Significance of grouting for controlling leakage in water tunnels - a case from Nepal

The rock itself is a significant barrier against groundwater movement if it has low porosity and is unweathered. The existence of discontinuities in the rock mass however increases the permeability and it may vary widely. In the Himalayas, active tectonic movement and shearing have made rock masses weathered and fractured and increased the permeability. Having such rock masses has direct impact on stability caused by water inflow and leakage in tunnelling. If the tunnel is excavated for conveying water, the leakage problem is not limited only to the tunnelling phase, since there is also high risk of water loss through unlined tunnels during operation. The paper discusses the role that preinjection and postinjection grouting played for controlling leakage through the unlined headrace tunnel of the 60 MW Khimti I hydropower project in Nepal Himalaya.