Preconditioning blasting for a deep blind sink shaft excavation

Authors: Hall, A; Simser, B; Cai, M Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2465_80

Cite As:
Hall, A, Simser, B & Cai, M 2024, 'Preconditioning blasting for a deep blind sink shaft excavation ', in P Andrieux & D Cumming-Potvin (eds), Deep Mining 2024: Proceedings of the 10th International Conference on Deep and High Stress Mining, Australian Centre for Geomechanics, Perth, pp. 1221-1236, https://doi.org/10.36487/ACG_repo/2465_80



Abstract:
In 2024, Glencore successfully completed an internal winze from 1,150–2,635 m below the surface at Craig Mine in Sudbury, Ontario, Canada. The shaft was sunk in brittle hard rock, which at the depths of construction resulted in seismicity, stress fracturing, pervasive spalling, and rockbursting conditions. The high-horizontal in situ stress meant adverse conditions manifested both in the shaft walls and the bench face. For comparison, a typical lateral development round throws muck away from the face, leaving it partially unconfined and this allows for stress redistribution to occur immediately after the blast. On the other hand, blasted muck from a shaft blast will fill the void created, which confines the bench and inhibits large-scale stress fracturing from occurring. As confinement is reduced from mucking out the round, there is an increase in strainburst risk when operators are required to mark bootlegs and prepare for drilling/loading the next advance. Due to the limited working area associated with a shaft sinking operation, development is highly dependent on physical labour and handheld mining equipment. Compared with lateral mechanised development, fewer tactical controls can be used while shaft sinking to mitigate the risk of rockburst to operators. Preconditioning blasting became a critical control for managing high stress conditions in the shaft sink. There are limited guidelines in published literature for preconditioning blasting in shaft sinking operations and less evidence that preconditioning is providing a benefit. A customised preconditioning blasting strategy was developed based on visual inspections, seismic monitoring, and numerical modelling. The number of holes and location of the ‘de-stress’ charges were adjusted according to the rock mass conditions. It was also essential to institute controls on the shaft bottom mucking to prevent mucking beyond the planned break, so that the stress-fractured material that confined the highly-stressed rock ahead of the bench face was not removed. The experience learned from this project should be beneficial to other future shaft sinking projects at depth.

Keywords: preconditioning blasting, de-stress blasting, strainburst, deep mining, shaft sinking, winze

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