Rock Slope Stability and Water tightness Assessment of Arjo Didesa Dam site, Western Ethiopia

Abstract Arjo Didesa dam is an earth and rock fill dam which is under construction on the Didesa River in western Ethiopia. However, the dam encountered engineering geological problems that affected the water tightness and slope stability of the dam abutments and reservoir area. To solve these problems, different methods such as discontinuity survey, packer test, sampling and laboratory tests were applied to evaluate the engineering geological conditions of materials along the dam abutments and reservoir area on slope stability and water tightness. Slope stability analyses were performed by kinematic, limit equilibrium and finite element methods using laboratory and in-situ tests. Kinematic analyses revealed that both the left abutment slope 1(LAS1) and the right abutment slope 1(RAS1) sections were unstable for planar mode of failures. Further stability analyses using deterministic methods indicated that both sections were unstable only during saturated conditions. Similarly, stability analysis using limit equilibrium and finite element method revealed that both the left abutment slope 2(LAS2) and the right abutment slope 2(RAS2) sections were unstable under saturated conditions. This study also analyzed the permeability of soils and rock masses and the result showed that the permeability values range from 2.693*10− 7cm/s − 6.687*10− 5cm/s and 3.19*10− 5cm/s − 1.3*10+ 1cm/s for soils and rock masses respectively. Integration of surface hydraulic conductivity and subsurface packer permeability tests showed the presence of potential leakage through the dam abutments. Remedial measures such as rock bolts, anchors and shotcrete for slope stabilization and grouting and clay blanketing are recommended to control this leakage.