitations chemo-m??caniques ?? l'??chelle du laboratoire. Des essais de traction-compression cyclique ??tudient la refermeture d'une fissure. Ils indiquent que les d??formations in??lastiques seraient gouvern??es en partie par les frottements g??n??r??s lors du r??-embo??tement des l??vres de la discontinuit??, non correspondantes du fait des contraintes internes. Des joints rocheux alt??r??s chimiquement sous sollicitation tangentielle sont ??tudi??s au travers d'essais de cisaillement direct : le comportement des joints d??grad??s est profond??ment modifi?? du fait de la diminution des propri??t??s m??caniques du mat??riau de part et d'autre de la discontinuit?? et de l'accentuation de la non-correspondance des profils rugueux. Le comportement d'une discontinuit?? est mod??lis??e par le couplage d'un mod??le ??lasto-plastique endommageable continu avec une r??solution discr??te du probl??me de contact/frottement (code calcul aux ??l??ments finis Cast3M). Les r??sultats num??riques confirment les ph??nom??nes constat??s exp??rimentalement.//Abstract: To probate the technology of CO[indice inf??rieur 2] geological storage, the integrity of the site must be assure over time. This industruial problematic involves the study of the mechanical properties alteration of geomaterials in the presence of CO[indice inf??rieur 2]. The scenario at the origin of this thesis illustrates the possibility of a CO[indice inf??rieur 2] leakage on a fault located in the caprock. This geological problem is complicated by the many parameters to consider: in situ temperature and pressure, scale effect, heterogeneities of the geomaterial, geofluide composition, chemical reactions ... These works focus on the behavior of a discontinuity in a geomaterial solicited chemomechanically at the laboratory scale. They were realised in cotutelle between SIAME laboratory at the University of Pau and Pays de l'Adour (France) and the laboratory of rock mechanics and engineering geology from the University of Sherbrooke (Quebec, Canada). The first part of the experimental program was defined to characterize the reclosing of a crack under cyclic uniaxial stress. The second experimental campaign has studied the shear behavior of a rock joint chemically degraded. The data obtained were used to model the behavior of a discontinuity by the finite element method. The mechanical behavior of a crack under normal stress is assessed with cyclic tension-compression tests. Stress curve showed hysteresis during opening and closing cycles of a discontinuity in concrete, it indicated inelastic deformations The analysis of displacement field by image correlation indicated that theses deformations were partially governed by the friction generated during the closing of the discontinuity lips. Frictional phenomena are due to asperities mismatching induced by the internal stresses in the concrete. The shear behavior of a rock joint chemically damaged was studied through direct shear tests. Rough surfaces were immersed in acid solution during 6 hours at constant pH. Digitalization of these surfaces befor and after immesion, with a lase profilometer, indicates little modifications of the geometry induced by dissolution of material. Results of tests pointed out significant modifications for altered joints illustred by a of the peak shear strength and an increased of contractancy. They are induced by: 1) the mismatch enhancement of the rough profiles of the discontinuity and, 2) the degradation of the mechanical properties of the material on both sides of the discontinuity due to the chemical attack. Numerical contribution of the thesis lies in modeling the behavior of a discontinuity by the coupling of an continuous elastic-plastic damaged model with a discrete resolution of the contact/friction problem. The model is developed with the finite element code Cast3M. Geometries lips discontinuities are modeled directly from the roughness profiles from experimental scans. The numerical results correctly represent the friction phenomena observed experimentally. Finally, a model of the shear test altered joints is performed by coupling the mechanical model with chemical damage model.

