ENGINEERING GEOLOGICAL EVALUATION OF RESERVOIR SLOPES

Abstract

Koteshwar dam project is a 400 MW hydro electric project being executed on the river Bhagirathi, a tributary of Ganga in Tehri district of Uttarakhand. The project is located about 22 km downstream of Tehri dam and the reservoir extending to 17 km upstream and nearly up to the steel girder bridge which is located close to the Tehri dam. One of the major structures involved in such projects is the dam behind which the reservoir is formed. In the event of failure of any slope around the periphery of the reservoir and the consequent landslide that can occur, a large chunk of rock mass can collapse in to the reservoir body and generate waves, which travel towards the dam and can give a big impact on the body of dam and damage it. It becomes therefore essential to ensure the stability of hill slopes all around the periphery of the reservoir. In the Himalayas, subtle variation in lithology and local changes in the orientation of rock discontinuities can make rock sliding extremely varied. Due to this variability, rock slope failures in some domains are so inevitable and certain that practical remedial measures are sometimes quite ineffective and extremely costly. An attempt has been made in the present study to discuss in detail the geotechnical investigation conducted at the site of Koteshwar reservoir, field and other studies undertaken for studying the stability of all the hill slopes around the periphery of the reservoir. Field studies included i) collection of all the geological data related to rock mass characterization of the slope material all around the periphery of the reservoir along Bhagirathi river, ii) conducting scan line survey at some typical slope sections and iii) collections of representive samples of rock and debris material forming the various slopes. The reason behind the rock slope failure is when the shear stress acting along a critical failure surface in a rock slope exceeds the shear strength of the slope mass in that slope. As far as rock mass classification systems is concerned the properties of discontinuities are the main input parameters in most of the classification schemes. The CSIR classification system of Rock Mass Rating (RMR) has been used for classification of jointed rock masses. Slope Mass Rating (SMR) technique for stability assessment of rock slopes, is primarily based on the application of (RMR)basic and the orientation of discontinuities. ii Entire test data was analyzed to assign rock mass rating at various rock slope sections identified and to classify all the debris/talus samples at debris/talus slope sites. Debris/talus slopes were then analyzed using the software packages-SARC and SAST for possible circular failure or a talus slide type of failure. Stability studies were carried out for computations of the factors of safety in static and seismic conditions for dry, partially saturated and fully saturated conditions of the slope mass. For all the unstable slopes, an attempt has been made to suggest the remedial measures, which in most of the cases, are in the form of re-profiling of the slopes using the method of benching and cutting. For rock slopes, possible mode of failure for every slope was identified using stereo plots prepared for every rock slope on basis of the geological data collected at the site. Kinematic analysis of rock slopes using Hoek and Bray charts was carried out for getting a priory idea about the condition of the rock slope. Similarly, Slope Mass Rating (SMR) system was also employed for obtaining the idea about the rock slope stability. Subsequently, detail kinematic analysis of rock slope was carried out using the software packages like SASP for plane wedge type of slide and SASW for three dimensional wedge failure. For all the unstable rock slopes, an attempt has been made to suggest the remedial measures in the form of cable anchors. The main objective of the research work in this study is to identify the unstable slopes around the periphery of the reservoir and study the stability aspects of unstable slopes of Koteshwar dam reservoir which has been taken as a case study in the present research. The stability of slopes in the reservoir area is very important for the safe functioning of the dam/ barrage. Unstable slopes in the rim area of the reservoir may lead to slope failures during the operation of the reservoir. On the basis of the entire study, it was observed that two slope sections in the left bank and five slope sections on the right bank of the Koteshwar reservoir are in critical condition. Attempt was also made to predict the possible height of wave which would be generated due to un-avoidable landslide and the corresponding wave height at the dam location when this wave travels upstream towards the dam. The values of maximum wave height generated at the location of these slope sections range from 0.7 m to 4.1 m. When these waves travel to the dam axis, the wave height left out varies from 0.0 m to 1.3 m. It is therefore clear that this wave height is not a matter of concern. This data is useful while adopting the preventive/ remedial measures.