Abstract:It has been proved that the ground water plays a fateful role in the safety factor of slopes. However,in the stabilization design of the TGRZ slopes,the methods adopted for determining the ground water table and hydrodynamic forces during reservoir drawdown are mostly empirical and varied from one project to another. To improve the analysis methods of landslide stability under drawdown conditions,based on the Boussinesq’s differential equation of unsteady-seepage,and Laplace’s transform,a simplified formula is obtained of phreatic surface in landslide mass under drawdown conditions. The simplified formula takes into account the factors of hydraulic conductivity,velocity of drawdown,aquifer thickness and height of drawdown. According to the free surface in landslide mass,the boundary water pressure is defined utilizing the flow net property with orthogonal flow line and equipotent line. It is proved that the seepage force is in equilibrium with the water weight and the water pressure around the slice. The formula of seepage forces is established in slice methods. Finally the effect of above factors on landslide stability is discussed with examples. The result shows that there is a minimum safety factor when the reservoir level is at about 1/3~1/4 of the slope height.