On présente tout d’abord une série d’essais de cisaillement direct 2D monotones et cycliques sur l’interface sable de Fontainebleau-plaque rugueuse et lisse, `a contrainte normale constante (CNL) et à rigidité normale imposée (CNS). Le but de ces essais est de simuler la situation mécanique le long de pieux soumis à un grand nombre de cycles d’origine environnementale ou anthropique. Ces cycles (typiquement 10000) de faible amplitude (10`a 40 kPa en terme de contrainte de cisaillement) ne sont pas cens´es produire de rupture prématurée. Ces tests incluent une série de cycles d’amplitudes (successives) variées. Le problème de la perte de sable entre la boite et la plaque est trait´e avec attention. Nous avons interprété l’effet de la position du ”centre des cycles” dans le plan de contraintes (variables cycliques moyennes) et de la densité initiale. Plusieurs facteurs tels que l’indice initial de densité (ID0), la contrainte normale cyclique moyenne (_n cm0), le niveau initial moyen de contrainte de cisaillement (_cm0), l’amplitude cyclique réduite (__) et la rigidité normale imposée (k qui dans cette thèse, va de 1000 `a 5000 kPa/mm), influencent les déplacements relatifs cycliques moyens normal ([u]cm) et tangentiel [w]cm) et sont pris en considération.On observe soit de la dilatance, soit de la contractance en accord avec l’état caractéristique développé par Luong. L’influence du chemin de contrainte (CNL ou CNS) est également analysée. Un modèle phénoménologique et analytique de comportement d’interface sur chemins cycliques CNL est propos´e. C’est également le cas pour le comportement monotone sur chemins oedométrique et CNL, la variable de mémoire unique étant la densité d’interface (sous contrainte) ou le déplacement relatif normal. Cette formulation permet de traiter, par incréments analytiques finis, les chemins comportant une variation d’amplitude cyclique, et les chemins CNS, ce qui introduit la notion de nombre de cycles équivalent. On notera que les chemins CNS sont toujours contractants. Ces essais sont utilisés pour aborder la simulation par éléments finis, avec le logiciel Plaxis, selon une approche de pseudo-viscoplasticité, le nombre de cycles tenant lieu de temps fictif. L’essai de cisaillement monotone`a la boite est modélisé en densités faible et forte, ainsi que deux essais de pieux modèles centrifugés, l’un en traction, l’autre en compression. Des recommandations sont proposées pour le calcul courant des pieux sous sollicitations cycliques. Cette thèse a été soutenue par l’ANR SOLCYP et le programme national ” recherches sur le comportement des pieux soumis à des sollicitations cycliques”. / A series of monotonic and cyclic 2D direct shear tests on sand (Fontainebleau)-rough/smoothmaterial interfaces under constant normal load (CNL) and constant normal stiffness (CNS)conditions are presented. The aim of these tests is to simulate the situation along the shaftof piles subjected to a large number of cycles due to environmental or anthropic loadings.These cycles (typically 104) are of small amplitude (10, 20 and 40 kPa in terms of shearstress) as the service loads are not supposed to produce an early failure. These tests in-clude the series of changing the cyclic amplitude in succession. The problem of loss of sandbetween the box and the rough plate, typical phenomenon in this type of test, receives aspecial attention. It is interesting to observe, according to the initial density, the positionof the ”center of cycles” in the stress plane (mean cyclic variables). Several factors such asinitial density (ID0), initial normal stress (_n cm0), level of initial mean cyclic stress ratio(_cm0), reduced cyclic amplitude (__) and imposed normal stiffness (in this thesis, k =1000, 2000 and 5000 kPa/mm) that influence the intensity of mean cyclic normal ([u]cm)and shear ([w]cm) displacements are considered. Along CNL paths either dilation or con-traction is exhibited, in agreement with the characteristic state developed by Luong. Theinfluence of the stress path (under constant normal stress or prescribed normal stiffness)is also highlighted. It should be highlighted that CNS paths are ever contractive. Themodel of monotonic interface behaviour under CNL and oedometer paths is fully analyt-ical and based on the rate-type framework with the normal relative displacement or theinterface density as unique memory parameter. While the analytical formulations for iden-tification are first proposed to describe the interface behaviour under cyclic CNL condition,the variation of cyclic amplitude and CNS condition are modeled by applying the analyticalformulations for validation using finite analytical increments and introducing the notion ofequivalent number of cycles. For numerical simulations by the FEM (Plaxis) these tests areinterpreted and formulated according to a pseudo visco-plastic framework, the number ofcycles being a fictitious time. The direct shear tests and two centrifuge pile tests (pull-outand compression) are also modeled. Recommendations are proposed for the calculation ofreal usual piles under cyclic loading. This thesis is one part of the national French project”ANR and National Program SOLCYP” (Research on behaviour of piles subjected to cyclicloading).