Engineering & Mining Journal

JAN 2019

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Page 29 of 59

BLASTING 28 E&MJ • JANUARY 2019 Since the first application of black pow- der for blasting in 1627, blasters and en- gineers have been using powder factor as a design tool. Powder factor is the ratio of pounds of explosive used in a blast to the total volume of rock blasted; a typical range for surface blasting is 1.0 to 2.0 pounds per cubic yard (lb/yd 3 ). During the first few centuries of blast- ing the thought process was that each rock, depending on type and structur- al geology, had a specific powder factor that when achieved would give optimal fragmentation. After equations were de- veloped to determine burdens, the blaster could then isolate the powder factor as the key performance indicator and adjust the spacing to achieve the proper value. This design approach allowed many blasters to begin to standardize specif- ic patterns to achieve approximately the right powder factor. This is where patterns such as equilateral triangle patterns orig- inated, giving the blaster an easy method to lay out a blast with minimal calcula- tions. Under this design approach a blast- er would not have a definite spacing and the powder factor would slightly vary with bench height. This also led to the belief that benches that were two to three times the burden would be the lowest cost benches; this has since been disproved. As the bench height increased, and the spacing was held constant, the powder factor would increase along with the bench height. During this time blasters could not understand why this powder factor ap- proach worked in some situations, but in others it failed miserably. Geology must be to blame and that site was just different was the common excuse. In Europe in the 1970s, Kuznetsov, a Russian mining engi- neer, completed his powder factor studies. Kuznetsov received grants from the gov- ernment to study the 'ideal powder factor' for all rock types to achieve the best frag- mentation. His studies lasted for nearly a decade and, at the end of his studies, he determined that no ideal powder factor ex- ists, and many variables go into a blast to determine the fragmentation and the other outcomes of a blast. This was known in the USA well before the 1970s. One would then think since powder factor was debunked that it would not be used further. Powder factor and standard- ized patters, however, are still prevalent in the mining industry today. Not only are they commonly used, but these de- sign approaches falsely lead operators to increase operating expenses. This article will discuss why these design approaches fail and how modern approaches to spac- ing and blast design dramatically improve performance of blasts, and how blasts and entire mines should be designed to minimize total operation cost. The Physics of Spacing In the early days of blast design it was believed that a blast broke in an equal radius around a borehole and that de- pending on the powder factor of a blast the fragmentation and throw of a blast could be changed. This thought process led to the modification of spacing to achieve increased powder factors, and in many situations led to blasters having a smaller spacing than burden. As research increased it was discovered that spacing was an extremely important variable in blast design, and Richard Ash was quoted in 1963 saying, "It can be generally as- sumed that uniformity in sizing is a direct result of the spacing. If on firing a single hole the rock is satisfactorily broken and cleanly removed without excessive dis- placement, it may be assumed the burden is satisfactory. Too often then blasters re- duce the burden rather than extend the spacing in their desire to eliminate boul- ders or to make rock sizing more uniform." Since the 1960's it has been discov- ered that spacing is a critical variable that relies on a 4-D design approach which includes the physical constants of a blast and the timing of the blast. The spacing will also affect, to a large degree, the uniformity of the fragmentation, the fragmentation sizing, and the throw of the material from a blast. The spacing of a blast is determined by the burden, the bench height, and the hole-to-hole timing of a blast. Should the spacing be too small for the timing of the blast, the boreholes will not break into the Powder Factor: The Ineffective Design Tool More modern approaches to spacing and blast design dramatically improve performance of blasts By Anthony Konya, M.S., and Cal Konya, Ph.D. Figure 1—Spacing to bench height for various powder factors (example is with a 4-in. borehole).

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