Kansanshi Mining plc (KMP) experienced a number of multiple-bench instabilities in saprolite zones of the open pit walls. Although detectable with current monitoring techniques, these instabilities caused production delays and necessitated a re-design of weathered slopes. This process started through back-analysis of old failures to validate the suitability of material properties. Calibration of 2D models with field observations resulted in a downrating of the material properties. Different combinations of bench heights, berm widths, bench face angles, and overall and inter-ramp slope angles were analysed. Based on the modelling results, it became evident that for the mine to maintain larger bench heights, in order to increase stability, shallower slope angles would be required. Given the financial implications of shallower slope angles, design objectives were re-evaluated and modified. Smaller (bench)-scale instabilities require the catchment capacity of the berms to exceed the expected failure volume. This allowed slopes to be steepened by 1.5°. A ‘double pre-split’ technique has been implemented so that the mine is able to blast two benches simultaneously. This enables the mine to operate efficiently through reduced cleaning, improved equipment efficiency, and a reduction in stoppages for blasting activities.
Kansanshi Mining plc (KMP) operates a copper mine in the North-Western Province of Zambia (Figure 1). Mining activities occur in two open pits; the Main Pit is the larger of the two, with a current depth of 170 m below surface and a strike length of 2.5 km, while North West Pit is shallower and smaller. The climate is subtropical with an average annual rainfall of 1.26 m occurring between the months of November and April. The final planned depth of excavation will be 530 m below surface with highly weathered rock masses extending up to 230 m from the surface. The deposit comprises a repeated sequence of phyllites, schists, and marbles with varying degrees of mineralization and weathering, both of which increase with proximity to veins and faulting.