“Embankment dam seepage and slope stability analysis: The case of Arjo-dedessa dam, Ethiopia

Abstract

Dams have one of the most important roles in utilizing water resources. An embankment dams are more common than any other type of dams. Seepage and slope stability failures cause completely failure of embankment dams. Dam failures not only risk public safety, they also can cost our economy millions of dollars in damages. The prime objective of this research was to assess Arjo-Dedessa embankment dam, highlighting on seepage and slope stability analysis. The study area is found in Oromia regional state between East Wollega and Bunno Bedele Zone. The dam is located on Dedessa River, tributary of Blue Nile River. Analysis of seepage quantity was done by Darcy’ s phreatic line, and SEEP/W software model for both homogeneous and zoned dam. The expected quantity of seepage estimated with the SEEP/W software model analysis that includes foundation seepage is 6.6*10-6 m 3 /sec which was compared with the quantity of seepage estimated at the designed document that is 4.16*10-5 m 3 /sec. Therefore, the design document has no problem of quantifying the expected quantity of seepage. But seepage is visible at the downstream berms of dam and downstream face of the existing Upstream Coffer Dam. The document stated that, the shell material used for the design was larger than the required size of shell material which may create Wet spots or seepage problem in the embankment. Even though, the dam has a problem data on construction history. Slope stability analysis is to contribute to the safe and economic design of upstream and downstream slope of the dam. Factor of safety was calculated under the standard loading conditions for limit equilibrium methods using entry and exit trial slip surface. The factory of safety obtained by Morgenstern-Price method for end construction is (FOS=1.99), for steady state (FOS=2.03) and rapid drawdown (FOS=2.91).The result shows that dam was safe when compared with international standards and design document. However, at the design document shell material hadn’t been designed properly. From field observation, there was also the oversized stone of shell materials and wrong placement of stones in the downstream of the dam which may leads to Face (slope) failure on the downstream slope of the dam. Finally the paper concluded that the possible remedial measures for seepage controls are impervious blanket, grouting, and for downstream slope failure by removing the weak zone and fill with similar graded material, regular maintenance and cover the downstream slope with grass.