Design And Construction Of Tunnels In Ethiopia Lessons Learned From The Case Of Gilgel Gibe Iii Hydropower Tunnels

Tunnels are long linear undertakings with few opportunities to perform the work at more thanrnone location. Tunnels are also a series of repetitive operations each of which usually must bernfinished before the next can be started. Tunnel engineering makes possible many vitalrnunderwater and underground facilities. Unique design and construction techniques are involvedrnbecause of the necessity of protecting the constructors and users of these facilities from alienrnenvironments.rnIn Ethiopia, tunnelling has been recently practiced only for one of the utility tunnels, i.e.,rnhydropower tunnels. The purpose of this research is to evaluate the design and constructionrnapproaches of the GG III hydropower tunnels, and to generate lessons which could be appliedrnfor proper design and construction for future tunnels in Ethiopia. To achieve the objectives,rnefforts were made to evaluate the design and construction approaches, including literaturernreview, geological, hydrogeological and geotechnical investigation reports, design reports,rnmethod statements, various pictures and drawings, challenges and remedial work reports, siternvisit and interviews.rnIn this research the author has found design and construction problems related to the appliedrnrock mass classification system. The problems in the design include: using inappropriate GSIrnvalues which lead to uneconomical primary support design and inconsideration of stand-up timern(one of the design parameters). The problems in the construction include: using incorrectrnconstruction procedures of NATM, installation of improper ground excavation supports, and norninstallation of instrumentations and drainage holes (in the diversion tunnels). Due to thesernproblems, rock and shotcrete wedges failure occurred at various locations of power andrndiversion tunnels, and a tunnel section collapsed in the central diversion tunnel. A worker hasrnbeen lost due to the rock wedges failure in the manifold of the right power tunnel. Due to therncollapsed tunnel section, the unexpected quantity and various types of rock supports, groundrnimprovement materials, various construction techniques, a construction of additional 63.25mrnaccess tunnel (adit), etc. are used in the remedial works. The remedial works took almost ninernmonths, from May 2007 to February 2008. As a result of this collapsed section, the constructionrncost and the construction time of the project obviously increased.rnDue to lack of proper design of the primary supports in the project, the author has performed thernproper design of ground excavation primary supports using up-to-date Q-support design method.

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