Geotechnical Problems of the Underground Excavation in the Deccan Basalts Sarovar (Narmada) Project, Gujarat, India

The Sardar Sarovar (Narmada) Project, Gujarat State envisages the construction of an underground power house (6x200 MW) and its ancillary structures in the Deccan basalt. The basalt lava flows in the area are intruded by dolerite dykes and sills and dissected by fractures, shears and faults. These features have posed varied geotechnical problems like block falls, wedge failures, roof collapses and water seepage during the excavation of machine hall, access tunnel, draft tube tunnels and exit tunnels. The adequacy of support system designed on the basis of Barton’s and Bieniawski’s rock mass classification is constantly monitored and reviewed from time to time. The main power house cavern (210x23x58m) is being entirely supported by rock bolts and shotcretes with wiremesh. In the shotcreted upstream and downstream faces of the power house cavern cracks for maximum height of 22 m has been observed and are under evaluation. The rib supports have been introduced in tunnels passing through slacked zones in dolerite dykes and sills traversed by faults and shear zones