Electrical resistivity imaging and engineering geological investigation of Gereb Segen dam, northern Ethiopia

Abstract

The paper presents results of electrical resistivity imaging and engineering geological investigations conducted at the Gereb Segen dam site and reservoir to evaluate water tightness and stability. A programme of 2D resistivity imaging, discontinuity surveying, core drilling and Lugeon testing was conducted to assess the engineering geological performance of the dam and reservoir area. The geology of the dam site comprises alluvial deposits, dolerite, micritic limestone and marl-limestone-shale intercalation units. The investigations revealed dissolution cavities up to meters in size showing a preferred alignment along the valley; the separate lines of investigation correlated well to provide confidence in the findings. A karstified zone was identified at the base of the marly limestone unit at the dam site. A layer of gypsum found at shallow depth (9.5 m) along the dam axis may pose a dissolution hazard to water tightness and ground quality. Lugeon test results define very-low- to low-permeability units (shale with gypsum lenses and shale-mudstone) and high-permeability units (marly limestone and dolerite) and very-high-permeability zones associated with the karstic features. The presence of discontinuities and/or weak zones at the abutments and cavities at the dam foundation will affect the stability of the dam and facilitate leakage. Multiple line grout curtains for the karst zones and grouting for the jointed limestone at the dam axis and both abutments are recommended to reduce the possible leakage that will occur at the dam.