Assessment on stress induced instability in a tunnel project of the Himalaya

In the Himalaya, the majority of long tunnels face severe tunnel instability problems. One of the major instability is related to rock stresses. The headrace tunnel of Parbati II hydroelectric project is one of the long tunnel systems that is facing severe stress induced instability. Especially, this has been the case in the tunnel segment passing through Manikaran quartzite. It is known fact that the extent and type of stress induced instability vary greatly upon the rock type, deformability properties of the rock in question, and jointing and inter-bedding characteristics in the rock mass. This paper reviews most important rock mechanical properties that may have direct link on the nature of stress induced instability. It also back calculates magnitude of in-situ stress state in the project. An attempt is made to establish tectonic component of the horizontal stress. It is hoped that calculated stress magnitude may be useful for further stability assessment.