Engineering & Mining Journal

JAN 2019

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

Issue link: https://emj.epubxp.com/i/1070726

Contents of this Issue

Navigation

Page 32 of 59

BLASTING JANUARY 2019 • E&MJ 31 www.e-mj.com 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.

Articles in this issue

Links on this page

Archives of this issue

view archives of Engineering & Mining Journal - JAN 2019