Design of an open-pit gold mine by optimal pitwall profiles

A. Lapworth,J. Samardzic,D. Gregory,S. Utili,A. Agosti,A. Prawasono

doi:10.1080/19236026.2021.1979382

Abstract

ABSTRACT The overall steepness of pitwalls significantly influences the financial return of an open pit mine. In current practice, pitwall profiles are planar in cross-section. In this paper, a new geotechnical software, OptimalSlope, is employed to determine depth-varying optimal pitwall profiles for each slope sector of the mine. OptimalSlope solves a mathematical optimization problem where the overall steepness of the pitwall is maximized for the assigned stratigraphy, rock properties, and Factor of Safety (FoS). Bench geometries (bench height, bench-face inclination, and minimum berm width) are incorporated into the optimization as constraints that bind the maximum local inclination of the sought optimal profile together with any other constraint related to any geological discontinuities that may influence slope failure. The optimal profiles are always steeper than their planar counterparts—that is, the profile exhibiting the same FoS, generally up to 8 degrees, depending on rock type and constraints. To showcase the financial gains that can be achieved via OptimalSlope, the design of a gold mine in a complex geology dominated by weak rocks was initially carried out for planar pitwalls and then for optimal pitwall profiles. The pit has been divided into five geotechnical sectors, each requiring a different pitwall profile design. Adopting optimal slope profiles led to a 52.7% higher net present value and reductions in the carbon footprint and energy consumption of 0.0613 million tonnes CO2 eq and 31.3 million MJ, respectively, due to a 3.5% reduction of rock waste volume.

Highlights

In open-pit mining, there is a clear trend of excavat ing mines of increasing depths (Figure 1a)
To showcase the financial gains that can be achieved via OptimalSlope, the design of a gold mine in a complex geology dominated by weak rocks was initially carried out for planar pitwalls and for optimal pitwall profiles
Adopting optimal slope profiles led to a 52.7% higher net present value and reductions in the carbon footprint and energy consumption of 0.0613 million tonnes CO2 eq and 31.3 million MJ, respectively, due to a 3.5% reduction of rock waste volume

Summary

INTRODUCTION

In open-pit mining, there is a clear trend of excavat ing mines of increasing depths (Figure 1a). In the case of a profile to be excavated in a uniform c φ geomaterial, the optimal shape calculated by OptimalSlope (Utili, 2016) turns out to be partly con cave and partly convex (Figure 2c) and is significantly different from the purely concave shapes considered in previous literature. Another perhaps even more impor tant limitation of all the aforementioned methods is the assumption of uniform slope, which prevents the appli cation of these findings to real open-pit mines, which typically involve complex lithologies and multiple rock formations of different mechanical strengths and with various geological discontinuities. With regard to capital costs, here we account only for the costs to perform the mine expansion considered (Table 3)

METHODOLOGY

G EN ER ALMINING SOF TWARED at am ine St udio OP

RESULTS

CONCLUSIONS