Coal Age

MAR 2019

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26 www.coalage.com March 2019 blasting Forget Powder Factor for Blast Design Design should be based on economics and blast performance, not an arbitrary range of weight of explosive per volume of rock by anthony konya and dr. calvin j. konya When explosives were first used in 1627, blasters and engineers began looking at methods to better design blasts for their specific environ- ment. At this time, there was little understanding about the various effects of explosives, and a majori- ty of the blasting conducted at this time was the duplication of pat- terns that were working in other mines with minor modifications, which were supplied throughout the underground mines of Hunga- ry by the ministerial councilor and the engineer who developed the blasting, Caspar Weindl. Eventually the Hungarians began implement- ing new means to control the blast, such as drilling machines in 1683 to drill deeper and faster, stemming in 1685 to confine the gas pressure to advance farther per blast, and pa- per cartridges to package the black powder in 1689. During this time, they also sought to develop better methods for designing these blast variables, such as the distance between bore- holes, the amount of stemming to use, and the total powder load. This started the development of pow- der factor, which was nothing more than the total pounds of explosive used per cubic yard of material blasted (or kg/m 3 ). Working off pre- vious patterns, these blasters and engineers would use experience to determine how hard a rock was and what was an appropriate amount of explosive to use to break through this rock mass. This method of powder factor design worked much better than the previous methods of adjusting old patterns and as the technology of blasting spread from Hungary to Germany, England, and the rest of the world, this method of design also spread. At this time, there was no way to measure fragmentation, heave, ground vibration and air overpressure. There was no way to study the rapid expansion of explo- sive gases and its effects on rock. It is also nearly impossible to tell when a blast has improved in frag- mentation by even 10% with just the naked eye, so blasters and engineers could not optimize patterns, but in- stead sought methods to ensure that patterns would function and break the rock. New explosives were developed and new powder factors were dis- covered, but science of blasting barely progressed from the late 1600s when it left Hungary to the late 1800s when new research began. From 1870 to 1914, a tremendous amount of research was conducted on blasting and new design methods were being invented from the study of blast mechanics. Methods of opti- mizing blasts and improving the eco- nomics and performance of blasts were coming at record pace and the use of powder factor as a design tool had practically disappeared, leaving almost no trace in the literature as new design techniques came into existence that worked much better than powder factor. Following World War I and World War II, the explosives industry was decimated, as from 1914 to 1945, there was nearly no major work completed in explosives, and the knowledge from the 1870s onward was nearly completely lost to the world. Blasters and engineers be- gan anew, starting with the basis of powder factor as a design tool based off the blaster's experience. It is from this day that powder factor design still looms in the industry today after it had been eliminated as a design tool. It is time to leave powder fac- tor design to the 1700s blasters and move into a new age where design is based off of economics and per- formance of a blast, not an arbitrary range of weight of explosive per vol- ume of rock. How to Design With Powder Factor Powder factor design begins with the assumption that a certain amount of explosive put into a specific volume of rock will gener- ate breakage. While this is true, the range of explosive that can gener- ate breakage is large. For surface mining and construction, it varies between 0.5 lb/cubic yard (yd 3 ) to 2 lb/yd 3 . This is a wide range because explosives are particularly effective at breaking rock, so with an optimal design, powder factor can be very low. However, many different things can influence the powder factor of a blast. Every design variable selected will impact the powder factor. Typi- cally, a poor design will raise powder factor to large levels because more explosive energy will be required to overcome the strength of the rock. In addition, the desired breakage and throw of material is important in determining the powder factor. This will vary based on the type of rock. In the late 1940s, it was be- lieved that a specific powder factor would be appropriate for every rock and one could correlate the powder factor and rock strength to deter- mine the exact fragmentation from a blast. Throughout the entire 1950s,

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