Stability of Slopes for Dam Excavation by Slope Mass Rating in the Pare Hydroelectric Project Sylvia Kashyap Department of Civil Engineering Assam Engineering College Guwahati, India Ranjeet Bahadur Singh Department of Civil Engineering Assam Engineering College Guwahati, India Pragati Goswami Department of Civil Engineering Assam Engineering College Guwahati, India Diganta Goswami Department of Civil Engineering Assam Engineering College Guwahati, India Pawan Kumar Singh Department of Civil Engineering Assam Engineering College Guwahati, India Dibyajyoti Kalita Department of Civil Engineering Assam Engineering College Guwahati, India Abstract—Dam is an important element in all river valley projects, and excavation of both banks of the river, where the dam is abutted, is a very important consideration for the total cost of the project. Thus, the stability analysis of the side slopes of the rock mass is a very important exercise, for a feasible hydro-electric power project. The study area lies in the Papunpare district of Arunachal Pradesh, India which is represented by Upper Siwalik sub-group of the Sub- Himalayan range. In this paper the stability of rock slopes for a stable excavation for dam construction, is analyzed with the help of Slope Mass Rating (SMR) by using Basic Rock Mass Rating. Support measures, where ever required has also been recommended based on SMR. The satisfactory performance of the excavated slopes at the dam site of Pare H.E. P. project establishes that SMR method can be used with confidence for rock slope stability analysis. Keywords—SMR; RMRbasic; adjustment factors; stability I. INTRODUCTION Concrete gravity dam of a Hydro Electric Power project is abutted to the fresh rocks on both banks of the river. A stable, yet the most economical slope, needs to be designed for the purpose. In this paper, Slope Mass Rating system (Romana, 1985) as in [1] is used to study the stability of the rock slopes for abutting the dam of the Pare H.E.P. project at Papunpare district of Arunachal Pradesh. The study area lies within the upper Siwalik sub-group of the Sub- Himalayan range and are characterized by soft tertiary sedimentary sandstone/ siltstone. Four drill holes, namely, DAH 2, 3,4 & 5, drilled by NEEPCO Ltd. along the dam axis, exhibits repetitive sequence of coarse to fine grained, greyish coloured, soft, friable, moderately fresh sandstone, pebble impregnated sandstone, sand rock and pebble beds, on either banks. Geology of the dam site, as mapped by geologists of NEEPCO Ltd. are presented in the following sub-section. II. GEOLOGY OF DAM SITE The left and right rocky abutment was indentified with three sets of joint oriented in different directions. The dip/strike of the joint sets are given in the table I. TABLE I. DIP/STRIKE OF JOINT SETS Joints Left Abutment Right Abutment Dip Strike Dip Strike J1 60°-75° N235°-250° 52°-70° N230°-240° J2 45°-75° N150°-180° 28°-46° N140°-170° J3 32° N195° 32° N195° III. ROCK MASS CLASSIFICATION In a river valley project, excavation of a dam requires the modified rock slopes to be stable. The improper excavation methods on natural slope conditions and modification of cut slopes can develop new cracks which may lead to instability of the rock slope. Therefore the preliminary approach of a dam excavation should include the understanding of the geological and geomechanical parameters to prevent slope failure during and after excavation. For various engineering designs and stability analysis, Rock Mass Classification systems form the cornerstone of empirical design approach and are extensively implemented in rock engineering. Rock Mass Classifications have been recently employed in underground constructions, tunneling and in mining projects (Barton 1974, 1988; Bieniawski 1989; Laubscher 1990) as in [2][3][4]. A rock mass is characterized by comparing the values obtained from visual inspection or by simple tests with some standard sets of descriptions and values for some specific parameters. Thus with the help of standard qualitative data and guidelines, rock masses described by different geologists and engineers at different locations can be compared and analyzed. International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 http://www.ijert.org IJERTV9IS020352 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Published by : www.ijert.org Vol. 9 Issue 02, February-2020 640