Abstract: The flow motion law of the underground seepage of plane sliding landslide saturated under heavy-rain is studied; and then a new improved method for slope stability analysis of such landslide is presented, considering the coupling effect of hydrostatic pressure and hydrodynamic pressure. Using the principle of groundwater dynamics, on the basis of some assumptions to simplify the analysis process, the underground seepage flow equation under heavy-rain is derived. Based on the slice method theory, taking the non-uniformity of the flow activity at each point into account, the traditional calculation formula for slope stability coefficient is improved; and the critical condition of failure is also given. The proposed formulas are applied to Ermanshan landslide occurred in Hanyuan county of Sichuan province; and the obtained results prove the rationality of the method. The research indicates that the stability of plane sliding landslide under the heavy rain mainly depends on the internal friction angle of saturated rock and soil, the permeability coefficient, the scale of sliding mass (thickness and length) and the relative water-head between the slope crest and the toe. Such research results have a certain practical significance that can be used to guide the emergency rescues and disaster relief work.

Key words: groundwater seepage, slope stability, critical condition of failure, Ermanshan landslide