Tunnel support design in fault zone in hydropower project in the Nepal Himalaya: a case study

Tunnelling through faulted rock mass is associated with long lasting displacements, stability problems, overburden squeezing, high deformations and dynamic stress conditions. With the dominance of faults in the Nepal Himalaya, determination of behaviour of the rock mass around tunnels and physical parameters for tunnel support is very challenging and demanding. Tunnel support and rock mass stability in tunnel are always dominated by fault. In this paper a hydropower tunnel in the vicinity of a fault has been used as a case study for the design and application of tunnel support. Rock mass is characterized, behaviour of the unsupported rock mass is evaluated, and support is designed. Geological Strength Index (GSI) is used to characterize the rock mass and a methodology has been used to design the support using numerical analysis The system behaviour of the rock mass and the tunnel support are analysed for the various combinations of geomechanical ground conditions. The displacement, stress, yielded elements and plastic region are studied for the influence of the fault and its stress conditions of the designed tunnel supports.