Effects of discontinuities on stress redistribution and rock failure: A case of underground caverns

Excavation of the left bank underground powerhouse caverns of the Baihetan hydropower station has been undertaken in a deep rock mass with discontinuities under high in situ stresses. Rock failures such as spalling were encountered during the excavation of the large caverns. A discrete element model containing discontinuities was established and used to investigate the failure mechanism of the rock masses using 3DEC to reproduce the evolution of the displacement and stress fields. Based on the modeling, a risk boundary, significantly affected by discontinuities, is proposed and applied to evaluate roof stability. Owing to the discontinuities, the excavation of the top arch induced spalling at the crown arch and upstream sidewall and opening of the rock mass at the downstream sidewall and floor. Excavation of the subsequent sections increased stress at the crown arch, revealing the arching effect; simultaneously, the stress release and deformation of sidewalls were facilitated by the discontinuities. The simulation results were verified by on-site monitored data. Based on the study it was concluded that discontinuities influence stress redistribution, inducing the arching and sidewall effect. The result of this study is expected to aid in the design of excavation and support systems for underground engineering projects.