Koysha Dam, located in the Southern Nations, Nationalities and Peoples’ Region of Ethiopia, is the fourth plant of the Gibe-Omo cascade comprising Gilgel Gibe (IP=200 MW), Gibe II (IP=420 MW) and Gibe III (1’870 MW) all in operation. The plant includes a 200 m high RCC gravity dam, a gated spillway on the left bank and an open-air powerhouse housing 6 Francis turbines fed by 2 steel penstocks crossing the dam body. The right abutment of the dam is partly founded on conglomerate, composed by a variable proportion of cobble and gravel sized basalt and rhyolite-trachyte sub-rounded elements surrounded by a weak matrix of fine sand and silt particles. The geotechnical characterization of such a complex material resulted in a very challenging task due to the substantial impracticality of collecting high quality, undisturbed and representative samples. Therefore, the mechanical behaviour of the conglomerate has been assessed by means of in-situ large scale shear tests carried out into a 70 m long inspection tunnel. The test results show that in the range of selected confinement pressures (0.3 to 1.0 MPa) the conglomerate exhibits a softening behaviour. Peak friction angle and cohesion are coherent with data and empirical models presented in the literature which indicate a strong correlation between volumetric block proportion and mechanical properties of the Block-In-Matrix (BIM) rock.
Koysha dam is located on the Omo river, 130 km downstream of Gibe III dam in the Southern Nations, Nationalities and Peoples’ Region of Ethiopia. The project includes a 200 m high RCC gravity dam, a gated spillway on the left bank and an open-air powerhouse housing 6 Francis turbines fed by 2 steel penstocks crossing the dam body. The owner of the project is the Ethiopian Electric Power Company (EEPC) whereas the main contractor is WeBuild SpA (Italy). Koysha is the 4th plant on the Omo river, downstream of the operating Gilgel Gibe, Gibe II and Gibe III. With its 1620 MW of installed power and 6260 GWh of annual energy production, Koysha is one of the most important projects in the Ethiopian Government’s commitment to meet the country’s present and future power requirements.
The project presents many geotechnical challenges, one of the most important being the presence of heterogeneous Conglomerate outcropping on the right dam abutment. The Conglomerate is a melange formation composed by unweathered to moderately weathered gravel and pebble clasts into a silty and sandy matrix. From a geotechnical point of view, the Conglomerate represents a typical block-in-matrix rock (bimrock) [1], [2] and [3]. These geological mixtures exhibit a complex behaviour with considerable spatial, lithological and mechanical variability and common fundamental engineering problems related to their geotechnical characterization [4] and [5].
