Mechanism of core discing in the relaxation zone around an underground opening under high in situ stresses

Core discing is a brittle rock failure during borehole drilling in intact rock subjected to high in situ stresses. This failure mode has been recognized as an important index for assessing underground projects under high in situ stresses. With increasing depth of underground space exploitation, core discing occurs not only in highly-stressed intact rock, but also in the relaxation zone of the surrounding rock masses. In the latter case, the disc thickness is even smaller, and the mechanism is significantly different from that of core discing in highly-stressed intact rock. In this study, the correlation between the regional geological structures and the disc thickness in the relaxation zone is analyzed based on the deep auxiliary tunnel of the Jinping hydropower project. A “structural arch” model, applicable for analyzing the mechanical behaviors of fractures and rock blocks in the relaxation zone, is proposed. The model is then verified by laboratory similarity tests and numerical simulations. Finally, it is revealed that the “discontinuous” stress field in the relaxation zone is the main cause of core discing in the relaxation zone around underground openings subjected to high in situ stresses.