LAUSR

Based on stress evolution induced by mining activity, triaxial confining pressure unloading tests with multiple failure state under multi-level axial stress were conducted on sandy mudstone, which were from the roof above roadside backfill area in gob-side entry retaining. Stress–strain curves and damage evolution curves of rock specimens were analyzed as well as the mechanical properties during different confining pressure unloading stages. The results show that sandy mudstone suffers mechanical properties damage and dilatancy deformation along the unloading direction during the confining pressure unloading tests under both first level axial stress and second level axial stress. Both of the corresponding axial strain of the first failure state and that of the second failure state increase with the increase of confining pressure, and the corresponding circumferential strain and volume strain increase with the decrease of confining pressure. In particular, the confining pressure unloading factor of rock specimen was introduced to describe the damage evolution characteristics of mechanical properties. There is remarkable negative linear correlation between deformation modulus, cohesion, and friction angle and confining pressure unloading factor, whereas positive linear correlation between dilatancy angle and confining pressure unloading factor is obtained. Meanwhile, shear failure characteristics were observed in triaxial confining pressure unloading tests with multiple failure state under multi-level axial stress.