ABSTRACT
Teesta Stage-VI Hydroelectric Project was lying abandoned from 2012 later taken over by LTHPL (subsidiary of NHPC) in October 2019 as a financial distressed asset. The project has twin tunnels 13.8km long each and 9.8 m finished diameter. Faces 3A and 3B are located beneath a perennial stream ‘Kalej-khola’ with low cover of 55-60 m. Face 3A (RD 493m) was already in collapsed state. As the excavation at Face-3B (RD 527m) commenced in November 2022, it encountered very poor geological conditions and heavy water ingress resulting in a collapse. There was gushing inflow of loose fragments of material mixed with overburden/river borne material. The conventional pre-grouting from inside the tunnel was not successful in stopping the heavy water ingress. Permeation grouting using TAM pipes was carried out from the surface of Kalej khola over the entire area Face-3B which was extended in Face-3A also which is detailed in this paper.
