Abstract：Seepage is an important factor in the failure of rock slope. The extended discontinuous deformation analysis (DDA) method is presented by considering the seepage in fracture network and the coupling of seepage and deformation of rock. The local safety factor along the given path can be calculated. By the extended DDA method，the failure process of a tunnel in Japan is simulated. It is shown that the direct reason of the failure is raining and icing，which raises the level of underground water and causes the propagation of fracture. The result agrees with the investigating conclusion by Accident Investigation Committee of Japan.
