Geophysical study of bedrock nature and behaviour at Chatra dam site, Dehradun and Uttarkashi districts, Uttar Pradesh

The paper presents the results of electrical resistivity and seismic refraction surveys conducted for delineation of nature and depth of bedrock before constructing a 200 m high rock filled darn-across the river Tons for generating 80 MW hydroelectric power in Chatra area (30°58 '27N) 77°56'28E), Dehradun and Uttarkashi districts, U.P. Characteristic P-wave velocity and electrical resistivity values of the quartz-mica schist/massive quartzite/bedrock have led to its successful delineation. Appreciably larger depths varying from 30 m to 60 m in the flood plain and maximum overburden thickness of the order of 80 m over terraces near the road gutting at the proposed dam axis have been inferred in contrast with the exposures on both the banks. The bedrock profile along the dam axis shows two depressions possibly linked to palacoglacial activities in the area, one of which is along the Chatra Gad and the other along the dam axis. No bedrock intersection down to the depth of 44 m in the drill hole very accurately corroborates the interpreted depths between 44 m and 52.6 m in the river bed. ― .