Empirical Solution for Predicting Soil Rim Slope Deformation under Cyclic Reservoir Water Level Fluctuations

Cyclic water level fluctuations have been suspected to be the main cause of many reservoir-induced landslides. Rim slope failures are a constant hurdle in the smooth functioning of the hydro-power projects. Understanding of mechanism involved in reservoir-induced landslides, and the assessment of landslide displacement due to water fluctuation is very complex. There is a need for a quick assessment tool for landslide displacement to ensure safe operation of hydro-power plants. In the present article, effect of cyclic fluctuation in reservoir water level on the stability of rim slope has been studied through physical modeling. A small-scale physical slope model test (PSMT) apparatus was developed indigenously and results of tests conducted on 45 degrees modelled soil slope have been discussed. The modelled slope was made using soil samples collected from an actual rim slope situated between Tehri and Koteshwar Dam (India) and was subjected to 30 cycles of water level fluctuations. Total head values and deformations were recorded after every cycle drawdown. Based on the experimental results, a hypothesis on initiation and propagation of reservoir-induced landslides is suggested. A dimensionless term called Spread Index (Si) is defined as the ratio of Spread (horizontal displacement at toe) and Hydro-Fluctuation Belt of the cyclic fluctuation in the reservoir water level. A novel simple approach is proposed based on the spread index to predict displacement of soil rim slopes subjected to cyclic fluctuations as an extended application of the present study. The proposed solution will be helpful for rough estimation of displacements along rim slopes.