Engineering Geological Appraisal of Mid-Level Outlet Tunnel for Zarima Dam; Tigray Region, Northern Ethiopia

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Addis Ababa Universty


The Tekeze River and its tributaries have been selected by Ethiopian Sugar Corporation as one of the key irrigation development area for sugar cane crops cultivation. As such, Zarima Dam is constructing across Zarima River in lower Tekaize River Basin to irrigate about 40,000 hacters land for Woilkaite Sugar Development Project in western Tigray Region of northern Ethiopia. The irrigation head works comprise; 153 meters high central asphalt core rockfill dam, diversion conduit, central clay core rockfill coffer dam, spillway, irrigation intake tunnel and dam midlevel outlet tunnel of 600 meters in length. The mid-level outlet tunnel is designed to be excavated along left side slope of Zarima dam abutment, and the principal aim of providing this tunnel is to assure the possibility to accelerate the reservoir impounding program with control of the overtopping during dam construction works. The tunnel has an overburden rock of thickness reaching from 12 m to 105 m. The geology through which the tunnel is excavated is composed of Meta volcanic rock of low grade metamorphic unit, affected with variable degree of rock masses weathering. Engineering geological studies along the tunnel alignment is the main scope of the present research work. This includes geological mapping at the scale of 1:5000, geostructural surveying, rock mass classifications, and determination of rock properties. From the results of the engineering geological studies, it is found that the full length of the tunnel will be excavated through Meta-volcanic rocks of low grade metamorphic unit. Based on the engineering properties of the rocks which are directly related to the degree of rock mass weathering, nature of discontinuities and intact rock strength, three geotechnical units (Gtu.1, Gtu.2 and Gtu.3) have been identified along the tunnel alignment. The results of the rock mass classifications conducted in the study area suggests five types of underground excavation rock supports such as; support type A, B, C, D, and E. These rock support design is slightly more on the safe side if compared with the one suggested with the Q system for selecting temporary rock supports. Key Words: Detailed geological mapping, geo-structural survey, rock mass classifications and laboratory rock property tests



Detailed geological mapping, Geo-structural survey, Rock mass classifications and laboratory rock property tests