Abstract:

The Overall Slope Angle (OSA) of pitwalls plays a crucial role in the financial return of an open pit mine. In current practice, pitwall profiles are designed to be planar in cross-section within each rock layer, i.e. the profile inclination across each layer tends to be constant. The presentation will showcase a novel design methodology where non-planar optimal pitwall profiles are employed without compromising mine safety. A new slope design software, OptimalSlope, is employed to automatically determine the geotechnically optimal profile shape to be employed for the pitwall of each mine sector.

OptimalSlope can be used by mining engineers in the strategic planning stage of mine design to work out the pitwall profiles needed as input by the pit optimiser software of choice. OptimalSlope seeks the solution of a mathematical optimisation problem where the overall steepness of the pitwall to be designed is maximised for an assigned stratigraphy, rock properties (unit weight and strength) and Factor of Safety. The geometry of the benches (bench height, face inclination and minimum width) is provided as input to OptimalSlope. The other input parameters required by OptimalSlope are the rock mass strength of each layer which can be expressed either in terms of Mohr-Coulomb or Generalised Hoek-Brown parameters, rock unit weight and desired Factor of Safety.

The results obtained from the design of three real open pit mines, a gold mine ownwed by Kinross (Agosti et al., 2021), a Chilean copper mine (Utili et al., 2022) and the McLoughlin gold mine (Agosti et al., 2021), will be illustrated. To quantify the improvement obtained by adopting geotechnically optimal profiles, we performed two designs: one employing planar pitwalls and another one adopting the optimal pitwall profiles determined by OptimalSlope. In determining the Ultimate Pit Limit and pushbacks, we sought to maximise the Net Present Value (NPV) and achieve an annual production schedule as uniform as possible over the mine lifetime. The FoS adopted for both planar and optimal pitwall is the same, with verifications performed by 2D Limit Equilibrium Method analyses (Morgenstein-Price method) run in Rocscience Slide 2 and finite difference method with shear strength reduction technique analyses run in FLAC on the pitwall of each mine sections. Also full 3D stability analyses of the entire Utimate Pit Limit were performed in FLAC.

For each mine, we assessed both financial gains (in terms of NPV and IRR) and environmental gains (measuring the reduction of carbon footprint and energy consumption). It emerges that the adoption of optimal profiles realises gains of up to 52.7% NPV and reductions of mining carbon footprint and energy consumption up to 18.7%.

References

Agosti A et al (2021) Design of an open pit goldmine by optimal pitwall profiles. CIM Journal, 12(4): 149-168.

Utili S et al (2022) Optimal pitwall shapes to maximise financial return and decrease carbon footprint of open pit mines. Mining Metallurgy & Exploration.

Agosti A et al (2021) Optimal pitwall profiles to maximise the Overall Slope Angle of open pit mines: the McLaughlin mine. SSIM2021, 69- 82. doi:10.36487/ACG_repo/2135_01

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Cristobal Valderrama