The steepness of the slopes of an open pit mine has a substantial influence on the financial return of the mine. The paper proposes a novel design methodology where overall steeper pitwalls are employed without compromising the safety of the mine. In current design practice, pitwall profiles are often planar in cross-section within each rock layer; i.e., the profile inclination across each layer tends to be constant. Here instead, a new geotechnical software, OptimalSlope, is employed to determine optimal pitwall profiles of depth varying inclination. OptimalSlope seeks the solution of a mathematical optimization problem where the overall steepness of the pitwall, from crest to toe, is maximized for an assigned lithology, geotechnical properties, and factor of safety (FoS). Bench geometries (bench height, face inclination, minimum berm width) are imposed in the optimization as constraints which bind the maximum local inclination of the sought optimal profile together with any other constraints such as geological discontinuities that may influence slope failure. The obtained optimal profiles are always steeper than their planar counterparts (i.e., the planar profiles exhibiting the same FoS) up to 8° depending on rock type and severity of constraints on local inclinations. The design of a copper mine is first carried out employing planar pitwalls, secondly adopting the optimal pitwall profiles determined by OptimalSlope. The adoption of optimal slope profiles leads to a 34% higher net present value and reductions of carbon footprint and energy consumption of 0.17 Mt CO2 eq and 82.5 million MJ respectively due to a 15% reduction of rockwaste volume.

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This paper investigates the financial and environmental consequences stemming from the introduction of a carbon levy applied to mining and processing activities. The novelty is twofold: (1) the effect of a carbon tax, proportional to the emissions produced by all relevant mining activities, is accounted for in the determination of the Ultimate Pit Limit (UPL), i.e. the environmental costs are not applied a posteriori to pit optimization but included concurrently to Net Present Value (NPV) maximization, allowing to investigate the relationship between carbon tax value versus NPV, amount of ore extracted and carbon emissions; (2) we use a new software, OptimalSlope, to automatically determine geotechnically optimal profiles for the mine pitwalls. The Marvin copper deposit (block model data publicly available from MineLib repository) was adopted as a case study. Several pit optimizations were performed based on four different values of carbon tax and adopting either traditional planar or non-linear optimal pitwalls. It emerges that the relationships between carbon tax value versus NPV, amount of ore extracted, and carbon emissions exhibited linearity in both cases of planar and optimal pitwall profiles. Moreover, the adoption of optimal profiles realizes gains up to 215 million AUD, without compromising the safety of the UPL.