Longhole open stoping (LHOS) is an efficient method of underground mining used for dipping orebodies. For its effective implementation, two major parameters should be balanced by the engineers, which are the recovery of ore and the amount of dilution. Ore recovery is the percent of economical minerals extracted according to the initial plan. Dilution means an unexpected addition of waste rocks to the extracted material, which reduces the grade of the extracted ore. The sequence of stopes’ extraction determines the results of both of these metrics.
The sequence strategy impacts the redistribution of stress in the surrounding rock mass. With the extraction proceeding, the concentration of stress occurs in the adjacent unmined pillars. The combination of stochastic mine planning and sequential simulations of high order allows optimizing the sequence, so that the schedule takes into account geomechanical changes (Carelos Andrade & Dimitrakopoulos, 2024). The proper choice of primary and secondary sequences ensures the absence of stress damage to the rock wall.
Dilution performance assessment will require evaluation of unplanned overbreak in relation to the designed boundaries. Through structural analysis and modelling, geotechnical engineers make predictions about how different extraction sequences will affect the stability of the global system (Machuca et al., 2015). In cases where extraction sequence causes stopes to be in highly-stressed abutments, failure will occur and cause falling of the rock mass inside the drawpoint, hence high unplanned dilution. In order to determine that quantitatively, there is need for reconciliation between the mucked volume and the theoretical designed model.
In contrast, assessing recovery performance is through evaluation of the amount of ore that has been planned but remains in place, often referred to as underbreak. Improper planning of extraction order may result in premature production of adjacent ore pillars, which makes mining such pillars impossible and leads to poor recovery. In addition, LHOS makes continual mapping impossible, and thus good underground grade control is a must for bulk extraction (Dominy et al., 2010).
The analysis for modern mining operations requires specialized tools to do so in an effective manner. The cavity monitoring system (CMS), which involves laser scanning of the cavity of empty stopes after excavation, provides the necessary information for this purpose. The comparison between the original stope wireframe and the CMS point cloud makes it possible to calculate the exact percentage of overbreak and underbreak. The design of the sequences is modified through empirical methods based on the information from the CMS.
An approach whereby there is prediction through modeling and evaluation after mining is used to determine the efficiency of various LHOS sequences. If there is a LHOS sequence that makes it possible to have good recovery and poor dilution, it is going to be expensive for the mill, while too conservative will lead to loss of ore.
References
Carelos Andrade, L., & Dimitrakopoulos, R. (2024). Integrated Stochastic Underground Mine Planning with Long-Term Stockpiling: Method and Impacts of Using High-Order Sequential Simulations. Minerals, 14, 123. https://doi.org/10.3390/min14020123
Dominy, S. C., Platten, I. M., Xie, Y., & Minnitt, R. C. A. (2010). Underground grade control protocol design: case study from the Liphichi gold project, Larecaja, Bolivia. Applied Earth Science, 119, 205–219. https://doi.org/10.1179/1743275811y.0000000016
Machuca, L., Sutton, M., Grow, R., & Andrews, P. (2015). Geotechnical approach to stope and pillar optimisation at Granny Smith Mine. Proceedings of the International Seminar on Design Methods in Underground Mining, 215–232. https://doi.org/10.36487/acg_rep/1511_10_machuca
