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  Managing Geological Complexity in BVC and PAC of Tehri PSP, India -A case study

  Tehri hydroelectric project is located on the left bank of River Bhagirathi in the state of Uttarakhand. Tehri Pumped Storage Project (PSP) comprising 4 Nos of reversible pump -turbine units of 250 MW each, involves construction of an underground Machine Hall, Upstream Surge Shaft with Chambers - 3 & 4, a Butterfly Valve Chamber (BVC), a Penstock Assembly Chamber (PAC), the Downstream Surge Shaft with Chamber - 3 & 4, a pair of Tail Race Tunnels (TRTs) and the Outlet structures. Execution of large infrastructure projects cannot be imagined without high- level engineering explorations, the prerequisites to increasing the economic efficiency of capital investments. Prior Estimate of uncertainties and risks involved in the construction process is central to decision making in the underground projects. In such Projects, a significant part of uncertainties results from unknown geological conditions. Timely calculation of risks and action required therein is vital in mitigating geological risks in the underground construction and tunnelling projects. Continuous updating of geological models, readjustment of excavation and support to actual ground conditions are the measures that control all geological risks at the site. In short, adopting appropriate excavation technology, proper sequences, dependable support, monitoring behaviour and auxiliary measures mitigate the geological risks to a great extent. The present paper discusses advance identification of geological risks based on continuous updating of geological model of the site, design updating, and measures applied to reduce the risks in the Twin Chambers viz. Butterfly Wall Chamber (BVC) and Penstock Assembly Chamber (PAC) of Tehri Pumped Storage Hydro Project in the Indian Himalaya. The Paper also underlines the meticulous construction planning, on the spot resolution of the problem and visualisation of geological risks involved at every stage of design and execution.

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