SYNOPSIS:

The paper presents the investigation conducted in two limestone opencast mines for the design of ultimate pit slopes. These mines cater to a cement plant and are situated in the state of Tamil Nadu in India. The main issues, which were taken into account were the presence of highway, village and other structures near mine boundaries. The investigation mainly included rock mass classification using Rock Mass Rating and Slope Mass Rating approaches, structural mapping for identifying the likely mode(s) of failure and the stability analysis for slope designs. The rock masses of these mines were classified as poor to good, with RMR varying from 38 to 62. The stability analysis was performed for plane and circular failure conditions using limit equilibrium approach. The overall slope angles in one mine were suggested at 51 - 530, 50 - 510 and 58 - 600 for ultimate slope heights of 37 m in north side (towards highway), 45 m in east side and 37 m in other sides respectively. In another mine, the overall slope angles for an ultimate slope height of 55 m were suggested at 50 - 510 for slopes facing the highway and village road and 55 - 560 for other slopes. Further, it was suggested that a minimum distance of 25 m be maintained between the ultimate slope crest and the highway and/or village road in these mines. This study has demonstrated that scientifically designed ultimate pit slopes help in the optimum recovery of mineral, which is otherwise locked up in the barriers.

1.0 INTRODUCTION

The stability and design of slopes have become vitally important from the view points of safety and economics of opencast mines with the growing significance of opencast mining in mineral production. The increasing pit depth and production requirements from opencast mines subject the design engineers and planners to the pressure of working under the constraints of two conflicting requirements of safety and economics. This scenario poses a big question as to how to achieve an optimum design - a compromise between a slope that is flat enough to be safe and steep enough to be economically acceptable. The practical approach to slope stability is guided by the basic geological data, geo-technical information, ground water details and a good measure of engineering judgment (Jhanwar & Chakraborty, 2009). The factors, which mainly influence the stability of an opencast slope, are the shear strength parameters of slope mass, the presence of structural features, their characteristics and orientation vis-à-vis the slope and ground water conditions. Structural features in the form of joint, faults, etc. play an important role in defining the failure characteristics and the stability of slopes. The other extraneous factors that influence the slope design are presence of surface structures near the ultimate pit limits, influence of blasting etc. A study was conducted in two opencast limestone mines each having two separate pits and working under the constraints of important structures in the form of highway, village road, etc..

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