Evaluation Of Slope Stability And Seepage Analysis Using Pore Water Pressure, A Case Study Of Malka Wakana Dam

Abstract

When a dam fails, large quantities of water are suddenly released, creating major flood waves capable of causing disastrous damage to down-stream. It may cause loss of life, erosional damage, spoiling of agricultural land, and adverse ecological and environmental impact. This thesis deals with slope stability evaluations and seepage analysis for Malka Wakana embankment dam carried out by commonly used limit equilibrium and finite element methods. The study utilize a limit equilibrium software SLOPE/W due to its simplicity and better prediction of factor of safety and finite element based software SEEP/W was used to analyze the saturated or unsaturated soil region. A limit equilibrium analysis was carried out using the SLOPE/W software for the stability of the dam slope. The geometry was created in the SEEP/W program and transferred to SLOPE/W by setting the model for steady state condition stability analysis. The dam is analyzed under normal water level taking the water elevation at 2516 m. This is used as upstream boundary condition and zero pressure boundary condition at the downstream horizontal toe drain. The soil properties parameters such as cohesion value, internal friction angle, unit weight, hydraulic conductivity and water content, assigned to prepared regions and for seepage analysis the saturated or unsaturated model was used while for slope stability analysis the Mohr-coulomb model was used. The steady state seepage analysis used as parent analysis for slope stability analysis that means the software use the pore water pressure comes from the initial steady state analysis as piezo-metric line in the body of the dam. Also for Transient seepage analysis the steady state analysis used as initial pore water pressure condition. For sudden drawdown slope stability analysis the transient seepage analysis used as initial pore water pressure condition that means the software reads the pore water pressure condition from previous analysis. Then the analysis type selected and slip surface drawn for slope stability analysis by using entry and exit and it follow a right to left path for upstream slope and for downstream slope it follow a left to right path. In SLOP/W different methods were available for slope stability analysis, among these methods Morgenstern-price, Bishop, Spencer and Janbu method are some of them. Among these the Morgenstern-Price analysis and half-sine function for interslice forces was selected because this method is satisfy both the force and moment equilibrium condition and also gives the better results for factor of safety as compare to Spencer, Bishop and Janbu analysis methods. The overall minimum stability factor of safety for the steady state condition was equal to 1.985, which means the slope is stable under this condition. Also for transient slope stability analysis the minimum factor of safety obtained were 1.613 for rapid draw down and 1.958 for slow drawdown. These means the slopes were stable for rapped and slow draw down condition. In this study, the seepage through the dam as per the SEEP/W software model that includes foundation seepage is 0.022m3 /day per1m of length and the exit gradient at the downstream toe is less than 1.0. The slope is potentially stable throughout the steady state and transient analysis and the exit gradient is always less than one which means the dam is overall stable under all conditions.