Target Audience

Geotechnical engineers and mine design professionals.

Abstract

In open pit mining, there is a clear trend of excavating mines of increasing depths, from less than 50 m deep in the 1920s to more than 1 km in recent years. Owing to the increased efficiency of mining equipment and improved exploration techniques and technology, the orebodies left to be exploited reach depths of even 2,000 km from the ground surface. The deeper a mine, the higher the effect of pitwall steepness on the amount of waste rock excavated and, therefore, mine profitability. As such, designing pit walls to be safe and as steep as feasible at the same time has never been more important. The steepness of mine pitwalls significantly affects not only mine profitability, but also carbon footprint since mining operations are primarily responsible for carbon emissions.

Traditionally, the design of pitwall inclinations is carried out by geotechnical teams with little interaction with mining engineers. Usually, the geotechnical engineer establishes the maximum inclination of any pitwall to be excavated within the various geotechnical domains of the mine. The design is performed following a prescribed Factor of Safety (FoS) chosen based on the risk profile of the mining company. The mining engineer then uses various software packages, e.g. Datamine Studio NPVS, Geovia Whittle, Hexagon MinePlan, Maptek, etc. to compute pushbacks and the ultimate pit limit by performing a strategic pit optimisation with the maximum acceptable pitwall inclination acting as a constraint. No further interaction occurs until the geotechnical team is asked to verify the stability of the final pitwalls produced by the pit optimisation software. However, it is well known that the maximum inclination of a slope is a function not only of the prescribed FoS but also of slope height. Therefore, imposing a pitwall maximum inclination, irrespective of the pitshell depth, implies that the current design methodology gives rise to suboptimal designs where often the pitwall inclination is either less steep (design overly conservative) or steeper (a below-target FoS is adopted) than what it could be.

To overcome current limitations, participants will be introduced to a design methodology where pitwall inclinations are selected, accounting for the pit depth, and integrated into the strategic mine design (Utili et al. 2022).