Abstract
The paper showcases how substantial increases of the financial returns for open pit mining projects can be obtained by the adoption of steeper non-linear in elevation geotechnically optimal pitwall profiles. The pitwall profiles are determined by a novel slope optimization software which determines the optimal slope shapes. The optimal slope shape is defined as the one maximizing the overall steepness of a pitwall given the geotechnical properties of its rock, namely the Hoek-Brown or Mohr-Coulomb strength parameters and the unit weight, and for the prescribed factor of safety and pit depth.
Data (mine block model and geotechnical parameters) from a real copper open pit mine currently under development were obtained. We redesigned the mine using the same pit optimizer originally employed (Lersch & Grossman with Milawa NPV) but prescribing optimal non-linear pitwall profiles featured by the same Factor of Safety as the planar pitwalls of the original design. To this end a depth varying angle, obtained as output from the novel slope optimization software, was prescribed as geometrical constraint for the pitshells produced by Whittle instead of a depth constant overall slope angle. All the input economic parameters, e.g. commodity price, MCAF, annual discount rate, etc., were assumed to be the same for sake of comparison as well as the design philosophy employed for the choice of the pushbacks and the Ultimate Pit Limit, i.e. maximization of the NPV together with a realistically quasi-uniform production rate over the years.
From our analysis it turns out the design employing the optimal pitwalls exhibits an increase of NPV of around 30% which is mainly to be ascribed to a similar reduction percentagewise of rockwaste volume. Such a reduction of rockwaste in turn results in a substantial reduction of the mine carbon footprint.