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International audience; The migration of CO2-rich fluid in fractured rock masses can cause processes such as mineral dissolution and precipitation, chemically induced weakening, which can affect the long-term mechanical and transport properties of the rock mass as well as the stability of fault systems. Some numerical approaches are already available in the literature for modelling the dissolution/precipitation phenomena in fractures (e.g. Yasuhara & Elsworth 2007) as well as subcritical crack propagation (e.g. Park et al. 2007). Generally, the dissolution is supposed to increase the rock porosity and, in this way, decrease the rock strength. Some experimental data are available for the variation of rock strength and stiffness parameters with the porosity and so as a consequence of dissolution process (Bemer et al. 2004). Also the effect of chemical processes on the mechanical stability has been studied and modelled numerically in the framework of continuum materials and the context, in particular, of weathering in underground galleries (Ghabezloo & Pouya 2006). In the context of fault systems, a complete numerical modelling of the stability evolution with the flow of a reactive fluid has not yet been done. In this paper we present a simplified, but complete, set of equations for a whole system of coupled hydro-chemo-mechanical process of reactive fluid flow inside a fault. These equations have been implemented in Porofis, a FEM numerical code specially conceived for HCM processes in porous fractured media. We show how this numerical method allows to model the coupled HCM processes in the fault and the evolution of the mechanical stability in presence of in situ stresses and reactive fluid flow.

International audience

International audience; This paper investigates the two-dimensional flow problem through an anisotropic porous medium containing several intersecting curved fractures. First, the governing equations of steady-state fluid flow in a fractured porous body are summarized. The flow follows Darcy's law in matrix and Poiseuille's law in fractures. An infinite transversal permeability is considered for the fractures. A multi-region boundary element method is used to derive a general pressure solution as a function of discharge through the fractures and the pressure and the normal flux on the domain boundary. The obtained solution fully accounts for the interaction and the intersection between fractures. A numerical procedure based on collocation method is presented to compute the unknowns on the boundaries and on the fractures. The numerical solution is validated by comparing with finite element solution or the results obtained for an infinite matrix. Pressure fields in the matrix are illustrated for domains containing several interconnected fractures, and mass balance at the intersection points is also checked.

International audience; In this paper, 3D steady-state fluid flow in a porous medium with many intersecting fractures is derived numerically by using collocation method. Fluid flow, in the matrix and fractures, are described by Darcy's law and Poisseuille's law, respectively. The recent theoretical development presented a general potential solution to model the steady-state flow in fractured porous media under a far-field condition. This solution is a hypersingular integral equation with pressure field in the fracture surfaces as the main unknown. The numerical procedure can resolve the problem for any form of fractures and also takes into account the interactions and the intersection between fractures. Once the pressure field, and then the flux field in fractures have been determined, the pressure field in the porous matrix is computed completely. The basic problem of a single fracture is investigated and a semi-analytical solution is presented. Using the solution obtained for a single fracture, Mori-Tanaka and self-consistent schemes are employed for upscalling the effective permeability of 3D fractured porous media.

International audience; An Equivalent Continuum Model (ECM) is proposed in order to represent the hydromechanical behaviour of the fractured Excavation Damaged Zone (EDZ) around deep underground galleries excavated in claystone. The fractures observed in these galleries show a regular trend that makes possible elaboration of ECM based on theoretical homogenization methods. The ECM established first for plane fracture surfaces is then extended to curved, conical shape, fractures surfaces based on some simplification assumptions. The stress and displacement fields have been compared around the EDZ by using the ECM in numerical simulation or by introducing the fractures individually as discontinuities in the model. The results show that however the size of EDZ is too small compared to that of Representative Elementary Volume (REV) considered in homogenization approaches, the ECM obtained in this way seems reproducing well the fractured EDZ behaviour.

International audience; La perméabilité effective des roches fracturées ou des matériaux poreux fissurés a des applications importantes aux problèmes d'intérêt environnemental ou industriel. Elle a été intensivement étudiée ces dernières années par différentes méthodes. Cet article expose une méthode simple de modélisation de l'écoulement dans des milieux poreux fissurés et en déduit une méthode d'estimation numérique de la perméabilité effective de ces milieux.

Colloque avec actes et comité de lecture. Internationale.; International audience; La déformation et l'endommagement des argilites sous sollicitation hydrique sont observés au MEB environnemental. La déformation est mesurée par corrélation d'images à l'échelle micrométrique des zones argileuses entre les grains carbonatés et de quartz et révèle un couplage hydromécanique complexe. On montre l'apparition d'une déformation irréversible lors d'un cycle hydrique, ainsi qu'un endommagement sous la forme d'un réseau de microfissures localisées dans la phase argileuse ou aux interfaces grain-matrice, dont la morphologie dépend de la vitesse et du sens du chargement hydrique.

International audience; The objective of the present work is to study the effects of CO2 injection on the hydromechanical behavior of a fault running across a reservoir in view of safety storage risk assessment. At the fault zone scale, the equivalent permeability of the statistically distributed fractures is modeled by singular integral equations method. At the site scale, a fault is modeled as a shallow layer of filling material surrounded on both sides by a series of hydromechanical joint elements. The fluid flow across the fault due to the injection is studied. The failure probability on the fault, due to effective stress change can also be estimated by the model.

