The study of the hydromechanical behaviour of fractured rock masses requires a detailed knowledge of the isolated fractures. The aim of the paper is the simulation of the evolution of the void space, located between both walls of a fracture, during a shear test under constant normal stress. Results of the mechanical simulation model are first validated then analyzed in detail. We observe that, during the shear process, there is an opening of the fracture with the creation of preferential channels. These channels are determined at the early phase of the shear process (pre-peak phase), and we observe during the post-peak phase that void areas increase in size rather than the creation of new apertures.

Die Untersuchung des hydromechanischen Verhaltens eines zerlegten Felsmassivs erfordert genaue Kenntnisse der einzelnen Spalten. Ziel dieses Artikels ist die Simulation der Evolution des Hohlraumes zwischen den beiden Wanden eines Spaltes wahrend eines Abscherungsexperimentes unter konstantem senkrechtem Druck. Die Ergebnisse der Modellierung werden zunachst mit dem Abscherungsexperiment verglichen und dann im Einzelnen ausfuehrlich analysiert. Wahrend des Abscherungsprozesses tritt eine Öffnung des Spaltes auf, die die Bildung von bevorzugten Fliesswegen fördert. Diese Kanale werden bereits in einer fruehen Phase des Abscherungsprozesses (pre-peak Phase) festgelegt. In der post-peak Phase tritt dann eher eine Vergrößerung dieser Kanale, als die Bildung neuer Hohlraume auf.

L'etude du comportement hydromecanique d'un massif rocheux fracture ne peut se faire sans une connaissance approfondie des fractures le constituant. L'etude presentee dans cet article concerne la simulation de l'evolution de l'espace des vides present entre les deux epontes d'une fracture au cours d'un essai de cisaillement à contrainte normale constante. Les resultats du modèle sont tout d'abord valides puis analyses en detail. On constate que, au cours du cisaillement, il apparaît une ouverture de la fracture permettant la creation de chenaux preferentiels. Ces derniers sont determines très tôt dans le cisaillement (phase pre-pic), et on observe dans la phase post-pic davantage un accroissement de la taille des chenaux plutôt que la formation de nouvelles zones de vides.

Introduction

The study of the hydromechanical behaviour of rock fractures has today a lot of applications in various fields of geology (petroleum reservoirs, seismology, hydrogeology, geothermy) and civil engineering (foundations, tunnelling). In order to understand the behaviour at the rock mass scale, we need to have information at the fracture scale.

Fluids through rough rock fractures flow essentially by channelling. The existence of channels comes from the geometry of the void space between both walls of the fracture. When normal or shear stresses are applied to fracture surfaces, an immediate consequence is the evolution of channelling. Various authors have worked on this subject by different ways (Esaki et al., 1992; Power & Durham, 1997; Yeo et al., 1998a and 1998b; Matsuki et al., 1999; Lamontagne, 2001; Olsson & Barton, 2001).

The aim of the study is to analyse the evolution of the void space as a function of applied stresses. This study is based on a detailed characterisation of the morphology of fracture surfaces and on simulations.

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