Temporary tunnel with a lightweight lining at the Kurpsa hydroelectric station

1. Consideration of the hydrological situation in the basin of the Naryn River and the presence of the unfilled upstream reservoir of the Toktogul hydrostation made it possible to reduce the design discharge during construction at the Kurpsa site and to reduce the cross section of the temporary tunnel. 2. A careful analysis of the engineering-geologic conditions along the route of the temporary tunnel (made after driving the upper level) and investigations of the cohesion of gunite with the rock mass made it possible to use a lightweight gunite lining instead of the earlier planned reinforced-concrete lining over a considerable length of the tunnel. 3. The use of the lightweight gunite lining made it possible to dam the Naryn River 8 months earlier and to start on the construction of the main structures. A considerable reduction in the cost of the finished tunnel was obtained. 4. Experience in operating the tunnel of the Kurpsa hydrostation after it was put into service made it possible to introduce corrections (design cohesion of gunite with rock) in the “Instructions on Designing Hydraulic Tunnels” SN-283-73.