Analysis of the failure of Gilgel Gibe II hydropower head race tunnel using numerical approach

The Gilgel Gibe II Hydro-power concrete lined headrace tunnel was constructed crossing a highly weathered basaltic geological formation in the southwestern part of Ethiopia. During the construction of the tunnel, failures have been encountered regularly causing the overall delay of the project. This tunnel also failed soon after the beginning of the normal operation and the event was seen as one of the most difficult tunnel failures. The purpose of this paper is to investigate the observed tunnel failure using numerical modeling. Parametric studies have been carried out to explore the effect of geotechnical and geometrical properties on the observed tunnel instability. The model results show that the tunnel experienced severe squeezing problems. The influence of rock material properties on the observed failure is noted. Finally, the implemented remedial measure, concrete lining, seems inadequate to improve the stability of the headrace tunnel.