Seepage and Slope Stability Analysis of Earthen Dam: A Case Study of Koga Dam, Ethiopia

Evaluations of an earth-fill dam throughout its service life must ensure the stability of it against seepage and slope failure. This study presents the seepage and slope stability of the Koga earth-fill dam. The analyses were carried out using a finite element based PLAXIS 2D software, and covers the whole dam body; including 20 m of foundation depth. The behavior of both the body and the foundation of the dam were described using the Mohr-Coulomb criterion. Assessments of safety factor and quantity of seepage through the main body of the dam and foundation were carried out at different critical loading conditions. In this study, seepage analysis was undertaken of flow rate, pore water distribution and location of phreatic line. Additional actual field data measurements and observatory investigation were also carried out. From the simulated results, the average flow rate of seepage for the entire length of the body of the dam at normal pool level was equal to 0.06085 m3/s, whereas the figure for that through the foundation of the dam was 0.01937 m3/s. Moreover, total seepage through the main body of the dam at the current reservoir level was 0.04982 m3/s, while the actually measured quantity of seepage accumulated at the downstream toe of the dam was 0.04644 m3/s. The simulated and measured seepage discharges are 93.2 % similar. Based on the result of this study, the resulting factor of safety values during end of construction, steady state condition and rapid drawdown condition were 1.6221, 1.6136 and 1.2199, respectively. Using recommended design standards: United States army corps of engineers (USACE), British dam society (BDS) and Canadian dam association (CDA), the slope stability analysis of the Koga earth dam at all critical loading conditions are safe.