Mechanical responses and failure mechanism of hydrostatically pressurized rocks under combined compression-shear impacting

Accurately characterizing the mechanical responses and failure mechanism of hydrostatically pressurized rocks under combined compression-shear impacting are of great significance for construction safety in deep underground engineering. Using a split Hopkinson pressure bar (SHPB) modified with axial and radial confining systems, oblique cylindrical specimens with different hydrostatic confining pressures of 7, 14, 21 and 28 MPa are tested under loading rates varying from 1200 to 3200 GPa/s. The dynamic force equilibrium of the oblique specimen in the triaxial SHPB tests can be satisfied by using the pulse shaping techniques. A complementary discrete element method (DEM) simulation is conducted to describe the failure behaviours of hydrostatically confined rocks under combined compression-shear loading. The experimental and numerical results show the effects of confining pressure and loading rate on the compression-shear impact responses and failure mechanism of rocks. Under different hydrostatic confinements, both the dynamic compressive and shear strength of rocks increase linearly with equivalent stress rate, while the rate of increase slows as confinement increases. Post-mortem examination of recovered specimens and the DEM simulation show that the failure pattern of hydrostatically pressurized rocks under compression-shear impacting changes from two parallel shear bands with small inclination to a single shear band along the short diagonal of oblique specimen as confinement increases. Fragmentation analysis shows that the rocks are more fragmented under a higher loading rate and that confinement enlarges the average fragment size and its distribution range.