A large power house cavern (22m x 50m x 250m) is proposed to be excavated for installing a 1000 MW hydroelectric power plant for Tehri Dam Project. The gorge chosen for the dam is very narrow and the topography is rugged and inhospitable. This dampened the process of preconstruction investigations. A number of tunnels were constructed for various project structures in the vicinity of the cavern. This provided an excellent opportunity to study the rock mass behaviour. Tunnel closure and rock loads were monitored with the help of instruments. Deformation moduli were also determined for different rock grades with the help of various techniques. A voluminous data had been obtained. This data had been analyzed thoroughly and values of various parameters had been suggested for the final design of underground cavern and the supporting structures
