Impact of Drawdown Speed on Slope Stability of Tendaho Dam Under Static and Pseudo Static Earthquake Loading Condition

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Date

2018-09

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Addis Ababa University

Abstract

Dams are hydraulic structures used to store, control, divert water impounding it behind the upstream side of dam in a reservoir for different purposes, like hydropower generation, water supply, irrigation, navigation and transportation, etc. Although dams have many advantages, the risk that may happen due to the failure still exists. One of the most dangerous conditions causing failure of earthen dams’ upstream slope is speed of drawdown condition when the countervailing upstream water pressure has disappeared with fastest speed; it causes a danger to the upstream slope. In this work Tendaho earthen dam, low permeability construction material, which is located on the highly seismically active zone, Eastern Ethiopia Rift Valley was studied. The overall objective of the study was assessing the impact of drawdown speed on Slope Stability of Tendaho dam under static and pseudo static earthquake loading Condition by using different methods. Traditional limit equilibrium methods and a finite element software, GeoStudio 2007 and 2012 computer program were utilized in the slope stability analysis. Seep/w, Slope/w and Sigma/w GeoStudio sub program were used to analysis and develop pore pressure curve and factor of safety curve for purpose of reservoir water level operation monitoring. The developed factor of safety curve shows that safety of upstream slope of the dam decrease during drawdown condition and seismic load effect. The upstream slope of Tendaho dam is stable during different drawdown speed and reservoir water level fall from FRL to MDDL. But, the worst case scenario occurs when reservoir water falls below 387m from FRL (408m) within recommended values of evacuation periods. In addition, during earthquake force was applied to the model the critical drawdown level and evacuation period is 394m and 40 days, respectively. It is also found that drawdown situation of Tendaho dam under reservoir operation have not affect on upstream slope of the dam and emergency drawdown is not required till now. However, assessing the dam slope stability under different drawdown speed and earthquake loading condition is important to avoid the upstream slope sliding during drawdown condition in the future.

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Keywords

Earthen Dam, Finite Element Method, Limit Equilibrium Method, Factor of Safety, Earthquake Force, Slope Stability, Drawdown Speed

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