An Uncertainty-based Transition from Open Pit to Underground Mining

There are some large scale orebodies that extend from surface to the extreme depths of the ground. Such orebodies should be extracted by a combination of surface and underground mining methods. Economically, it is highly important to know the limit of upper and lower mining activities. This concern leads the mine designers to the transition problem, which is one of the most complicated problems in mining industry. The transition problem is categorized as a strategic one and is formulated in the form of long-term production scheduling problems. This implies that the transition problem is highly affected by the uncertainties that are rooted in the quantity and quality of an explored orebody. The current study aims to evaluate the effects of geological uncertainty on transition depth. To this aim, an integer programming (IP) model was executed on different simulations of an orebody. The results indicate that the net present value (NPV) of the deterministic solution is greater than that of the basic alternative. However, the uncertainty-based solutions show that the NPV of the whole mining operation is lower than the basic and deterministic solutions mostly (more than 72% of the simulations). Nevertheless, there are some rare cases in which the NPV of the operation may increase ideally up to 2.5 % due to development of the pit bottom downward. Finally, because of a negligible difference between the average NPV of the simulations and that of basic alternative, it is expected that the primitive pit bottom would play the role of transition depth.