Geotechnical aspects of the Dam foundation

Tehri dam is an earth core rockfill dam across Bhagirathi River in Uttarakhand. At the dam site, the Phyllites of Chandpur Formation of Jaunsar Group are exposed which have been further categorized as phyllitic quartzite massive (PQM), phyllitic quartzite thinly bedded (PQT), quartzitc phyllite (QP) and sheared phyllite (SP). The major tectonic dislocations having continuity of more than 1 km were systematized and were assigned order I, II, III and IV. The bedding and foliation planes are dipping in SSE direction, i.e., towards downstream. In the block tectonic model, evolved for Tehri dam site, eleven longitudinal and six diagonal major shears were delineated. The diagonal shears define the boundary of the different blocks in which different structures are located. The key parameters lithology, petrography, structural discontinuities, weathering grade, destressing, permeability and geomechanical properties were considered for defining the rock mass at the dam site. Extensive surface and subsurface explorations, including the geophysical profiling, drilling, and drifting, were carried out on either abutment of Tehri dam, and the data revealed that the thickness of river-borne material varied from 4 to 14m. The geological logging of the under-river-bed drift (at El 574 m) disproved the existence of any riverbed fault below the dam seat, which saved voluminous excavation of hard rock. After stripping to the designed levels, foundation of the core and shell areas was geologically mapped and geotechnically assessed. All important shear zones were given dental treatment and the areas occupied by sheared/deformed rock mass were lowered and back filled with suitable strength concrete.