ANALYSIS OF SEEPAGE AND ITS IMPACT ON EARTH DAM: CASE STUDY GEREBSEGEN EARTH DAM, TIGRAY, ETHIOPIA

Abstract

Gerebsegen dam, zoned earth type of dam with height of 46m was constructed for the purpose of irrigation and Mekelle city water supply. It is located in the Southwestern part of Mekelle city, capital city of Tigray regional state, Northern part of Ethiopia. The dominant geological formations of the site are dolerite shale and marl-limestone intercalations, characterized by cavernous, joints and karastic features which can be responsible for the excessive seepage and presents serious problems in the dam foundation, reservoir and abutments that may be so extensive as to make corrective measures impracticable. The main intention of this thesis work was to evaluate the performance of the dam related to seepage and slope stability, for which Geo Studio-2007 software (SEEP/W and SLOPE/W) was used throughout the analysis. Prior to the analysis using the model, seepage dominantly flowing through the foundation and abutments was measured using current and float method. The measured value was 0.2053 m3 /sec and the annual amount of water loss due to seepage computed was 6,474,340.80 m3 /year. By selecting three different sections of the dam during the model analysis, the average seepage flux computed at the normal pool level (NPL) and actual reservoir level (ARL) for every meter length of reservoir were 3.304E-05 m3 /sec/m and 2.252E-05 m3 /sec/m respectively. But the value computed during designing of the project where only one section of central foundation was considered, was 1.031E- 07 m3 /sec/m, which ignores the two left and right foundations which are found to be pervious and critical to the water tightness of the dam. The simulation results of SLOPE/W revealed that the downstream under steady state seepage, upstream and downstream under steady state seepage with earthquake, rapid draw down and rapid draw down with earthquake slope factor of safety are 1.805, 1.423, 1.366, 1.883 and 1.203 respectively, indicating the dam is safe against slope failure and the amount of shell materials used for the construction of the dam were excess. But the uncontrolled excessive seepage can have an adverse impact on the constructed earth dam; for which the stability of the dam against the excessive seepage needs to be assessed regularly