Geological And Geotechnical Investigations Of Axum Dam Site Tigray Northern Ethiopia

Geological and geotechnical study was conducted in concrete gravity dam which is planned to be constructed in the Maychew River 40 km south of Axum town for the purpose of water supply for the town. The objectives of this research were to map geology of the area to characterize geological defects within and around dam site to evaluate the water tightness of the dam site and to determine the bearing capacity of the dam foundation. The research involved review of different literatures lithological and structural mapping characterizing rock masses by using different rock mass classification methods interpretation of subsurface data geophysical core drilled data test pit data etc.. Results of the study indicate that the area is underlain by Quaternary sediments metasedimentary and metavolcanic rocks. The Quaternary sediments are characterized by low permeability low plasticity and are poorly graded nature. Metasedimentary rocks are found covering the right abutment of the dam whereas at reservoir area it is found intercalating with metavolcanic rocks. These rocks are moderately jointed and sheared with faulting and folding noticed due to these they have a relatively high permeability. Metavolcanic rocks which are found covering the left abutment are strong less permeable and fractured. Most of discontinuities such as fractures bedding and foliation in the study area are oriented E-W NNW-SSE and NNE-SSW. The VES tomography and drilled core result revealed that the potential problems seepageleakage could occur due to presence of faults joints karstified black limestone lithological variations groundwater depth and topography at right abutment. Differential settlements may also occur because empirically estimated moduli of deformation Ed of rock masses indicate that for right abutment much less than left abutment and different geological defects across the dam axis. To minimize these problems contact grouting and consolidation grouting are recommended as mitigation measurements.