Head Race Tunnel Evaluation Through Engineering Geological and Remote Sensing Approach-a Case of Wabe Hydropower Project, Central Ethiopia
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Date
2020-08-08
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Addis Ababa University
Abstract
The present study was carried out along head race tunnel (HRT) alignment of Wabe hydropower project which is situated in Welkite, Gurage Zone, Southern Nation Nationality and Peoples regional state in Ethiopia. The project is accessible via Addis-Welkite-Jima asphalt road and is about 161 km west of Addis Ababa. The project is proposed to be constructed across Omo River with installed capacity of 450 MW. It envisages a 4m diameter modified-horse shoe shaped, 15821mlong horizontal head race tunnel (HRT), a pressurized penstock with 3.2m diameter, and 1394 m long horizontal and 300m vertical shaft tunnel. In order to evaluate the suitability of HRT alignment, tunnel inlet and penstock shaft integrated approach through engineering geological, geophysical and remote sensing methods has been utilized in the present study. The main purpose of this study is to evaluate and characterize the rock mass along the proposed tunnel alignment and to recommend supports for the proposed head race tunnel including slope stability condition at the tunnel inlet portal and, the slope around the surge and penstock shaft. These objectives were met by following systematic approach by utilizing secondary and primary data. The primary data was generated through investigations carried out in the present study area which include, surface mapping, terrain evaluation through remote sensing of satellite images, rock mass classification, in situ testing on rock properties, and laboratory testing on representative samples. Moreover, secondary data from previous investigation was also utilized to analyze and characterize the rock mass along HRT. This includes geotechnical and geophysical investigation reports. Litologically, HRT will be excavated through fresh to moderately weathered aphanitic and porphyritic basalts) (BA-3) and moderately weathered to completely weathered aphanitic, porphyritic and vesicular basalt (BA-4). In order to characterize and classify the rock mass along HRT alignment, rock mass classification systems i.e. rock mass rating (RMR), tunnel quality index (Q system) and geological strength index (GSI) were used. The results of rock mass classification through Q system indicates that the rock mass quality in general varies from poor to very good whereas classification through RMR system indicates that the rock mass quality in general varies in the range of Fair to good. The GSI value for rock mass ranges from 67 to 81 which suggests rock mass ranges from good blocky un disturbed to very good massive. Further, 2D resistivity imaging was carried out along the HRT alignment with the objective to characterize geological features and to assess general ground condition prevailing along the headrace tunnel alignment. Based on 2D resistivity imaging analysis and remote sensing interpretations seven normal faults and six fracture zones were identified. Accordingly, these faults were observed to be striking northwest and southwest, dipping from 65 to 80°. These faults were characterized to have vertical displacement (throw) ranging from 10 and 20 m. Furthermore, remote sensing analysis was performed in order to aid the study by providing valuable information for evaluating geological and topographical consideration of the study area. Finally, the study forwards recommendation for tunnel support and Suitability based on the findings.
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Keywords
Headrace Tunnel, Rock Mass Classification, Rock Mass Characterization, Tunnel Support