Micromechanisms of inelastic deformation in sandstones: an insight using x-ray micro-tomography

This study investigates the grain-scale mechanisms that lead to failure by strain localisation in specimens of Fontainebleau sandstone with different degrees of cementation. While the effects of inter-particle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of sandstones during triaxial compression to failure. The material investigated was Fontainebleau sandstone, a quartzite formation from the Paris basin (France), which can be found as very hard, tightly cemented sandstone or more porous and less cemented material. Specimens with porosities of 6 and 21% were investigated. Triaxial compression tests at confining pressures of 2 and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter and 22 mm height. Three-dimensional (3D) images of the full specimens were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution 3D tomographic images have a voxel size of 8·5 μm (0·033d₅₀), allowing clear identification of the grains. This analysis suggests that dilatancy of the material, which depends on the degree of bonding between grains, plays a fundamental role in the failure mode of granular media. Insights into bonding rupture mechanisms and grain damage by inter- and intra-granular cracking are presented.