Mechanism study of abrupt deformation of underground powerhouse induced by excavation unloading under high in-situ stress

The mechanism of abrupt large deformation in the sidewall of underground powerhouse is analyzed. The surrounding rock mass dynamic loosening is concerned as a main cause of abrupt deformation in underground powerhouse. Based on the monitored data at Pubugou hydropower station, numerical analysis is conducted with finite element method; and horizontal displacements induced by excavation load quasi-static and dynamic unloading are calculated respectively. The result itself and comparison to monitored data show that sliding and opening on structural surfaces induced by excavation load dynamic unloading are the major causes of abrupt large deformation in underground powerhouse, but which is not so obvious under quasi-static unloading. In actual projects, the transient effect of in-situ stress unloading should be considered; and the disturbance in the surrounding rock mass during each operation should be minimized to avoid potential abrupt large deformation, or even instability.