Rainfall induced landslides occur on steep slopes owing to water infiltration and subsequent changes in pore pressure and shear strength of the soil. Water is one of the main factors causing slope damage, and it is easy to soften the surface material and to reduce the strength, then increase the pore water pressure and the effects on the slope stability. If the soil is in the unsaturated state, the failure is due to rainfall infiltration. The infiltration typically addressed by calculating the factor, safety or critical depth for an infinite slope to seepage (Collins and Znidarcic, 2004).
Some scholars have pointed out that the most serious environmental factors to the natural slope are persistent rainstorms, which can easily lead the erosion and collapse of the slope. At the same time, the infiltration of rainwater will promote the elevation of groundwater level causing the instability of the slope. The distinction between the erosion and the pore water pressure is illustrated that soil erosion takes place by the removal of the layers of the soil and the pore water pressure is depicted to be the permanent writing point which indicates the minimal point at which the soil is able to hold water (Fredlund and Rahardjo, 1993).
Adverse effects of groundwater on slopes include: water pressure acting on vertical fractures and resulting in horizontal thrusts, which lead the slope to be pushed downward, the pore water pressure (u) effect acting on shallow sliding surfaces that it reduces the effective stress and friction of surface, the ground water acting on the sheeting minerals especially the clay minerals will produce the lubrication action. Then it will cause sliding and it will change the physical and chemical properties of soil and rock, making rock and soil deterioration, reducing strength and finally influencing the stability of the slope (Zhao, He and Wang, 2003).
After the weathering of slope and rocks, the strength is greatly reduced. Thus, the slope lies at one-side that brings about the increasing of erosion and then re-exposed of internal rocks leading to further erosion and weathering.The collapsed ground acceleration caused by the earthquake will increase the sliding force of the slope and reduce the resistance. The horizontal shock wave released by the earthquake will easily destroy the slope which lacks of the lateral shear force (Lu and Godt, 2008).