Determination of optimum overall slope angle for an open pit iron mine

Akdağ, Selahattin
Currently, vast majority of the mineral extraction is conducted by open pit mining operations. With the improvements in mining industry, mines have become progressively deeper leading to slope stability problems. Therefore, slope stability assessment has gained more significance for geotechnical engineering. Moreover, deepening the open pit mines has revealed the necessity of designing optimized slopes with regard to the economic viability. Steepening the ultimate slope of an open pit as much as possible minimizes the amount of waste rock which results in reduced production cost under a prerequisite of ensuring the mining safety conditions. Hence, slope stability evaluation is important to keep the balance between safety of slopes and economic efficiency. In this study, optimum overall slope angle for open pit mines in Bizmişen region by conducting slope stability analyses is aimed to be determined. Additionally, the study presents the slope design chart constituted for iron ore mines in Bizmişen region to contribute the further designs of open pits. Within the scope of this research, geotechnical fieldwork was carried out to gain geotechnical relevant data about the rock mass characteristics around the mine sites. Q-system, Rock Mass Rating (RMR) method and Geological Strength Index (GSI) system were used to characterize the dominating lithological units observed in the field. Besides, GSI ratings were assigned directly by field observations for the lithological layers within the mine sites. Moreover, mechanical properties of the rock mass around the region was obtained by laboratory experiments conducted on the cored samples taken from the exploration drillings and rock blocks taken from the mine site. Afterwards, according to the mining plans, the most critical cross-section within the mine site was determined by considering the instability conditions and for the deepest part of the open pit. Slope stability analyses were performed for various overall slope angle schemes with the combined use of limit equilibrium methods and numerical modeling which increases the reliability and accuracy of the stability analyses. Due to the topography and the orebody orientation, mining depth differs around the open pit. Therefore, optimum overall slope angle for varying mining depths were also determined by considering the results of slope stability analyses. These analyses were carried out to determine the factor of safety (FOS) and strength reduction factor (SRF) values that are the indicators of slope stability. The results from the limit equilibrium methods and numerical modeling were compared, and optimized safe overall slope angles, that satisfy minimum FOS and SRF values of 1.2, were estimated for different mining depths. Additionally, slope performance chart has been created for the region by considering the overall slope angles and corresponding mining depth. The developed chart can also be applicable for the iron ore mines in Turkey showing the same geotechnical characteristics with Bizmişen region.