Mitigating loose rock fall and cavity formation in Adit-2 tunnel of Rammam III hydroelectric project in Himalayas, India: Challenges and Solutions.

The turbulent geological history of “The Himalayas” arguably poses the most challenging ground conditions than anywhere in the world for underground excavations works. Cavity formation, loose fall, rock busting, high ingress of water and squeezing ground conditions are common phenomenon that encountered along tunnel profile in Himalayas. The Rammam III Hydroelectric Project, situated in Sikkim Himalayan belt lies in Main Crystalline Thrust zone (MCT) and is facing its fair share of Geological challenges during the underground excavation process.

The primary objective of the Rammam Stage-III Hydro Electric Project is to generate 120 MW power through 8.2 km of HRT (Head Race Tunnel).

Excavation of the Adit-2 to HRT has been challenging because of its proximity to the Jhepi Khola, a fault-incised stream. The tunnel’s progress was impeded by various obstructions of small and large magnitude caused by several sympathetic shears along joints and other geological complexities.

This paper will focus on the challenges faced in mitigating loose rock falls, cavity formation in Adit-2 from RD 312.0m to 320.0 m. To address the issue, an effective excavation philosophy known as DRESS (Drainage-Reinforcement-Excavation-Support-Solution) was adopted. The solution proposed is highly effective in addressing water-bearing issues in the Himalayas.

The treatment method adopted not only achieved the desired level of safety and smooth construction progress, but also provides a valuable reference for future similar cases of loose fall and cavity formations.