96. Construction of Access Tunnel for an Underground Hydroelectric Project

The Pykara Ultimate Stage Hydroelectric Project (an underground project) with an installed capacity of 3x50 MW is being constructed in the Nilgiris district of Tamil Nadu. In this project, an Access tunnel of size of 6.5 x 6.5 m, 1503 m long has already been constructed to provide access to the underground powerhouse complex located at about 532 m below the surface level. The size, shape, length and slope of the tunnel have been finalised based on the location of the powerhouse from the portal, maximum size and weight of generating machinery package likely to be transported, etc. The tunnel passes through a rock media of hornblendebiotite gneiss, migmatite and charnokite traversed by dolerite, pegmatite and quartz veins. The tunnel has been excavated by conventional drilling and blasting adopting heading and benching method. After excavation, the rock mass along the entire length of tunnel was classified into four categories and supported as per the support system required for each category. The in situ stresses have been determined at the end of the tunnel close to the powerhouse. During excavation of powerhouse, rock fall has occurred in the tunnel between Ch. 1355 m and 1365 m. The zone where rock fall occurred was supported properly to prevent further loose fall. This paper attempts to describe construction aspects of access tunnel and problems faced during construction.