The breccia

a typical unfavorable geologic body in Wudongde hydropower station with low strength

poses a potential threat to the stability of underground cavern in left bank. Firstly

physico-mechanical properties of breccia

as well as the strength

cementation with limestone

dolomite and marble

and fusing contact characteristics are obtained by in-situ investigation and borehole television

and experiments in laboratory. Following that a 3D numerical simulations

which consider the left hill and underground cavern and are suggested mechanical parameters

are performed to simulate the excavating of main powerhouse for revealing the mechanical responses of breccia in roof during excavation. The simulated result provides useful information for detail design of rock supporting by showing the calculated failure approach index. The responding supporting design including the prestressed rock bolt

shotcrete and prestressed anchor cable. In-situ practice involving rock excavation and reinforcement indicates that the breccia in the roof remains stable

which is also confirmed by in-situ monitoring deformation with convergence pattern. This analysis indicates that the breccia can be adopted as load bearing rock in view of the acceptable self-stability and compressive strength related to the diagenesis process. Detail excavating and supporting schemes which utilizes the relaxing-inhibition supporting idea are applicable and practical.