Abstract: 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.

Key words: breccia, mechanical properties, geology condition, engineering prevention, relaxing inhibition