125 International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp. 125-127 #020410131 Copyright © 2011 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved Effect of Variation in Vertical Core Thickness on Stability Analysis of Earth and Rockfill Dams Khanna Rajesh ARO, CSMRS, New Delhi – 110016, Email: rajesh12khanna@yahoo.com Datta Manoj Director, PEC, University of Technology, Chandigarh -1600121, Email: director@pec.ac.in Ramana, G.V. Professor, IIT, Delhi -110016, Email: ramana@civil.iitd.ac.in ABSTRACT: Construction of earth and rockfill dams allows the use of available soil of different gradations in appropriate zones of dam cross section. Moreover the flexibility of the materials provides excellent seismic stability to the structure. Thus it requires evaluating the stability analysis of zoned dam with different materials and hydraulic properties of the construction materials. The stability of the dams depends on its geometry, materials properties and the forces to which it is subjected. These forces include the effects of water in the form of pore water pressure and seepage forces. This paper presents stability analysis carried out on 185 m high earth and rockfill dam with upstream slope of 1V:2.50H and downstream slope of 1V:1.80H. Thickness of the vertical core was varied from 1V:0.25H to 1V:2.65H and its effect on slope stability was analyzed under three critical conditions i.e. during and end of construction, steady state seepage and rapid draw down for both upstream and downstream slope of dam in static condition by Limit Equilibrium Method using SLIDE 5.0 software. It was observed from the analysis that by varying the thickness of vertical core of earth and rockfill dam, thinner core up to 1V:1.30H gives more stability and further increase in thickness of core tends to sharp decline in factor of safety both for upstream and downstream slope under different critical conditions. KEY WORDS: Factor of safety, End of Construction, Steady State Seepage and Rapid Draw Down, Core thickness. INTRODUCTION Slope stability analysis of high earth and rockfill dam is very important to ascertain the stability of the structure. As the geotechnical and hydraulic properties of the material in different zones of the earth and rockfill dam differ to a great extent, the stability of both upstream slope and downstream slope under static and pseudo static conditions under different conditions are to be analyzed. Zoned dams, specifically earth and rockfill dams allow the use of available soils of different gradations in appropriate zones of the dam cross section. Moreover the flexibility of the materials provides excellent seismic stability to the structure. An earth and rockfill dam should be stable under various conditions. The stability of earth and rockfill dam depends on its geometry, its components, materials, properties of each component and the forces to which it is subjected. In this paper one of geometrical factors i.e. varying thickness of the vertical core and its effect on the stability analysis was analyzed under different conditions . LIMIT- EQUILIBRIUM METHOD Limit-equilibrium method provides a general theory whereas other methods can be viewed as special cases. The elements of statics used in the limit- equilibrium method for deriving the factor of safety are the summation of forces in two directions and the summation of moments about a common point. These elements of statics, along with the failure criteria, are insufficient to make the slope stability problem determinate. Either additional elements of physics or assumptions regarding the direction or magnitude of some of the forces are required to render the problem determinate. Limit- equilibrium method utilizes the assumption regarding the direction of the interslice forces. This approach has been widely adopted in limit- equilibrium method. Limit- equilibrium methods are the most commonly used approaches for analyzing the slopes. The popularity of the limit equilibrium method is that it is relatively simple, ready ability to evaluate the sensitivity of stability of various input parameters, and the experience geotechnical engineer have acquired over the years in interpreting the factor of safety. STABILITY ANALYSIS The purpose of slope stability analysis is to provide a quantitative measure of the stability of the slope or a part of the slope. Traditionally it is expressed as a factor of safety against failure of that slope, where the factor of safety is defined as the ratio of the restoring force to the disturbing force, such that factor of safety greater than one shows stability and the factor of safety less than one denotes failure. Earth dam, in general, should have seepage control measures, such as interior drainage trenches, downstream pervious zones or drainage blankets in order to keep the line of seepage from emerging on the downstream slope. In zoned embankments, consideration should be given to the relative permeability and gradation of the embankment materials. ANALYSIS Dam section comprises of 185 m high vertical core earth- rockfill dam. The u/s and d/s slopes of the dam are 2.5H: 1V and 1.80H: 1V respectively. Length of the dam at the crest level is 585m. Thickness of the vertical core was