Limitations of Transient Seepage Analyses for Calculating Pore Pressures during External Water Level Changes

Abstract

External water level variations, such as rapid drawdown or flood loading, change the internal pore pressures in an embankment as a result of changes in the boundary conditions. Three different effects result: (1) changes in water pressure on the slope change the seepage boundary conditions, (2) changes in the total stress applied by the weight of the water on the slope change the confining pressure on the soil, and (3) changes in the stabilizing load from the reservoir cause changes in shear stress. Uncoupled transient seepage analyses, which consider only the first effect, are increasingly being advocated as an appropriate means to calculate pore pressures for effective stress stability analyses following water level changes. This paper discusses the limitations of uncoupled transient seepage analyses for calculating pore pressures during drawdown, explains the requirements for more appropriate analyses, and gives examples that show the errors incumbent on the use of uncoupled analyses.