Computer Solutions for the Stability of Reservoir Slopes

Abstract

Prediction of the stability of slopes under various water conditions and for a wide range of loading and soil conditions can be made with the help of a digital computer. Reservoir slopes may reach a very critical state immediately after very rapid drawdown. At present, this condition is analyzed assuming an “instantaneous” drawdown event. This paper presents a method for obtaining the slope stability under a drawdown condition with a defined time-elevation history. The analyses are based on the simplified form of Bishop’s model with circular failure surface and vertical slices. Two options are available for determining the pore pressure. In the time-dependent drawdown analysis, a differential equation equates the volume of water that flows out of the slope in a given time with the product of the velocity of flow, area of flow, and time interval. The solution of this equation yields the position of the phreatic surface with time. This information helps determine the pressures along a trial circular sliding surface, and the variation of the factor of safety with time. The factor of safety decreases rapidly with the lowering of the reservoir level, followed by a slow and nonlinear increase to the steady state value.