ABSTRACT
Tunneling in Himalayas is a tough challenge attributed to complex geology brought by the complex orogenic processes, resulting in complicated folding, faulting, metamorphism and displacement which have brought the litho-units of different chronology and genesis in juxtaposition to each other giving rise to complex geology, where in unrelated rock masses of contrasting engineering properties has been brought in close association. The 6.5m diameter and 13.521km long Head Race Tunnel of Mangdechhu Hydro-Electric Project negotiated through the rocks of Higher Himalayan Crystalline comprising of strong granitic gneiss, quartzite and weak mica/biotite schist. The complex geological conditions, necessitated continuous modification and optimization of excavation methodology and support system to maintain a steady progress, which was achieved by implementing advanced excavation methodology comprising of pre-grouting, fore polling, pipe roofing, controlled blasting, adequate support, monitoring of rock mass behavior by geotechnical instruments and probe hole drilling to infer rock mass conditions ahead of the.
