ABSTRACT
In the present study, vulnerable slopes have been identified along the road between Tehri dam and Koteshwar dam, stretching about 20 km. This road is important from the point of view of livelihood of local habitats in this Garhwal region, tourist movement and smooth operation of the hydro-power generations in these two dams maintained by THDC India Limited (Formerly Tehri Hydro Development Corporation Limited). The case study area is situated in the Lesser Himalayas which is located in between the Shiwaliks in the South and the Greater Himalayas in the North and runs almost parallel to both the ranges, ranging 60–80 km wide and about 2400 km in length. The rocks exposed in the study area belongs to the inner as well as the outer Lesser Himalaya. The Inner Lesser Himalaya, in the study area is represented by the rocks of Rautgara Formation of Damtha Group, Deoban Formation of Tejam Group and Berinag Formation of Jaunsar Group. On the other hand, the rocks exposed in the outer Lesser Himalaya belong to the Chandpur and Nagthat Formation of Jaunsar Group and Blaini, Krol and Tal Formations of Mussoorie Group. The Tehri region is located near the Central Himalayan Seismic Gap, a major geologic fault zone. This region was the site of a 6.8 magnitude earthquake, known as the Uttarkashi earthquake in 1991, with an epicentre 53 km from the Tehri dam. Hence, it becomes significant to carry out risk assessment of the existing road connecting Tehri to Koteshwar. For this, field visits have been made and geological maps and profile sections of vulnerable slopes along this road have been prepared. Rock and soil/debris samples were also collected from field for laboratory assessment of shear strengths and other physical and geo-mechanical parameters necessary for stability evaluation. For the stability assessment, different limit equilibrium methods have been employed and stability analysis of the selected slopes have been performed for static and pseudo-static loading conditions at different water saturation levels to account for the regional seismicity and rainfall. From the analysis, risk to the road from any impending landslide has been evaluated and sliding or critical slopes have been identified. For one such critical slope, a suitable mitigation measure scheme has been analysed and the design is proposed for implementation. This study will help in keeping the road operational throughout the year and thus, will support the local community as well as the economy of the area.
