GEOTECHNICAL INVESTIGATIONS OF VISHNUGAD-PIPALKOTI HYDEL PROJECT, GARHWAL, INDIA

Abstract

The demand for energy has increased many folds in the recent times in India due to tremendous industrial growth and rapid urbanization. India is endowed with enormous water potential, which is confined within the high altitude glacial peaks of Himalaya. It is one of the most important prospective potential source forming hydropower reserves of the country. However, a large part of water resource in Himalaya is yet to be harnessed fully. This slow pace of developments related to hydropower projects can be attributed mainly to the difficult terrain characteristics related to Geology and Engineering. The Engineering Geological challenges during construction and post construction of dams in Himalayan terrain are many due to complicated geology, high seismicity, rugged terrain and high relative relief in addition to excessive seepage problems. The Vishnugad–Pipalkoti Hydroelectric Project, a run-of-the river (ROR) Scheme envisages construction of a 65m high diversion dam near village Helong (79°29’30” E and 30°30’50” N), a 13.4 km long Power tunnel (PT) and an underground power house to the south of village Hat (79°24’56” E and 30°25’31”N) to produce 444 MW of power (4 x 111 MW). The project is located on Alaknanda River, a major tributary of river Ganga, in Chamoli District in the state of Uttarakhand. A detailed Engineering Geological evaluation of the project has been carried out, to understand various Engineering Geological problems, which may arise during construction and to find suitable control measures. Hydroelectric projects have many irreversible geo-environmental impacts due to blocking of the water course. During dam construction, stability of hill slopes in natural condition as well as after dam stripping is an important consideration in the geo-environmental appraisal of the dam. The vibrations induced during blasting due to use of explosives to achieve maximum pull may often cause instability of hill slopes above the tunnel in addition to causing damages to houses, and other civil structures. In view of greater importance of these aspects, they have been given suitable consideration in the present study so that a proper geo-environmental evaluation of the project as a whole could be achieved. The present research includes Geological mapping and Engineering Geological evaluation of suitability of various project components on appropriate scales as well as identification of problems likely to be encountered during construction and immediately after construction. Geological 3D logging of exploratory drifts was carried out in addition to ii logging of drill holes done at the site. Extensive water pressure tests were also done in the foundation area to understand the seepage pattern below the dam. Based on collection of extensive field data, the geomechanical rock mass classification for different rock types forming the project components were evaluated through Rock Quality Designation (RQD) of Deere et al, 1967, Rock Mass Rating (RMR) of Bieniawski, 1989, Q-system of Barton, 1974 and Geological Strength Index (GSI) of Hoek and Brown, 1980 were used to obtain the rock mass properties of rocks exposed within the project area. Joint strength parameters were obtained based on Joint Roughness Coefficient (JRC) of Barton and Choubey, 1977 and Joint wall compressive strength (JCS) of Deere and Miller, 1966. Recent research in the field of Rock Mechanics shows some encouraging developments in stability analysis for surface and underground structures by providing graphical visualization programs. The facility of the programmes with enabled option to incorporate available field data and freedom in selection of method based on which the factor of safety (FOS) will be estimated. The extensive data collection from field and systematic laboratory studies help in better understanding of the graphical output generated from the softwares. Slope stability analysis of left and right abutments was carried out. The collected data was used to interpret dam abutment conditions including designing of stripping limits of the foundation and other foundation treatments. The problems that were likely to arise in different segments during construction of power tunnel were identified in detail. Since uncontrolled blasting causes damages due to excessive vibrations, safe limits of charges per delay for blasting were assessed so that the blast impacts can be minimized. A detailed Engineering Geological study was done on stability of power house cavern. On the basis of mapping of the power house area, the construction problems that are likely arise during and excavation and construction of underground powerhouse were identified. The pattern of unstable wedges, support pressures and support requirements for the powerhouse cavern were evaluated. The construction of a dam and impounding water behind it causes a major environmental feature that is reservoir, which is a standing water body. The fluctuations of water levels due to drawdown conditions cause instability of the hill slopes. Important unstable locations were identified and over all stability assessment of hill slopes around the rim has been carried out.