Uncertainty and risk assessment of leakage in water tunnels - A case from Nepal Himalaya

Safety and stability are the key issues in underground excavations. Making tunnels water tight plays an important role in this regards. Water leakage problems in unlined or shotcrete lined water tunnels are not new issues. In many occasions severe water inflow as well as leakage problems have been faced that not only reduced stability of the rock mass surrounding the tunnel, but also valuable water has been lost from it, causing safety risk as well as huge economic loss to the projects. In the Himalaya, due to active tectonics in this region, the rock masses are highly fractured, folded, sheared and deeply weathered. Tunneling through numerous zones of weakness, fractures and faults is thus a matter of reality. Moreover, the majority of these zones are in general highly conductive, representating potential sources of ground water aquifer as well as possible sources of water leakage from the completed unlined or shotcrete lined water tunnels. Thus, the degree of uncertainty and risk associated with water leakage is extremely high. Water leakage control in the tunnels plays a vital role not only in improving the rock mass quality, but also in increasing the safety as well as saving economic loss caused by large leakages. The real challenge is however accurate prediction and quantification of possible water leakage prior to tunnel excavation, so that cost consequences are incorporated well in advance. The main focus of this paper will be to discuss on a new approach of uncertainty and risk analysis that leads to better understanding on the quantification of possible water leakage from unlined or shotcrete lined tunnels. For this, factual data of the headrace tunnel of Khimti I Hydropower Project in Nepal Himalaya, where effective use of injection grouting was applied to control the leakage, has been exploited.