Engineering & Mining Journal

JAN 2019

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Page 32 of 59

BLASTING JANUARY 2019 • E&MJ 31 Low Bench Effects A low bench will be considered a bench whose stiffness ratio is below 4 and typi- cally above 1 as it is never recommended to fire a blast with a stiffness ratio below 1. Low benches present several problems; the first of these that will be discussed is their tendency to break under a cratering mechanism — which is a highly inefficient breakage mechanism. When a blast has a stiffness ratio of 1.0, it will function in a full cratering method showing vertical displacement instead of horizontal move- ment of material. The bench will also have backbreak possible up to many times the burden, the bench can have substantial breakage below grade, and the fragmen- tation will be varied with both fines and numerous large boulders. As the stiffness ratio increases, the breakage mechanism shifts to the borehole effect which com- pletely takes over at a stiffness of 4.0. Low benches are also extremely costly, not only in terms of the poor post-blast re- sults but also in the overall borehole utiliza- tion. One of the most basic rules of thumb in blasting is that the stemming in a bore- hole should be approximately 70% of the burden, therefore if the stiffness ratio is low then the borehole has almost no explosive in it. This creates a large amount of drilling for a relatively small amount of explosive. This is also true with initiators, an initiator costs the same amount whether it detonates 50 lb or 500 lb of explosive, therefore using a deeper borehole which is filled primarily with explosives leads to better utilization for the cost of initiators. This borehole utilization is also one of the reasons that a major increase is seen between a bench with a stiffness ratio of 1.0 and 2.5. As can be seen in Figure 6, when a bench changes from a stiffness ratio of 1 to 2.5, the borehole utilization is increased by around 33% leading to a large increase in total explosives. At the same time the increase in spacing from this change is small. One can then see that to further increase from a 2.5 to a 4.0 only an 8% increase in borehole utilization exists. Furthermore, any increases above a stiffness ratio of 4.0 show incremental changes to the borehole utilization. Another way to understand why the pow- der factor for a low bench quickly peaks at around a 2.5 and then declines is based on the relationship between the increasing weight in a borehole and the expansion in the pattern based on the spacing. As the stiffness ratio increases, the percent change in the weight of the explosive is re- duced. This is shown by the blue line in Figure 7. For example, at a stiffness ratio of 1.5 to 1.75 the weight of explosive in a borehole will increase by 23%. To further increase the stiffness ratio to 2.0 from the 1.75, only an additional 18% of the total weight of explosives will be added. There- fore for each increasing step in stiffness ratio less and less additional explosive will be added. After a stiffness ratio of 2.75 the change in charge weights for each increas- ing step is minimal and stays around 10% for each 0.25 increase in stiffness ratio. At the same time the expansion in spac- ing between steps of stiffness ratio remains similar, as can be seen from the orange line in Figure 7. As the stiffness ratio of a blast is increased from 1.5 to 1.75, the spacing of a blast expands approximately 3%. To further increase the spacing from 1.75 to 2.0 the spacing increases by 2.75%. While the spacing increase is slightly smaller at each increasing increment of stiffness ratio the total change is minimal, especially com- pared to the change in explosive weight. Perhaps the most simplistic way to view these changes is through what the authors have developed as a Spacing En- ergy Factor (SEF). This is a standardized method to view the changes from Figure 7 compared to the stiffness ratio. The SEF is a comparison of the change in weight to Figure 6—Borehole utilization to stiffness ratio. Figure 7—Charge weight and spacing changes based on stiffness ratio.

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