Tapovan-Vishnugad hydroelectric power project - experience with TBM excavation under high rock cover / Tapovan-Vishnugad Wasserkraftwerk - Erfahrungen mit TBM-Vortrieb bei hoher Überlagerung

NTPC Ltd. of India is presently constructing the 520 MW (4 x 30 MW) Tapovan-Vishnugad hydroelectric power plant in Uttarakhand in the Himalayas. As part of this project, an approximately 12.1 km head race tunnel (HRT) is to be constructed, of which approximately 8.6 km are being excavated by DS-TBM with an excavation diameter of 6.575 m. Construction of this HRT has been awarded to a Joint Venture (JV) of Larsen, Toubro Ltd., India, and Alpine, Austria. Geoconsult ZT GmbH is acting as a Consultant to NTPC Ltd. for the TBM part of the HRT. The overburden above the tunnel is up to 1, 100 m with the result that knowledge of the geology along the HRT alignment could only be based on projections made from surface exposures available in the area. Basically, the ground consists of jointed quartzite, gneiss and schist. Excavation of the HRT started in October 2008 and excavation rates of over 500 m per month were achieved in November 2009. However, in December 2009 the TBM encountered a fault zone along with high-pressure water inflow and became trapped. This paper outlines the present status of HRT construction and describes in particular the difficulties encountered during TBM excavation in fault zones with large high-pressure water inflows and how these problems are being dealt with.