Shear banding in drained and undrained triaxial tests on a saturated sandstone: Porosity and permeability evolution

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 a heterogeneous damage pattern along the shear band. At high confinement, pore pressure generation inside the band leads locally to fluidisation of the crushed material, which results into the formation of connected 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 [The effect of pressure on porosity and the transport properties of rock. J Geophys Res 1984; 89 (B11): 9425–31] 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.