Groundwater Ingress in Head Race Tunnel of Tapovan: Vishnugad Hydroelectric Project in Higher Himalaya, India

Abstract

In underground projects vast uncertainties are confronted in terms of hydrogeology and geology, more often in mountainous terrains like Himalaya. Groundwater is often the main source of problems in tunneling projects and it is more troublesome during construction. Groundwater problems like sudden water ingress or continuous seepages are considered to be the most difficult conditions to predict. The problem is often due to ingress of water in higher volumes than predicted, or worse, the unanticipated encounter of water during tunneling. In Tapovan—Vishnugad Hydro electric project (520 MW), a run of the river scheme, the Head Race Tunnel (HRT) is being driven by Tunnel Boring Machine (TBM). Tunnelling is through the rocks of Central Crystallines which are heavily stressed due to—the presence of Main Central Thrust (MCT) in close proximity and presence high rock cover (>1 Km.). The Tunnel Boring Machine (TBM) was stuck up at RD 3016 m (rock cover ± 990 m) due to major rock fall on the shield near cutter head, in December, 2009. This led to sudden ingress of water (600–700 L/s) in HRT, damaging the pre-cast lining in the crown portion. The water ingress was from quartzite rock through some wide open joints, fractures or fault which were progressively widened due to ground water movement and caused instability problems. The major problems associated with the heavy ingress of water were found to be—difficult working conditions, damage to the structures, stability problems and environmental impact to the ground water resources. The work was called off for more than 10 months in the year 2010. A bypass tunnel was driven to reach the location where TBM was stuck up and damaged so as to carryout repairs. The ingress of water was diverted from HRT through a drift to make the HRT workable.