Evaluation and analysis of potential instabilities along the road tunnel through the Lesser Himalayan rock mass – A case study

Underground construction in the Himalayan region is challenged by active tectonics, anisotropic stresses, and highly fractured rock mass, making it especially susceptible to block falls and stress-induced instabilities. This study aims to assess rock mass behavior resulting from the redistribution of stresses during the excavation of a proposed road tunnel connecting Pokhara with the Kaligandaki Corridor of Nepal. Rock mass quality, evaluated using the Rock Mass Rating (RMR) system and the Q-system of rock mass classification, revealed the presence of predominantly poor-quality rock mass. Whereas, sensitivity analysis indicated a five-point improvement in the Geological Strength Index (GSI) resulted in a reduction of approximately 24 % displacement. This suggests that structural integrity must be effectively maintained through proactive reinforcement, such as forepoling (spiling) to enhance rock mass quality before significant yielding occurs. Further, numerical simulation demonstrated that implementing a forepoling zone reduces deformation (>26 %), underscoring the necessity of presupport in weak ground. Tunnel stability was assessed through integrated analytical and numerical approaches, accounting for varying lithology and rock cover depth. Results predicted brittle failure in metasandstone formations with spalling depths reaching 1.34 m. Conversely, weak graphitic phyllite units exhibited severe squeezing, potential for rock mass collapse with a maximum predicted strain of 18.8 % (chainage 5 + 000 m), necessitating implementation of stabilization measures, including forepoling (spiling), systematic rock bolting, steel fiber-reinforced shotcrete, and pre-injection grouting. These findings offer critical insights into the behavior of heterogeneous Himalayan rock formations and provide a basis for adaptive design and planning of underground openings in such geological environments.