Pit Slope Stability Analysis at Rajpardi Lignite Open Cast Mine, Gujarat

Abstract

Mining is an arduous job and involve risk at each working stage. In open cast mining method, since it is the main focus as it contributes major portion of production, stability of the slope is of utmost importance. To avoid a slope from failure, working is to be carried out in accordance with the guidelines and safety standards. The factor of safety of the working slope has to be calculated and monitored from time to time so that safe working environment can be created and the slope can be avoided from failure. Factor of safety often calculated by the traditional deterministic analysis methods cannot exactly represent the stability of the slope. Case of Rajpardi Lignite Mine (GMDC) has been considered for this project. The working benches with height 3 m are very steep with individual bench slope angle varying from 80o to almost 90o. A total of 50 benches has been considered in a single formation for calculation of factor of safety with varying overall slope angle. The probability of failure of slope increases by increasing the overall slope angle of the working bench. A benchmark safety factor of 1.20 was set to consider the working conditions to be safe. Beyond this safety factor, working would be difficult and risky and probability of failure will be more. SURPAC software was used to make the solid and block model. Volume calculation, reserve estimation and average grade calculation was done using the borehole data of the mine. Followed by SURPAC, FLAC/Slope is used for calculation of factor of safety and numerical modelling of the slope. A total of five sections were considered to calculate the factor of safety with varying rock type. The sections were incorporated into FLAC/Slope from the block model that was made in SURPAC. Sections with ball clay was found to be less stable as compared to sections without ball clay.