Coal Age

MAR 2019

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March 2019 27 blasting continued the Russian engineer Kusnetsov completed a decade-long study try- ing to develop this relationship and determined that no such powder factor existed because just changing small variables would cause large changes to the fragmentation. It has been known since the early 1600s that one of the most import- ant variables in the blast design is the line of resistance, or what we call the burden today. Throughout the late 1940s and early 1950s, dozens of empirical equations were devel- oped by various researchers and ex- plosive companies to determine this burden based off of borehole pres- sure, modifications to the Haussler Equations, weight of explosive, and many more variables. The problem was that while they understood the spacing was also extremely im- portant, methods to determine the spacing did not readily exist due to the variations of spacing with bench height. Powder factor design was then used to fill the gap by setting an arbitrary powder factor, and after designing the burden, stemming, and subdril, the spacing would be adjusted to match this arbitrary powder factor. This design technique made an incorrect assumption that the rock would exhibit the same breakage and the blast would behave the same if the same powder factor is used, which was known to be false even in the 1870s. This was a de- sign technique based off of ease of design and experienced-based guessing at values. Modern design methods exist today to determine each blast-design variable based on the performance desired in a blast. The main problem in using any de- sign methodology based on powder factor (including energy factor) is that as multiple variables change in the blast, the powder factor wide- ly changes. This is not taking into account in powder factor design and the end result is the develop- ment of a poor spacing that creates problems in the blast, such as large boulders and fines and reduces the throw of the blast. A few of the variables that dramatically impact powder factor are the rock mass, the rock matrix, the charge dimension (which is influenced by the bench height and charge diameter), the hole-to-hole timing, the row-to-row timing, the stemming material, and the configuration of blast. For the remainder of this article, any numbers indicated with pow- der factor will be considered in lb/ yd 3 and next to it will be the met- ric-equivalent kilograms per cubic meter (kg/m 3 ) in parenthesis. An ex- ample is where a powder factor of 1 lb/yd 3 will be listed as 1 (0.59). Rock Matrix When looking at the rock matrix, the actual type of rock is of con- cern. It is easy to determine whether the rock is a sandstone or shale, but the difficulty lies in the deter- mination of what the exact powder factor should be for this rock. Since the 1940s, many have believed as the ancient blasters once did, that there was an exact number that could be used as base on the rock, or the rock's strength, to determine the total amount of explosive used. In the 1950s, Kusnetsov did more than a decade of research in Rus- sia with the attempt to determine the optimal powder factor for all rock types so that Russian mines could optimally blast with the best fragmentation. Kusnetsov tested multiple charges with variations to diameter, length, type of rock, type of explosive, stemming, etc. After a decade of study, he concluded there was no unique powder factor, but a range of powder factors. This range was not solely based on the rock matrix or the rock character- istics, but instead on all the other variables associated with the ex- plosive. This was for single-charge work, yet alone multiple charge blasts were spacing and timing are also of large concern. At best, a range of 0.75 (0.44) to 1.25 (0.74) was considered as an appropriate range for blasting. Weaker rocks are typically blasted between 0.75 (0.44) and 1.1 (0.65) and stronger rocks are blasted be- tween 1 (0.59) and 1.5 (0.89). The importance to design is extreme, es- pecially in the determination of the spacing where the change from 0.75 (0.44) to 1.25 (0.89) would change the spacing by more than 50% in many cases. Today, the powder fac- tor is still not defined enough for specific rocks that it can reasonably be approached as a design tool. Figure 1—Stiffness Ratio to Powder Factor based on the Independent Variable Konya Design.

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