• Open-pit slope geotechnical considerations and its effects on mine planning

      Enkhbayar, Bayasgalan; Chen, Gang; Ahn, Il Sang; Arya, Sampurna (2020-08)
      In open-pit mining, a stable pit slopes design is essential for safe operation and economic performance of the mine. However, a steeper pit is more desirable from an economic standpoint due to reduced overburden removal. As the mine deepens, the open-pit walls become increasingly prone to slope failure, which causes human and economic losses. Therefore, a feasible and stable slope mine design requires a serious geotechnical investigation. The optimization of this design requires steepening the overall slope angle as much as possible while maintaining mine safety for efficient and effective mining operations. The open-pit slope geotechnical investigation calls for detailed geological and geotechnical data and advanced numerical modeling. In this study, geological and geotechnical data are collected from the Erdenet Copper Mine of Mongolia. The collected information includes data from discontinuity face mapping, geotechnical core logging, groundwater condition, geological exploration cross-sections, pit map, and rock property lab test results. The open-pit slope stability is analyzed with geotechnical numerical modeling software FLAC2D, and the variation and distribution of factors of safety (FOS) are computed and studied. The stability of Erdenet mine’s North-West open-pit is simulated by dividing the pit into ten representative cross-sections, and subsequently, FOS is calculated for each cross-section. The simulation results show that each cross-section has a higher overall FOS value than the allowable mine FOS, set at 1.5 with an earthquake magnitude of 0.165g peak ground acceleration (PGA). However, the localized high shear strain on individual benches may still occur, which can cause potential failures. Parametric studies indicate that changes in the bench angles and rock mass properties will have various degrees of impact on pit slope FOS. The effect of bench angle changes appears to be more significant. The study of pit slope design on mine planning shows that a 1° increase on slope angle will reduce excavation volume by 5 M m3 and save $15 million in excavation cost, but will also reduce FOS by 0.12. Engineering judgment and decision will have to be made regarding this tradeoff for a safe and economical mining operation. Practice and analysis indicate that the computer simulation alone is not sufficient to ensure the accurate estimation of slope stability. It is recommended to use a combination of slope monitoring and computer simulation to provide verification against each other to detect any potential hazards in mine. Mine pit slope movement monitoring program setup and monitoring procedure are analyzed and proposed in this study. The above findings allow mining engineers to optimally design pit slopes under the given geotechnical conditions and minimize the risk of slope failures while improving the stripping ratio and enhancing production profit.