Rapid Drawdown Analysis - What is an Analyst To Do?

In the authors' experience rapid drawdown analysis of embankment dams is one of the most misunderstood slope stability analysis cases. As a result, inappropriate analysis methods have often been used. This paper begins with a review of the fundamental soil behavior during rapid drawdown. This is followed by a discussion of analysis methods that have been used, both appropriate and inappropriate methods. Rapid drawdown of an embankment dam occurs when the impounded reservoir is lowered rapidly enough that pore water pressures in some or all of the embankment soils cannot reach equilibrium with the new, lower water level. Instability of the upstream slope can occur in these circumstances, and, in fact, documented rapid drawdown failures have occurred. The susceptibility of an embankment to rapid drawdown instability is dependent on the rate at which the reservoir elevation is lowered and the hydraulic conductivity (permeability) of the embankment soils. As for any other stability analysis case, the available strength of the embankment soil is a function of normal stresses and soil cohesion and friction, according to the familiar Mohr-Coulomb equation. The pore pressures during rapid drawdown conditions are a function of both the changing seepage pattern caused by the drawdown and the response of the soil to changes in normal and shear stresses caused by the drawdown. The pore water pressure changes resulting from normal and shear stress changes are not always properly recognized, leading to inappropriate analyses. Rapid drawdown can subject soils to undrained shear loading - loading that is sufficiently rapid that pore water cannot drain from the soil during the loading, which results in either increases or decreases in pore water pressure in response to the loading. Analysis methods that do not account for the pore water pressure responses to stress changes in soils that cannot drain are not appropriate for rapid drawdown analyses. Methods which have been used, but fall into this category, include a) the use of transient seepage analyses to develop pore water pressures for use in stability analysis, and b) the use of an assumed phreatic surface coincident with the upstream slope for calculating pore water pressures for the analysis. Appropriate methods must include appropriate consideration of undrained strength behavior for those soils that cannot drain, and several such methods are available and will be reviewed. Both inappropriate and appropriate methods will be illustrated with analysis results from real embankments. 14 pp., 9 figures, 9 references.