In mountainous area one of the dominant rock weathering processes is crack propagation due to freezing-thawing cycles. This mechanism within a cracked rock matrix was reproduced in laboratory with notched specimens to study its kinetic. Experimental results show that the type of porous network of the rock has a direct ncidence on the stress generated by freezing in a crack., En milieu montagneux, un des processus d'altération prépondérants des massifs fracturés est la propagation du réseau de fissures existant par gel-dégel. Afin d'évaluer la cinétique de ce type d'altération, la génération de contrainte le longd'une fissure artificielle au cours du gel a été étudiée en laboratoire. Les résultats indiquent que le type de réseau poreux de la roche a une incidence directe sur la contrainte générée par le gel dans une fissure.

CFMR - Avancées en mécanique des roches expérimentales, LYON, FRANCE, 10-/06/2010 - 10/06/2010; Étude expérimentale de la propagation des fissures sous l'action du gel dans les massifs calcaires : Enjeux du gel dans les roches. Études de faisabilité. Dispositif et protocole expérimentaux. Analyse et modèle phénoménologique. Modèle empirique de la contrainte générée.

International audience

International audience

ARMS 2008 - 5th Asian Rock Mechanics Symposium, TEHERAN, IRAN, RÉPUBLIQUE ISLAMIQUE D', 24-/11/2008 - 26/11/2008; In most of mountainous area, particularly in France, the dominant weathering process lies in cracks propagation due to freezingthawing. To predict this risk in rock masses, it is necessary to know the stresses developed in freeze-thaw cycles in natural fractures. A reproduction of this phenomenon has been carried out on artificially notched specimens of different limestones. During these experiments generated stresses were assessed along the crack inner walls. The deduced stress profiles show a timedependant maximum value, which moves in-depth in the notch. The limit reached by this stress in this process depends on intrinsic properties of the material. A simple model for this evolution has been established. The generated stress by freezingthawing in a crack comes from a coupling between thermal propagation, water phase shift and water flow in the notch and in the porous rock matrix. An analytical assessment of the stress intensity factor at the crack bottom was made by using the model established for the stress in the crack. It confirms the possibility to initiate a rupture according to the principles of linear failure mechanics. This study gives encouraging results, which could be used to establish a predictive model for the failure of a rock mass subject to freezing-thawing cycles.

The "Rochers de Valabres" is a fractured rock slope located in the French Alps and afected by rockfall. This slope has been chosen as an experimental site for improving knowledge on mechanisms leading to rockfall. Since 2002 scientific investigations (field observations, monitoring and numerical modelling) have been carried out on the site. A seismic network has recorded a significant seismic activity correlated with existing discontinuities. Mechanical measurements using tiltmeters monitor the slope movements in order to improve the knowledge on thermo-hydromechanical process. Different geometrical and mechanical models have been performed. In 2005 the site was chosen to concentrate research on understanding the efffect of climate change on rock slope stability in the framework of a French national program named STABROCK. This paper presents teh results of field measurements and numerical modelling carried out on the experimental site and the complementary investigations will start within the new research. The aim is to study more accurately the impact of environmental factors on rock slope instability mechanisms and to assess the possible effect of the climate change. Technical results will be integrated in a process of risk management.

The "Rochers de Valabres" is a fractured rock slope located in the French Alps and afected by rockfall. This slope has been chosen as an experimental site for improving knowledge on mechanisms leading to rockfall. Since 2002 scientific investigations (field observations, monitoring and numerical modelling) have been carried out on the site. A seismic network has recorded a significant seismic activity correlated with existing discontinuities. Mechanical measurements using tiltmeters monitor the slope movements in order to improve the knowledge on thermo-hydromechanical process. Different geometrical and mechanical models have been performed. In 2005 the site was chosen to concentrate research on understanding the efffect of climate change on rock slope stability in the framework of a French national program named STABROCK. This paper presents teh results of field measurements and numerical modelling carried out on the experimental site and the complementary investigations will start within the new research. The aim is to study more accurately the impact of environmental factors on rock slope instability mechanisms and to assess the possible effect of the climate change. Technical results will be integrated in a process of risk management.

Colloque avec actes et comité de lecture. Internationale.; International audience; Afin d'améliorer les méthodes de prévision des chutes de blocs en milieu montagneux, la contrainte générée par les cycles de gel-dégel dans la fracturation naturelle des roches doit pouvoir être déterminée. Un tube de calcaire surcarotté a été instrumenté et soumis à des cycles de gel-dégel. L'analyse inverse des déformations orthoradiales mesurées a permis de définir le profil longitudinal des contraintes radiales dans l'entaille. A chaque instant, ce profil présente une valeur maximum qui avec le temps, se déplace en profondeur dans l'entaille et augmente en amplitude. L'effet conjugué des cinétiques de propagation thermique au sein de la roche, de changement de phase liquide-solide et de migration de l'eau serait à l'origine de cette allure ondoyante que nous avons essayée de décrire par des formules simples.