Stability Assessment of Homogeneous Embankment Dam Under Variable Drawdown Rates Using Unsaturated Soil Parameters

Abstract

Embankment dams are critical for sustainable water resource management, providing irrigation, hydropower, flood control, and water supply. One of the critical threats to an embankment dam stability is rapid drawdown, when reservoir levels fall faster than pore pressure dissipation in the upstream slope. This condition reduces shear strength and can trigger failure. One of the commonly used methods for rapid drawdown assessment is the three-stage approach developed in the early 1990s, which provides a practical framework by combining drained and undrained soil strength. However, these methods do not explicitly consider the influence of drawdown rate, which can strongly affect stability. This study evaluates the effect of drawdown rate on the stability of an embankment dam using unsaturated soil mechanics framework and compares the results with those from the three-stage method. Numerical analyses were carried out using coupled seepage and slope stability modelling. The soil-water characteristic curve of the clay core was estimated from index properties. Three drawdown rate variations were examined: half a meter per day, one meter per day, and two meters per day. The result showed that the three-stage method produced higher factors of safety than the unsaturated framework. Unsaturated framework also showed lower factor of safety for faster drawdown rates. This study highlights that the three-stage method may overestimate the stability of dam embankment during rapid drawdown. Incorporating unsaturated soil mechanics provides a more realistic assessment and offers insights for improving dam safety, particularly under conditions where faster drawdown may occur.