Engineering rock mass characterization – An integrated approach through Rock Mass Classification and Seismic Refraction; a case of TAMS Hydroelectric power Dam Project in Gambella, Ethiopia

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Addis Ababa Universty


In the present study, an integrated geotechnical and geophysical approach was followed to characterize the dam foundation of TAMS Hydropower dam, situated in South-western Ethiopia, in Gambella National Regional State, Gambella Woreda, 721 km south-west of Addis Ababa. The dam is being constructed for the purpose of hydropower generation and will have a height of 265 m. A systematic methodology was followed for the present study. This includes review of secondary reports on investigations and design and collection of primary data from surface on geological and geotechnical parameters at the dam site. The investigations carried out in this regard includes; surface mapping, Rock mass rating (RMR) data collection, insitu testing on rock properties, and laboratory testing on representative samples. Besides, secondary raw data on drill hole logs, water pressure test and Seismic refraction survey data was also utilized to analyze and characterize the rock mass at the dam foundation. Surface rock mass classification at exposed rocks have been conducted to characterize the rock mass at the dam foundation, accordingly, 50% of the foundation rock mass falls in RMR class 66 – 70, 40% falls in 61 – 65 and the rest 10% falls in class 70 – 80. The data analysis from the geological logs and the seismic refraction survey suggests that the bedrock in the dam foundation is fresh granite, granodiorite, metagranite and schist of different types which can be characterized as fresh, massive, strong, slightly fractured to unfractured rocks of high quality, represented by very high seismic velocities (>4500m/sec) from seismic refraction survey. With the exception of some very soft rocks and heavily jointed media, the majority of the rock mass can be classified as an excellent foundation material. Further, the permeability results from water pressure tests conducted in boreholes at both the abutments indicate very low to medium permeability. The permeability value on right abutment, in BH-3 borehole, demonstrated high to very high permeability in the top part of the dam foundation. The permeability in the intermediate reach on the right abutments is again high as the Lugeon value is more than 65. Therefore, the most effective means of checking seepage through the abutments and the river section would be a provision of a grout curtain. Finally, on the basis of findings of the present research, recommendations are forwarded to be adopted as solutions against geotechnical problems identified at the proposed dam site. Key Words: Rock Mass Rating, Seismic refraction, Lugeon, Permeability, Rock Quality Designation.



Rock Mass Rating, Seismic refraction, Lugeon, Permeability, Rock Quality Designation