Navigating the Geotechnical Complexities of the Himalaya: Innovative Tunneling Approaches in Rangit-IV Hydroelectric Project, Sikkim

Rangit-IV Hydroelectric Project in Sikkim, India, involves excavation of a 6.5 km long head race tunnel (finished dia. 6.4 m), through the geologically intricate setting of “Rangit Tectonic Window” within the Lesser Himalaya. This region is a fold-thrust belt which resulted in multi-phase deformation and tectonically disturbed strata. Rockmass in HRT comprised of sandstone, carbonaceous shale, and coal bands of Gondwana Group. Geological uncertainties during tunneling involved occurrence of harmful methane gas with thick, low-cohesive coal bands, friable sandstone bands, >2D cavities and distress conditions with >5% tunnel closure. HRT has suffered 21 cavities, 9 distress/squeezing ground stretches, and 430 m section of high methane concentration. To address these complexities, we employed site-specific innovations in excavation and in rock support measures. For low-cohesive coal bands, 50 mm provisional shotcrete with pot-belly toe support was applied at the face, while self-drilling anchors (SDAs) were used for rapid forepoling and pre-grouting of the crown. Methane mitigation included continuous gas monitoring, optimized ventilation with booster exhaust system, and a shift from steel ribs to lattice girders to minimize fire risk and improve efficiency. Additionally, mechanical excavation was optimized with light-charge blasting to suit rockmass conditions, achieving enhanced stability and safety of the tunnel crown. These adaptive methods enabled an average progress of 90 m per month from two faces, despite the tunnel being predominantly (74%) in Class IV/V as per geomechanical RMR. This chapter discusses these in-situ innovations and their role in overcoming geological uncertainties, providing insights into effective tunneling practices for similarly challenging geotechnical conditions.