The Jwaneng diamond mine located in the Republic of Botswana is an open pit mine that has been in operation since 1982. The mine has been developed in stages with mining taking place on the western and eastern sides of the pit. The Cut 8 stage is located on the eastern wall of the open pit where the bedding orientation of the rock units runs subparallel to the slopes, making this wall susceptible to instability events. Different slope design scenarios were developed for Cut 8, aiming to reduce the likelihood of slope instability in various critical locations, while maintaining the planned production targets to avoid significant economic impacts. Two slope design scenarios were selected for an objective comparison in terms of economic risks using a quantitative risk evaluation approach for slope design developed by SRK (Contreras, 2015). The purpose of the analysis was to assess the differential risk between a base case and an alternative design scenario, where the slope configuration improved the stability of certain areas of concern in the pit wall, but with some effects on the mine scheduling and the economics of the project.
The approach considered the results of the geotechnical evaluation of the pit wall available in the form of estimates of the probability of failure of 45 slope instability events for each design scenario covering the three main Cut 8 wall areas and six representative years of the 11-year mine plan. The estimation of the economic impacts associated with the failure events was carried out using a simplified cash flow model to quantify the effects of production disruption and additional costs. The probabilities of failure and associated impacts for each slope scenario were integrated using a probabilistic framework to define the respective economic risk maps. The risk maps were presented for annual impacts on profit and impacts on net present value during the life of mine. The risk map defines the relationship between the probability of exceeding an impact and the magnitude of that impact and accounts for different situations of occurrence of the events. These results were compared with acceptability criteria presented in the form of a risk matrix that defines four levels of risk: high (H), significant (S), moderate (M) and low (L).
The comparison of the risk maps for impacts on annual profits indicated that although a partial reduction of the risks was achieved with the alternative scenario, the deferral of the ore in this scenario caused a reduction of the effectiveness of the risk mitigation, due to the increase of potential impacts in the final years relative to the base case scenario. The results of the evaluation provided useful information to help the mine design team’s decision making process.
