2024 – Enhanced Stability Analyses and Pore Water Pressure Prediction during Rapid Reservoir Drawdown

Reza Mahinroosta, Damian Nott

Transient seepage analysis has been used in dam industry for a long time to predict pore water pressure in slopes in response to changing reservoir levels. However, the method has some issues when predicting pore pressure under a high rate of reservoir drawdown. In this paper, transient seepage analysis was used to
predict pore water pressure in the fissured foundation layer of a puddle core embankment during rapid drawdown, and the stability of the upstream slope due to drawdown was calculated. The transient seepage analysis overestimated pore water pressure in the low permeability, fissured clay foundation causing negative effective stress in the foundation at the upstream toe of the dam. This resulted in extremely low factors of safety for emergency drawdown. This occurred due to the failure to account for the volumetric change of the soil in response to the unloading.

To resolve this issue, two approaches were employed: adjusting permeabilities in the transient seepage analysis and using coupled hydro-mechanical analysis. Artificially increasing the permeability of affected zones resolved the high pore water pressures, but the parameter selection was arbitrary and therefore the findings were uncertain. The coupled analysis, which models both transient seepage and volumetric change simultaneously, was found to provide a more comprehensive method of assessment. The modified approach to the analysis significantly impacted the outcomes. This paper presents the analytical approaches suggested to overcome problems with transient seepage analysis of very rapid drawdown scenarios.