Engineering & Mining Journal

JAN 2019

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BLASTING 32 E&MJ • JANUARY 2019 the change in spacing of a blast with incre- mental changes in stiffness ratio. At a SEF above 5.0, the powder factor is increasing as the weight of explosive increasing in the borehole has a larger effect than the increase in spacing. At a SEF around 5.0 the powder factor is remaining about the same. At a SEF below 5.0 the powder fac- tor is decreasing. The slope of the SEF to SR graph indicates how rapidly the powder factor is increasing or decreasing. Figure 8 then shows that as the stiff- ness ratio is below 2.25 the powder factor is increasing as the charge weight per bore- hole is quickly increasing. Between 2.25 and 2.75, the powder factor then levels as the corresponding increase in charge weight is stabilized by the increase in spac- ing. Then from a stiffness ratio of 2.75 to 4.0, the powder factor then decreases as the increasing charge weight is less than the increase to spacing. The powder factor of a blast will then reach its minimum at a stiffness ratio of 4.0 as the bench transi- tions from a low bench to a high bench. High Bench Effects The transition between a high bench and a low bench occurs at a stiffness ratio of 4.0. At this stiffness ratio, with proper confinement, the cratering mechanism ceases to exist and the borehole effect is the primary breakage mechanism. The fragmentation of a high bench will be bet- ter and more uniform than that of a low bench, the throw will be improved, the ground vibration will be reduced, and the spacing of a pattern will be maximized. The effects of a high bench begin at a stiffness ratio of 4.0 and remain indefinite- ly. The use of higher stiffness ratios does not cause problems, until the drill hole de- viation becomes a major effect on the pat- tern design. The minimum powder factor of the bench will be at a stiffness ratio of 4.0; where spacing is maximized, follow- ing this is a slow increase in powder factor which occurs as the stiffness ratio increas- es. This is due to no increase in spacing, but small increases in charge weight. However, as the increase in borehole utilization is minimal, the powder factor increases at a slower rate and even at ex- tremely large stiffness ratio ranges (over 15) will not reach the same maximum powder factor that a bench between a stiffness ratio of 2.25 and 2.75 would have. In fact, after a slow build up of pow- der factor from a stiffness ratio of 4 to 12, almost no change is observed due to the fact that the additional weight of explosive added to the longer boreholes is minimal to the explosive already in the borehole. Ultimately operators' concerns with blasting are how can the cost of the pro- cess be reduced while the performance of the blast is improved. Old school de- sign approaches held these two goals at a constant tradeoff; however, today it is understood with modern blast design that these two are both able to be achieved. The theoretical best approach would be to design a blast with a stiffness ratio of 4.0, but other aspects such as the drilling and initiator costs can change this. Ideal - ly, all mines should design blasts to have a stiffness ratio of between 3.5 and 6.0, depending on actual on-site costs and conditions to produce a minimum cost while ensuring good performance. Case Study of Modern Blast Design This past year the two authors completed a project using the basic principles dis- cussed in this article at one of the largest surface mining operations in the middle east. This site was under strict government limits on the maximum powder factor they were allowed to use for both the overbur- den and ore in order to keep costs low; however, these powder factors were so lim- iting that over 10% of ore was discarded as oversize product and a large amount of the overburden required secondary blasting as the sites hydraulic shovels could not han- dle the boulders. The site had previously Figure 8—Spacing energy factor to stiffness ratio. Figure 9—Powder factor for high benches.

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