Numerical Interpretation of Phase-Wise Tunnel Rehabilitation in Faulted Himalayan Geology Using Ground–Support Interaction Analysis

In complex Himalayan geology, tunnel collapses in faulted, weak rock masses present severe challenges. A collapsed tunnel section (chainage 200–254 m) with adjoining deformed reaches (254–270 m) and overburden ranging 91–107 m was investigated. Laboratory tests on debris and muck revealed very low cohesion (e.g. c′≈0.037–0.12 kg/cm²) and moderate friction angles (~ 31–38°) under disturbed conditions. To reconstruct the tunnel, we propose a staged excavation sequence: firstly, plug the unstable face with sandbags and fiber-reinforced shotcrete to stop mud flow, followed by pre-support and drainage (long pipes and umbrella grouting). Subsequent excavation divides the section into two side drifts and a central drift, each immediately supported by steel ribs, shotcrete, and GFRP rock bolts. A detailed numerical model (3D finite element) was developed to simulate ground–support interaction, including the weak fault-zone with reduced stiffness (assumed c′=10 kPa, φ = 30° for phyllites). Modeling predicts that the proposed support scheme effectively limits convergence and settlements within acceptable limits. This combined empirical-numerical approach addresses Himalayan-specific hazards (fault zones, squeezing behavior) and aligns with best practices in tunneling. The study provides a comprehensive design and execution plan for safe tunnel rehabilitation under adverse ground conditions.