Success of post-grouting in controlling leakage through the headrace tunnel of Khimti I hydropower project in Nepal Himalaya

The likelihood of severe leakage from an ungrouted shotcrete lined/unlined water tunnel is relatively high. The excessive leakage from the tunnel not only may reduce the stability of the rock mass but also valuable water may be lost from the water conveying tunnel system. In addition, leakage may increase long term operational safety risk and may increase potential for huge economic loss to the hydropower plant. On the other hand, the use of full concrete lining as final rock support increases the construction cost and time by many folds. The best way to reduce leakage and control construction cost and time is the use of pre-injection grouting in the pressurized headrace tunnels built for hydropower projects. However, if no pre-injection grouting was done during tunnel excavation a post-injection grouting may also be considered an alternative solution, but with great care. The aim of this paper is to discuss and evaluate on the role and effectiveness of the post-injection grouting for controlling leakage through shotcrete lined/unlined low pressure headrace tunnel of the Khimti I hydropower project in Nepal Himalaya.