Design and Construction of Tunnels in Ethiopia: Lessons Learned from the Case of Gilgel Gibe III Hydropower Tunnels
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Date
2015-04-06
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Addis Ababa University
Abstract
Tunnels are long linear undertakings with few opportunities to perform the work at more than
one location. Tunnels are also a series of repetitive operations each of which usually must be
finished before the next can be started. Tunnel engineering makes possible many vital
underwater and underground facilities. Unique design and construction techniques are involved
because of the necessity of protecting the constructors and users of these facilities from alien
environments.
In Ethiopia, tunnelling has been recently practiced only for one of the utility tunnels, i.e.,
hydropower tunnels. The purpose of this research is to evaluate the design and construction
approaches of the GG III hydropower tunnels, and to generate lessons which could be applied
for proper design and construction for future tunnels in Ethiopia. To achieve the objectives,
efforts were made to evaluate the design and construction approaches, including literature
review, geological, hydrogeological and geotechnical investigation reports, design reports,
method statements, various pictures and drawings, challenges and remedial work reports, site
visit and interviews.
In this research the author has found design and construction problems related to the applied
rock mass classification system. The problems in the design include: using inappropriate GSI
values which lead to uneconomical primary support design and inconsideration of stand-up time
(one of the design parameters). The problems in the construction include: using incorrect
construction procedures of NATM, installation of improper ground excavation supports, and no
installation of instrumentations and drainage holes (in the diversion tunnels). Due to these
problems, rock and shotcrete wedges failure occurred at various locations of power and
diversion tunnels, and a tunnel section collapsed in the central diversion tunnel. A worker has
been lost due to the rock wedges failure in the manifold of the right power tunnel. Due to the
collapsed tunnel section, the unexpected quantity and various types of rock supports, ground
improvement materials, various construction techniques, a construction of additional 63.25m
access tunnel (adit), etc. are used in the remedial works. The remedial works took almost nine
months, from May 2007 to February 2008. As a result of this collapsed section, the construction
cost and the construction time of the project obviously increased.
Due to lack of proper design of the primary supports in the project, the author has performed the
proper design of ground excavation primary supports using up-to-date Q-support design method.
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Keywords
Construction of Tunnels, Design