Engineering Geological Appraisal of Mid-Level Outlet Tunnel for Zarima Dam; Tigray Region, Northern Ethiopia
No Thumbnail Available
Date
2016-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa Universty
Abstract
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
Description
Keywords
Detailed geological mapping, Geo-structural survey, Rock mass classifications and laboratory rock property tests