Spalling failure of deep hard rock caverns

Spalling is a typical brittle failure phenomenon of hard rock in deep caverns under high geostress. In this study, key issues are systematically studied concerning the spalling failure of deep hard rock caverns. First, the prismatic rock specimens with small thicknesses (i.e. width × thickness × height: 20 mm × 50 mm × 100 mm) are employed in our tests which not only successfully simulate the spalling failure of hard rock in the laboratory but also obtain a reasonable spalling strength similar to that of the rock mass. Then, a series of spalling experiments is carried out to investigate the mechanism of spalling failure of deep hard rock caverns. Our results show that the intermediate principal stress, weak dynamic disturbances, and rock microstructure have significant effects on the spalling failure. The spalling strength is approximately (0.3–0.8)UCS, where UCS is the uniaxial compressive strength of the cylindrical rock sample with a diameter of around 50 mm. The spalling strength increases first and then decreases with increasing intermediate principal stress. Moreover, an empirical spalling strength criterion and a numerical method of spalling failure are proposed. This numerical method can not only simulate the spalling failure zone formed by tangential compressive stress concentration after excavation under different intermediate principal stresses, but also successfully simulate the failure transition from tensile mode to shear mode associated with confinement change in deep hard rock caverns. Furthermore, an acoustic emission-based early warning method using neural network is proposed to predict the spalling failure. Finally, a technical roadmap for preventing and controlling spalling failure of deep hard rock caverns is presented after summarizing the successful experiences in a typical engineering case.