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

Abstract

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 m 3 /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: 1) Pit break back-analysis using i) industry guidelines; ii) an empirical approach based on historical slope instabilities; and iii) stability analysis using predicted post-closure phreatic surfaces. 2) Estimates of erosion potential. These results along with the well documented historical slope performance provided the basis for detailed Geotechnical Risk Assessments which addressed two periods. • Active closure with personnel undertaking rehabilitation activities in and surrounding the pits, and • Long-term closure, personnel and equipment not permitted within the long-term break back zone. 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.

A Suggested Methodology for Assessing the Failure Risk of the Final Slopes of Former Open-Pits in Case of Flooding

“…Even though there are numerous methods and methodologies for risk assessment [12,13,41,42], improving or simplifying them or developing new ones represents a step forward in science and is a special challenge. The proposed methodology was based on four important steps and some essential sub-steps as shown in Figure 6.…”

“…Studies conducted on failure risk of the final slopes of former open-pits in case of flooding are increasingly common [4,[12][13][14]. Prospective analyses have the role of highlighting areas with failure potential.…”

Weak rock open pit slope stability considerations to inform workplan for evaluating the position of permanent mine infrastructure: a northern Canada mine case study

“…Victor Mine is a northern Canada example of this process (de Graaf et al, 2019). Pit lakes and hydrogeological conditions are site-specific, and these conditions require investigation in the context of the stability workplan.…”