Evaluation of earthquake impact on magnitude of the minimum principal stress along a shotcrete lined pressure tunnel in Nepal

In situ stress condition in rock mass is influenced by both tectonic activity and geological environment such as faulting and shearing in the rock mass. This influence is of significance in the Himalayan region, where the tectonic movement is active, resulting in periodic dynamic earthquakes. Each large-scale earthquake causes both accumulation and sudden release of strain energy, instigating changes in the in situ stress environment in the rock mass. This paper first highlights the importance of the magnitude of the minimum principal stress in the design of unlined or shotcrete lined pressure tunnel as water conveyance system used for hydropower schemes. Then we evaluated the influence of local shear faults on the magnitude of the minimum principal stress along the shotcrete lined high pressure tunnel of Upper Tamakoshi Hydroelectric Project (UTHP) in Nepal. A detailed assessment of the in situ stress state is carried out using both measured data and three-dimensional (3D) numerical analyses with FLAC3D. Finally, analysis is carried out on the possible changes in the magnitude of the minimum principal stress in the rock mass caused by seismic movement (dynamic loading). A permanent change in the stress state at and nearby the area of shear zones along the tunnel alignment is found to be an eminent process. Keywords: Shotcrete lined pressure tunnel, The minimum principal stress, Three-dimensional (3D) numerical model, Geology, Tectonic activity, Himalaya