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

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

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