Probabilistic Stope Design
For an underground mine, the creation of value from mining open stopes is extremely sensitive to the quality of stope design and execution, particularly with regards to the performance of individual stope extraction in terms of dilution and ore recovery. For example, a stope with high dilution (high overbreak) or low recovery (high underbreak) will likely produce negative value for a mine, whilst maximum value will be obtained from stopes with minimum overbreak and minimum underbreak. The emphasis of this ACG research project was the optimum design of open stopes. This innovative and timely research project commenced in late 2017 and was completed in December 2019. The project team comprised Kyle Woodward, Yves Potvin, Johan Wesseloo (ACG) and Benoit McFadyen (University Laval).
Open stope design approaches and tools have not evolved significantly since the 1980s and the stability graph method (Mathews et al. 1980; Potvin 1988), is used extensively in industry. Amongst the advancements, the widespread application of the cavity monitoring system (CMS) (Miller et al. 1992) has allowed to capture accurate overbreak and underbreak data at most mines for well over two decades. Therefore, there is a wealth of data at most mine sites on stope performance, in particular stope overbreak and underbreak. However, it is argued that this data has not been used to its full potential. This is evidenced in literature by the continuous application of the same stability graph (Potvin 1988).
An exception is the concept of Equivalent Linear Overbreak Sloughing (ELOS) developed by Clarke and Pakalnis (1997). ELOS has offered a methodology to assess overbreak, initially in narrow vein mining. Capes (2009) has extended the original Clark’s database significantly and has also extended the applicability of the methodology to open stope design, with some forecasting capability.
Although some mines use the ELOS concept, unfortunately, overbreak forecasting based on sound statistical methods has not been widely embraced by industry and remains almost non-existent at mine sites. Given the extensive stope performance data available at most mine sites, there is a good opportunity to further the use of probabilistic forecasting methods for overbreak and open stope dilution.
Another obvious gap in open stope performance assessment and stope optimisation processes is the estimation and forecasting of underbreak. Potvin et al. (2015) noted that in the evaluation of stope reconciliation, very little attention had been given to the amount of underbreak. This is surprising given that the underbreak is likely as costly, if not more costly, than the overbreak. Potvin et al. (2016) highlight that the optimisation of overbreak and underbreak are generally conflicting and effort towards reducing overbreak may result in increasing the underbreak. This emphasises the importance of having good stope reconciliation tools and procedure that can translate into future stope performance optimisation.
The focus of this research was to develop and implement at five sponsor sites a method and associated tools to enable a probabilistic approach to stope design and reconciliation. The idea was to enable better stope design decisions and, therefore, to facilitate stope optimisation by informing designers on the probability of overbreak and underbreak outcomes from different design scenarios.
Capes, GW 2009, Open Stope Hanging Wall Design Based on General and Detailed Data Collection in Rock Masses Unfavourable Hanging Wall Conditions, PhD thesis, University of Saskatchewan, Saskatchewan.
Clark, LM & Pakalnis, RC 1997, ‘An empirical design approach for estimating unplanned dilution from open stope hanging walls and footwalls’, Proceedings of the CIM 99th Annual General Meeting, Canadian Institute of Mining, Metallurgy and Petroleum, Westmount, published on CD-ROM.
Mathews, KE, Hoek, DC, Wyllie, DC & Stewart, SBV 1980, Prediction of Stable Excavation Spans for Mining at Depths Below 1000 Metres in Hard Rock, report to Canada Centre for Mining and Energy Technology (CANMET), Department of Energy and Resources, DSS File No. 17SQ.23440-0-90210, Canada Centre for Mineral and Energy Technology, Ottawa.
Miller, F, Potvin, Y & Jacob, D 1992 ‘Laser measurement of open stope dilution’, CIM Bulletin, July-August 1992.
Potvin, Y 1988, Empirical Open Stope Design in Canada, PhD thesis, Department of Mining and Mineral Processing, The University of British Columbia, Vancouver.
Potvin, Y, Grant, D & Mungur, G 2015, ‘Towards a practical stope reconciliation process in large scale bulk underground stoping operations, Olympic Dam, South Australia’, CIM Journal, vol. 6, no. 2, pp. 102-110.
Potvin, Y, Grant, D, Mungur, G, Wesseloo, J & Kim, Y 2016, ‘Practical stope reconciliation in large scale operations part 2, Olympic Dam, South Australia’, Proceedings of the 7th International Conference and Exhibition on Mass Mining, The Australasian Institute of Mining and Metallurgy, Melbourne.
The ACG acknowledges the following project sponsors:
Agnico Eagle Mines Ltd., Canada
BHP, Olympic Dam
Glencore, Mount Isa Mines
Gold Fields Australia Pty Ltd, Granny Smith
Hecla Mining Company, Canada
IAMGOLD Corporation, Westwood Mine
Minerals Research Institute of Western Australia