The Gibe III Hydropower Project, being constructed on the Omo River, is a key project for Ethiopia, engaged in a strong development of its energy production means. It includes notably construction of a 246 m-high dam of roller-compacted concrete (RCC dam), to be the highest in Africa. The paper presents the unusual rock mechanics challenges that have been encountered during excavations of underground works and dam foundation, and the way they were dealt with. Geology of the country has been largely modelled by volcanism in relation with the African Rift System. The damsite is located within a gorge carved by the river through a thick trachyte body, having overlain former volcano-sedimentary formations. Unexpected rock mass conditions, in the form of cohesionless silty materials were found deeply seated over about 40 m length in the first reconnaissance borehole. Tunnelling of the diversion tunnels brought the additional surprises. During the excavation, a sudden, but long-lasting, flow of dry to slightly damp loose rock pieces, some of them rounded gravel, poured into the tunnel from one location in the top of the excavation front. The area was crossed using heavy support once the flow stopped. The trachyte rock itself appeared to present singularities. Although apparently massive, with joints often cemented by iron oxides, some cores from boreholes drilled in the dam foundation were found disintegrating with time into sandy-silty material. Comprehensive investigations showed that the trachyte had been injected by hydrothermal fluids, resulting in sometime extensive alteration of the rock matrix along discontinuities. Excavations for the dam foundation confirmed these conclusions, with dedicated investigations and special treatment of weak zones, whose frequency unexpectedly increased with depth. The implementation methods for the grouting curtain had to be modified, such as ensuring as much as possible removal of the weak material prior to grouting. The peculiarity of geological and hydro geological conditions can finally explain the observed phenomena, but required adaptation of design and construction methods while construction was progressing, resulting in a true "design as you go" process.

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