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De Beers is currently developing closure plans for two open pit mines. At first glance they appear quite similar; both are relatively remote, have operated for 10 years, have similar pit dimensions (250–300 m deep and 1.5 km wide) and have Palaeozoic sedimentary host lithologies with weak upper units overlying more competent lower materials. However, Victor Mine in the sub-Arctic Canada, is one of De Beers wettest mines (dewatering volume of 75,000 m3/d) and is hosted predominantly in good quality limestone with excellent final wall performance. While Voorspoed Mine, in semi-arid Southern Africa required virtually no dewatering, has poor wall performance associated many with mudstone and country rock breccia instabilities. Victor Mine is expected to achieve stable pit lake in less than 10 years (and less than 2 years with supplementation from a nearby river), while Voorspoed will take over 100 years to reach ultimate pit lake level (due to low groundwater inflows, high evaporation and limited opportunity for flow supplementation). This paper describes a process to determine closure stability design acceptance criteria (DAC) and characterise the zone of long-term surface disturbance surrounding the pit (i.e.: potentially unstable pit edge zone to define the closure exclusion zone). This involved: These results along with the well documented historical slope performance provided the basis for detailed Geotechnical Risk Assessments which addressed two periods. Risk-based monitoring plans were developed along with Trigger Action Response Plans (TARPs) to ensure that closure of the pit proceeds safely and efficiently while satisfying the regulatory requirements.

Geotechnical risk management for open pit mine closure: a sub-arctic and semi-arid case study