Slope Stability Analysis and Design of Open Pit Mine Slopes for Limestone Mine at Aruwakkalu, Puttalam, Sri Lanka

The largest open pit limestone mine in Sri Lanka which belongs to Holcim Lanka Limited is located at Aruwakkalu, Puttalam. The sub-surface of this site is comprised of an overburden of average thickness 24 m followed by a 9 m thick low grade limestone layer and a high grade limestone layer at the bottom which extends even below the seawater level.The overburden which is currently being excavated in five benches of near vertical cuts becomes unstable during the rainy season, which is a significant safety issue on this site. In this scientific investigation, the physical properties of each layer of material are determined by laboratory testing, in order to design optimum bench heights and slope angles for this open cast mine. Direct shear tests are carried out on undisturbed soil samples collected from the overburden, whereas rock-triaxial tests are carried out on the samples collected from low-grade and high-grade limestone layers, in order to determine design parameters: cohesion(c) and friction angle(ϕ) of these materials. The stability analysis is carried out by means of “SLOPE/W” software. The analysis of data for a three layer model reveals that the possible failures are confined to the overburden based on the outcome of this study. The overburden mine slopes could be optimized to three berms of each 8 m height, 6 m wide and of 700 slopes, having a sufficient safety factor which will enhance the safety and productivity of the future mining operations on this site