Blow-out analysis of head race tunnel: A case study of Solu Khola (Dudh Koshi) hydroelectric project, Nepal

This paper examines the analysis, rectification, and performance of the blowout that occurred at chainage 1+154.12 meters (critical section) in the Head Race Tunnel (HRT) of the Solu Khola Dudh Koshi Hydroelectric Project in Eastern Nepal. The Tunnelling Quality Index (Q) for the critical and adjacent to the collapsed section was derived using probe drilling results and face mapping. The rock support system was evaluated using the Q-system support chart and is validated with the finite element-based program Rocscience (Phase2). The parametric study was conducted to evaluate the support required for varying the overburden depth from 100m to 650m. The tunnel convergence at the critical section was observed almost for two years before application of final rock supports and was found to be a maximum of 4.277 mm at 27 days, which was quite low and within acceptable limits. The performance study has also been carried out for nearly two years and has been found to be in stable condition since its commissioning. This study presents practical rectification procedure for blowout analysis in Himalayan tunnels, and the findings of the present case study will be helpful to geologists, tunnel engineers, and designers in future tunnelling projects with similar site conditions and constraints.