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Hydromechanical behaviour of the granular rock and localization of the deformation

Hichem Ouffroukh 
Abstract : This work concerns the effect of pore fluid on the behaviour of the saturated granular rock in the pre and post localization regime. The hydromechanical behaviour of Fontainebleau sandstone is studied on the basis of isotropic and triaxial compression tests in drained and undrained conditions on water saturated samples. The effect of the evolution of the rock compressibility with the applied stress on the poromechanical parameters is shown. On the basis of micro-mechanical considerations, a new expression for the Skempton coefficient B is proposed as a function of the porosity, the drained bulk compressibility and the grain and fluid compressibility. The relation between rock deformation and pore-pressure evolution in undrained deviatoric tests is analysed. An elasto-plastic constitutive model with stress-dependent elasticity and damage is proposed to describe the behaviour of the rock and is validated through back analysis of drained and undrained tests. Detailed analysis of shear band formation and shear band microstructure formed in drained and undrained triaxial tests on Fontainebleau sandstone is presented. It is shown that under globally undrained conditions, local fluid exchanges inside the sample occur at shear banding resulting into heterogeneous damage pattern along the shear band. At high confinement, pore pressure generation inside the band locally leads to fluidisation of the crushed material which results into the formation of interconnected channels in the heart of the band. Image processing analysis is used for evaluation of porosity inside the shear band and estimation of the permeability is performed using the Walsh and Brace [1984] model. It is shown that, porosity increase as observed in the band at low confining pressure and porosity decrease as observed at high confining pressure are both accompanied by a reduction of permeability inside the shear band due to the increase of tortuosity and specific surface. However, this permeability reduction is much more important at high confining pressure and can reach values three orders of magnitude smaller than the permeability of the intact material. The final part of the document is devoted to a study of large scale shear band, which constitutes a geological fault. An analogy could be observed between the microstructure of shear bands as observed in Fontainebleau sandstone samples and the structure of the fault zone . The work presented have been developped within the frame of our participation in DGLAB-CORINTH European project on the seismic risks in the Gulf of Corinth.
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Submitted on : Thursday, August 4, 2005 - 8:00:00 AM
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  • HAL Id : pastel-00001354, version 1



Hichem Ouffroukh. Hydromechanical behaviour of the granular rock and localization of the deformation. Engineering Sciences [physics]. Ecole des Ponts ParisTech, 2004. English. ⟨pastel-00001354⟩



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