Geological Survey of India, Dehradun
Locating and executing the spillway structures, at different sites, posed certain geotechnical problems, which were tackled detailed geotechnical assessment and by providing suitable foundation treatments and well designed support system. These structures are located on different tectonic blocks of the block tectonic model of Tehri dam project. Along the chute spillway, different bands of phyllitic quartzite massive (PQM) and phyllitic quartzite thinly bedded (PQT) were encountered. Planar failure along the foliation joints and longitudinal (L) shears, due to day-lighting, were noticed and to arrest these planar failures, deeper rock bolts were provided. The junction of the chute spillway with the stilling basin was considered highly vulnerable because of the existence of sheared/deformed rockmass formed by the intersection of the major diagonal (D-1) and longitudinal shears. An 11m wide and 4m deep reinforced shear key was provided. Stilling basin foundation exposed different bands of PQM, PQT and sheared phyllite (SP). Numerous major and minor shear zones, L & D type, were delineated and were provided with dental treatment. The areas occupied by sheared/deformed rock mass were advised for local lowering and backfilling with suitable-strength concrete. The right bank shaft spillway area was assessed critical because of the poor rock mass around D-3 shear (near T-4 shaft) and multiple shear seams as projected along T-3 shaft. During the excavation of T-3 & T-4 shafts and their ducts, planar failures along longitudinal shears, wedge failure and problem of wall convergence was noticed. The junction of the main shafts with the de-aeration ducts was critical because of the reverse curve geometry. Rock bolting and chain link shotcreting were provided as the main stabilisation measures whereas spot bolting and steel rib support were provided in critical reaches. At the swirling device area of T-3 & T-4 shafts, planar failure along foliation joints were recorded due to day lighting of these joints and the area was stabilized by providing 8m long 32mm dia rock bolts along with two layers of shotcrete. No major geotechnical problem was encountered in the excavation of T-1 & T-2 shafts, and they were stabilized by providing rock bolts and shotcreting. The horse shoe shape intermediate level outlet (ILO) was driven through different bands of PQM & PQT. The excavation of the inclined glacis of ILO at the junction with T-3 shaft was very critical, and it was done by taking up smaller segments. Deeper rock bolts were provided on both the walls, and vertical steel channels were extended down to the increased wall sections, to prevent failure from the arch or wall sections, during the excavation at the junction.
