Abstract
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.
Highlights
In the last four decades, a clear trend of open pit mines being excavated at increasing depths is apparent (Fig. 1a)
A total of six iterations was performed for the traditional design based on planar pitwalls, while for the design based on optimal pitwalls, two iterations were enough to reach convergence
The factor of safety values of all the pitwall profiles were verified by performing a limit equilibrium method (LEM) analysis with the Morgenstern-Price method, which is a rigorous LEM method where all equations of equilibrium are imposed on all slices [52], in Slide2 [28] with non-circular failure surface and other default settings
Summary
In the last four decades, a clear trend of open pit mines being excavated at increasing depths is apparent (Fig. 1a). There is an increasing trend to excavate pits of significant depth in weak rocks such as saprolites [4]. The steepness of the pitwalls is lower than the one for pits excavated in competent rock to prevent slope failures. This in turn implies more waste rock is produced. Any gain in the overall slope angle (OSA) of pitwalls is more important than ever for the economic profitability of open pit mines
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