Unloading-induced acoustic and fracturing behavior of a fault interface at various depths: relevance to faulting-affected coal burst in deep mining

To investigate fault activation and slip-induced hazards at different depths in coal mining, this study conducted unloading experiments on inclined sandstone interfaces under confining stresses of 10 MPa, 18 MPa and 30 MPa using a true triaxial loading machine. Seismic signals over fault surface sliding were collected using a multi-channel acoustic emission (AE) system. Statistics on the AE energy and dominant frequency with fault activation were calculated, and seismic localization and fracture type during faulting were derived for different initial confinement conditions. Scanning electron microscopy (SEM) and confocal laser scanning microscope (CLSM) techniques were used to characterize the micro features of fault sliding. Experimental results indicate that the unloading-induced slip instability process of the fault surface under different confining pressures is basically consistent, and the duration of fault surface slip is positively correlated to the initial confining pressure. The AE events are distributed near the fault surface and mainly represent tensile fracture, but shear events significantly increase after fault macroscopic slip. Both the fracture intensity and pore abrasion of the fault interface are higher for a larger confining pressure and the surface is apparently smooth when confining stress reaches 30 MPa. Based on the experimental results, the mechanical behavior of unloading-induced fault sliding correlated to AE signal characteristics and surface fracture separately was discussed. Finally, implications of the findings on the seismic monitoring and early warning of faulting-affected coal burst in deep mining were presented.