Design and Construction of Tunnels in Ethiopia: Lessons Learned from the Case of Gilgel Gibe III Hydropower Tunnels

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Addis Ababa University


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.



Construction of Tunnels, Design