Construction of underground powerhouse at Tehri was inevitable because of non-availability of the space on the surface and the huge excavation (required to accommodate the powerhouse) of the steep back slopes. Excavation of the two huge cavities of the powerhouse complex i.e., machine hall and transformer hall was tackled carefully. Geotechnical assessment made earlier revealed that both these cavities are located in the most competent rock mass at Tehri dam site i.e. phyllitic quartzite massive (PQM) and phyllitic quartzite thinly bedded (PQT) Grade-I. Considering the geotechnical parameters including the in-situ stress measurements, the alignment of these cavities was preferred in N0209 direction. As the alignment was also against the dip direction, the excavation in these cavities was free from any major failure. These gigantic caverns were stabilized by means of rock bolting and shotcreting barring a patch in the crown of machine hall where a 5m band of deformed rock mass, associated with a major longitudinal shear, was encountered. The rock cover between the two cavities was insufficient (i.e. less than 2D), because of which problem of convergence was recorded during the excavation of the bus ducts in the common wall. Multiple bore hole extensometers (MPBAX) and load cells were installed to monitor the rate and extent of convergence, and a number of deep cable anchors (blind and through) were installed to stabilize the area.
