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Optimization of large-scale panel destressing for remnant pillar extraction

Rockbursts, destress blasting, pre-conditioning

Destressing blasting is one of several well-established techniques for rockburst control in underground coal and hard rock mines [Konicek et al., 2011]. The method of large-scale panel destressing is particularly useful in deep hard rock mines to enable remnant pillar or stope extraction in sublevel open stoping methods with delayed backfill [Andrieux et al., 2003]. To implement the method, a large panel that is close to the stope hanging wall is choke-blasted to cut-off the regional principal stress being applied to the stope pillar. Access to the panel is driven by extending the stope sill drives into the hanging wall of the remnant stope. Panel blasting prior to stope mining helps create a stress shadow in the stope pillar. Figure 1 presents a schematic illustration of the method [Vennes and Mitri, 2017]



The benefits of large-scale panel destressing are not immediately apparent when considering the obtained stress reduction, since the measured stress reduction is relatively small compared to pre-mining stresses [Vennes et al., 2020]. However, other pillar burstability assessment criteria such as ore at risk volume [Shnorhokian et al., 2014] and Energy release rate (ERR) [Mitri et al. 1999] elucidate the benefits of destressing when applied to a remnant pillar. In this study, the effect of panel geometry, far field stress magnitude, and far field stress orientation are quantified in terms of ore at risk and ERR on a simplified remnant pillar extraction sequence to determine the suitability of panel destressing under different stress conditions and panel geometries. The pillar-wide numerical model is constructed in FLAC3D and the linear-elastic constitutive model is applied. The holistic effect of destressing is simulated with the rock fragmentation factor (α) and stress reduction factor (β) in the panel zones, representing the instantaneous decrease in elastic modulus and stress tensor, respectively, which occur in the panel rock during the blast.



Finally, the Copper Cliff Mine (CCM) panel destressing program is presented as a case study to validate the benchmarks established in the parametric study [Vennes et al., 2020]. The ore at risk and ERR are calculated over the extraction and destressing sequence in the pillar with a pillar-wide numerical model. As can be seen from Figure 2, destress blasting reduces the peak energy release rates over the remnant pillar extraction sequence.



Acknowledgement

This work is financially supported by a joint grant from MITACS Canada and Vale Canada Ltd (Project IT07425), and the MEDA fellowship program of the McGill faculty of Engineering.



References

• Andrieux, P, Brummer, R, Mortazavi, A, Liu, Q, Simser, BP (2003) Large-Scale panel destress blast at Brunswick Mine CIM bulletin:78-87


• Saharan, MR, Mitri, HS (2009) Simulations for rock fracturing by destress blasting, as applied to hard rock mining conditions. VDM Verlag, Saarbrucken


• KONICEK, P, SAHARAN, MR, MITRI, H. 2011. Destress Blasting in Coal Mining – State-of-the-Art Review. Procedia Engineering, 26, 179-194


• MITRI, HS, Tang, B and Simon, R (1999) FE modelling of mining-induced energy release and storage rates. South African Institute of Mining and Metallurgy (Transactions of SAIMM Journal), Vol. 99(2): 103-110.


• Shnorhokian, S, Mitri, HS and Thibodeau, D (2014) A methodology for calibrating numerical models with a heterogeneous rockmass. International Journal of Rock Mechanics and Mining Sciences, Volume 70, pp. 353-367


• Vennes, I and Mitri, HS (2017) Geomechanical effects of stress shadow created by large-scale destress blasting. Journal of Rock Mechanics and Geotechnical Engineering. Volume 9, pp. 1085-1093. Elsevier.


• Vennes, I, Mitri, HS, Damadora, Chinnasane, R.D.  and Yao, M. (2020) Large-scale destress blasting for seismicity control in hard rock mines: A case study. International Journal of Mining Science and Technology. Vol 30, Issue 2, pp 141-149. Elsevier