Effect of Rate of Drawdown and Impounding in Reservoir Water Level on the Stability of Rim Slope

With the ever-rising need and pursuit of clean energy, there has been a substantial increase in hydro-power projects across the world. The rim slopes forming the reservoir for the dam projects experience water level fluctuations. Cyclic fluctuations in reservoir water level can have a significant effect on the stability of rim slopes. The instability caused may further aggravate potential landslides. Reservoir-induced landslides in the Three Gorges Reservoir (TGR) area are prime evidence of the instabilities caused by water level fluctuations of more than 30 m annually. The rate of water level fluctuation varies drastically with the changing climate around the year. It is important to investigate the effect of the rate of water level fluctuations to understand the failure mechanism as changing fluctuation rates. In this study, a numerical simulation of a 45° physical model slope subjected to drawdown and impounding in the laboratory is presented. Three different cyclic fluctuation rates were selected. The numerical model was first validated by comparing the outcome with that from the physical model for steady-state and immediately after drawdown conditions. Numerical simulation indicates that the maximum excess pore-pressure develops for the slope subjected to a rapid cyclic fluctuation rate, whereas the maximum rate of dissipation of excess pore-pressure occurs in the case of a slope subjected to a slow cyclic fluctuation rate. The model slope experiencing rapid cyclic fluctuation remains in an undrained condition even 1 h after the process of drawdown has ended. It is interesting to note that comparatively shallow failure is expected for the slope subjected to a slow cyclic fluctuation rate than in the case of the slope subjected to a rapid cyclic fluctuation rate. Lastly, the change in factor of safety with test time for all three cases has been discussed.