Abstract: Stability of surrounding rock mass of a cavern in a fractured rock mass is an important engineering geological problem to be solved in an underground plant of a pumped storage power station. Taking a pumped storage power station as an example, in order to accurately predict the unstable block position before excavation, we statistically analyzed the fractures in the hole, and determined the law of fracture development with combination of the fault development in the plant area. On the basis, the rock mass was divided into five tectonic zones:A, B, C, D and E. By the Unwedge program, the position of the potential block was determined and its stability was evaluated. The results show that the fractures are controlled by multi-stage tectonic faults, and are directional in a certain range. The occurrence of the dominant fractures in region A, E are close to the tectonic fault occurrences, and the dominant fractures in region B, C, D are cross the tectonic faults obliquely; six potential unstable blocks are formed on each side of the upper and lower side walls of the plant, ten potential unstable blocks at the top of the arch, and one potential unstable block at the right end wall.

Key words: pumped storage power station, block theory, fracture development law, stability of surrounding rock, Unwedge program