Inelastic compaction and failure mode of Bleurswiller sandstone under true triaxial compression

Previous investigations of the compressive failure of porous rocks under true triaxial compression have focused on the brittle faulting regime. These studies have underscored the dependence of the peak stress state on the interplay of the three principal stresses. In comparison, there is a paucity of systematic investigations of ductile failure under true triaxial compression. In this study we selected Bleurswiller sandstone, which has been extensively investigated in relation to the brittle-ductile transition under conventional triaxial compression at room temperature. Experiments were conducted in Wuhan on water-saturated samples with the size of 100mm×50mm×50mm at the minimum and intermediate principal stresses ranging up to 70 MPa and 170 MPa, respectively. Previous conventional tests have shown that the initial yield points of Bleurswiller sandstone fall on a linear cap relating the differential and mean stresses. Our new data show that initial yielding under true triaxial loading at a fixed Lode angle is also characterized by a Mises effective shear stress that decreases linearly with increasing mean stress, in agreement with the prediction of an elastic-plastic pore collapse model. Subsequent yielding was manifested by various degrees of strain hardening, that would culminate in a spectrum of failure modes (high-angle shear bands, conjugate shear bands, compaction bands, distributed cataclastic flow). The 3D complexity and geometric attributes of these failure modes have been characterized by X-ray CT imaging of the failed samples.