Simultaneous unloading of shear and normal stresses induces activation of naturally rough-walled sandstone fractures

The unloading mode of shear and normal stresses and the number of shear cycles have important influences on the unloading-induced slip of rock fractures. Clarifying the slip mechanism of rock fractures disturbed by deep excavation is critically important for the exploitation of underground energy and the mitigation of seismicity. In this research, the simultaneous unloading tests of normal stress and shear stress on the rough-walled sandstone fractures are carried out cyclically. Experimental results indicate that the ratio of unloading rate of shear stress to that of normal stress (i.e., vs/vn) mainly controls the slip modes of the fracture. When vs/vn is less than 0.66, the sandstone fracture will be activated to slip, which involves a quasi-static slip stage and a dynamic slip stage. And, a lower value of vs/vn tend to produce a larger duration of the quasi-static slip stage and a lower peak slip velocity at the initiation point, vice versa. The quasi-static slip stage is essentially dominated by the asperity damage process and the cycle number mainly affects the initial stress state of the fracture. Our observations suggest that adjusting the unloading rate ratio during the excavation process may reduce the possibility of fracture slip-induced dynamic hazards.

Automated summary

The unloading mode of shear and normal stresses and the number of shear cycles have significant effects on the unloading-induced slip of rock fractures. It is critically important to understand the slip mechanism of rock fractures disturbed by deep excavation for the exploitation of underground energy and the mitigation of seismic activity. The experimental findings suggest that the ratio of unloading rate of shear stress to that of normal stress mainly controls the slip modes of the fracture, and adjusting the unloading rate ratio during the excavation process may reduce the possibility of fracture slip-induced dynamic hazards.