Experimental investigation of the influence of pore pressure and porosity on the deformation of granular salt

The influence of pore pressure and porosity on the ductile deformation of granular salt was experimentally investigated under a isostatic compression at elevated temperatures. Confining and pore pressures were independently controlled while granular salt samples exhibited rate-dependent deformation that resulted in large decreases of sample volume and porosity. Over the experimental durations, which ranged from one to seven days, volumetric strain rates generally ranged from 10−2 down to 10−10s−1. The sample deformation rates were observed to correspond with both the sample porosity and the pressure difference between the confining and pore pressures. Experimental results show that the volumetric strain rate decreased when either the sample porosity or the pressure difference were decreased. Post-test observations revealed that the reduced porosity was caused by the solid particles deforming in a ductile manner into the pore volume. Insight into the cause of ductile deformation under isostatic compression was gained by considering the local stress distribution within a sample, which was approximated with the use of a representative volume element. Analysis of the local stress distribution showed how the magnitude of localized shear stress depends on both porosity and the pressure difference, which is consistent with the experimental results.