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