Due to high tectonic activity the rock mass in the Himalaya are fractured, sheared and deeply weathered. Tunneling through numerous zones of weaknesses, fractures and faults is a matter of reality. Hence, majority of the long tunnels face severe instability problems and construction challenges during excavation. Selection of appropriate construction methodology is thus crucial for successful and timely completion of tunnelling projects. The paper is a case study that reviews and evaluates the performance of TBM tunnelling along approximately 9 km long segment of the headrace tunnel of 800 MW Parbati II hydroelectric project, Himachal, India. Out of approximately 31 km headrace tunnel 21.23 km segment falls under civil work contract PB-Lot 2, where both Drill & Blast and TBM) methods of tunnelling are being used. The headrace tunnel passes through varying geological settings and Lot-2 contract is among the most critical concerning tunnel instability and construction challenges.

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