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  Melamchi Water Supply Tunnel Phase 1 Design and Construction Challenges

  The Phase 1 Melamchi Water Supply Tunnel is a 26 km, 4 m size tunnel that comprises the main component of a new water supply project to provide 2.8 m3/s of treated raw water for the city of Kathmandu, Nepal under full flow pressure conditions of 26 m of maximum head. The inlet and outlet of the tunnel are located about 40 km and 20 km northeast of Kathmandu respectively. The inlet headworks for the tunnel is located at an elevation of about 1425 m on the Melamchi River that is fed from the Langtang Mountain Range of Central Nepal. Design started for the project with studies in the late 1980’s and constructed started in 2010. The maximum cover along the tunnel was 1200 m and the geology along the tunnel alignment comprised mixed metamorphic bedrock of gneisses, mica-schists, pegmatites, and quartzites. The tunnel was constructed using traditional drilling blasting methods with two intermediate access adits for a total of six (6) working faces with initial tunnel support designated to the encountered rock conditions by the application of the NGI Q system. The final lining for the entire tunnel was shotcrete of variable thickness. Construction of the tunnel was delayed due to termination of the initial contractor for poor performance, the magnitude 7.8 Gorkha earthquake in 2015, bankruptcy of the second contractor, and tendering for the completion of the works by a third and final contractor.

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