Evaluation of the upstream slope stability of earth dams based on drawdown conditions - Khassa Chai Dam: a case study

Sudden drawdowns may cause instability in slopes without adequate levels of protection against failure. In this paper, a numerical approach utilising the finite element method (FEM) was employed to examine the seepage and slope stability of a typical earth fill dam. Finite element software (GEOSTUDIO 2012) was used to carry out both steady-state and transient seepage analyses on Khassa Chai Dam in Iraq to study the seepage and upstream slope stability after various upstream drawdown scenarios. To include water levels during evacuation, variable linear water heads with time were identified as boundary states in the transient seepage analysis. The quantity and direction of water flux, existing gradient, and safety factor were calculated for all scenarios. The results revealed that the stability of the slope during drawdown is highly impacted by how fast its pore water pressure dissipates. The results also showed that a minimum F.S. was achieved within 10 hr in the case of a 1-day water drawdown, with the F.S. reduced by 60.66%; this becomes critical when the water level in the basin drops by about 41.67% of its original height.