The overall steepness of the pitwalls massively influences the financial return of an open pit mine.
In current practice pitwall profiles are planar in cross-section. Sometimes rock layers exhibiting different strengths require the inclination of the profile to vary, but the inclination across each layer is constant.
OptimalSlope finds a depth varying optimal pitwall profile which maximises the Overall Slope Angle of the pitwall, from crest to toe at the pit bottom, for an assigned stratigraphy, rock mass properties and prescribed Factor of Safety. The optimal profile is determined as the solution of a deterministic mathematical optimisation problem where the geometric coordinates of the profiles are unknown variables and the maximum of an objective function representing the stability factor of the slope profile is sought. Among all the feasible candidate profiles, the optimal one is the one associated to the highest stability factor.
The benches geometry (face angle, berm width, and bench height) is assigned as a set of linear constraints to the optimisation problem and limits the maximum local inclination (αmax) of the sought optimal profile together with any other constraints such as geological discontinuities that may influence slope failure.
The optimal profiles are always steeper than their planar counterparts (i.e. the planar profile exhibiting the same FoS) of up to 8 degrees depending on rock type and severity of local constraints.
To showcase the benefit of the software, we have been working on 5 different case study of gold and copper mines (reported in publications). In these studies first the design adopting planar pitwalls is illustrated, secondly the design is carried out anew adopting the optimal pitwall profiles determined by OptimalSlope. The adoption of optimal profiles leads to increases of net present value of tens of USD millions, with increases up to 50%, and significant reductions of carbon footprint and energy consumption due to reduction of waste rock volume of up to 15%.
Prof Stefano Utili
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Mr Andrea Agosti