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Optimization of Presplit Using the Hybrid Stress Blasting Model
F. Orellana and R. Ortiz, Itasca S.A.
This paper deals with the numerical analysis of open pit pre-split blasting applied to the optimization of blast design parameters. The work also aims to improve understanding of the rock-explosive interaction and to assess the theoretical application of ANFO towards pre-split applications. The work is comprised of simulations carried out using the Hybrid Stress Blasting Model (HSBM) (Furtney et al., 2009). Blast simulations were carried out in fractured and continuous (scaled) rock mass with equivalent Hoek-Brown strength envelopes (Hoek et al., 2002), GSI being estimated as a function of logging parameters (Hoek et al., 2013a) and structure being randomly oriented. Numerical simulation results show that the damage field and particle dynamics are highly sensitive to rock mass scaling effect. It is concluded that blast design requires a different rock mass scaling methodology, better suited to the dynamic phenomena involved and that, in absence of such criteria, the rock mass should be represented as an assembly of intact rock blocks with explicit fractures, with as little scaling as possible.
The effect of structure strength parameters on pre-split blasting was also assessed, results suggesting that pre-split effectiveness and damage are both greatly sensitive to joint shear strength. Simulations suggest that pre-split blasting becomes considerably more difficult to implement with poor joint surface conditions, placing greater emphasis on the need for proper shear strength characterization. Models with better structure shear strengths presented favourable results with distinct generation of the desired fracture plane, as opposed to the models which represent smooth jointing, where damage to the surrounding rock mass became difficult to control. Once again, traditional rock mass scaling criteria such as GSI may need to be complemented with additional parameters to account for strain rate dependencies of shear and tensile strength.
Finally, the HSBM code was used to test the theoretical application of ANFO to pre-split blasting. Although practical considerations need to be dealt with, simulations suggest ANFO, characterized by lower peak and higher sustained borehole pressures than cartridge emulsions (Furtney et al., 2010), could be made to produce reasonable pre-split results if the design is adjusted to emulate emulsion borehole pressures.
Post-blasting results for case 1 showing fragments (in black) on the left and micro cracks (damage only) on the right. In this plan view the upper boundary corresponds to the free face. Although there is extensive micro cracking generated (represented by broken lattice contacts in red) free fragments (in black) are only seen near the blast holes, indicating inadequate fragmentation.