EXPERIMENTAL-OBSERVATIONS OF THE ELASTIC AND INELASTIC BEHAVIOR OF POROUS SANDSTONES

In order to investigate the mechanical behaviour of porous sandstones, a number of mechanical tests, including creep tests, were run on eight sandstones ranging from 17 to 30 per cent porosity, in both vacuum-dry and water-saturated conditions. To interpret the data it is essential to separate the elastic and inelastic parts of the total strain. Small stress excursions, along which the rock behaved elastically, were performed at discrete intervals along the main stress path. This allowed us to estimate the elastic moduli at various points along the stress/strain curve. We observed that the elastic constants increased significantly with increasing mean stress, but the magnitudes and pressure-dependence of the elastic constants were not consistent with presently proposed grain contact models. Knowing the elastic constants, we computed the elastic and, by difference, the non-elastic parts of the deformation. This study confirms that plasticity models such as those used in soil mechanics can account, at least qualitatively, for many features of the inelastic behaviour of porous sandstones. In particular, we observed the existence of a cap closing the yield surface in the hydrostatic pressure direction. In the relatively low stress range used here, the inelastic behaviour appeared to be controlled by frictional mechanisms rather than by grain fracturing as is commonly observed at higher stress levels. Non-elasticity seemed also to be closely connected to time dependence. Near failure, the creep rates were high and, furthermore, accelerated in the presence of water. Finally, comparison of the elastic moduli measured along different stress paths allowed us to detect and quantify mechanical anisotropy in some of the rocks considered.