Abstract: The discrete element method can be used to simulate the excavation of the fractured rock and conduct the fully-coupled analysis of hydraulics. So UDEC (universal distinct element code) can be used to simulate the changes of flow rate and the corresponding water pressure under the coupling conditions of the seepage and stress after the excavation of the fractured rock, and forecast the possible sudden blow disasters. The results show that the changes of the fracture aperture, water pressure and fracture permeability discharge of the fractured rock mass in the coupling conditions of the seepage and stress are interactive and interdependent. Because of the shortening of the fracture width, the hydraulic gradient and the permeability pressure of the fracture become large. It also promotes the expansion of cracks and fissure connectivity increases. The increase of the crack width will result in the increase of the infiltration capacity and seepage discharge and reduction of seepage pressure. Under certain conditions, changes in fracture aperture can lead to the formation of local water  access, the emission of high pressure head from the local place, and thus promotes the formation of the sudden blow disasters. The sudden blow easily occurs after the excavation of the diversion tunnel situated at the deep fractured rock where the ground stress and water pressure are very high in some hydropower station in the southwest of China. The simulation is done in this article, and some possible positions where the sudden blow occur are revealed.

Key words: fractured rock mass  ,   diversion tunnel  ,   the sudden blow  ,   UDEC