Engineering & Mining Journal

AUG 2018

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BLASTING AUGUST 2018 • E&MJ 35 www.e-mj.com materials was to the density of those ma- terials, the more energy from the shock wave was transferred. This became known as the acoustical impedance mismatch theory and the goal was then to have the impedance (density times the sonic ve- locity) of the rock match the impedance (density times the detonation velocity) of the explosive to maximize the energy of the shock wave going into the rock. However, this theory had a major prob- lem — rock and explosives have similar velocity components, but the density of rock is normally two to five times that of the explosive. The impedance could never be equal and would rarely be above 40%- 50% with minimal difference between ANFO and water gels. When this issue was raised, a very smart salesman coined the term "veloc- ity matching" and stated that the density was causing all the problems in the im- pedance mismatch and that looking at the velocity alone was a better approach. From a technical basis, this has no val- ue, even if shock breakage was truly the mechanism for rock breakage. But, this now gave the powder salesman a tool to easily sell higher velocity, more expensive water gels, emulsions, and dynamites in- stead of ANFO. This led to greater profits, but was terrible for the industry as it dra- matically increased costs for no reason. Shock Breakage Let's look at a few concrete examples to show how this has no bearing. First, let's discuss shock breakage and some very simple principles associated with it. To begin, let's look at a decoupled charge (such as packaged emulsion) in a borehole. Now the theory of impedance mismatch states that mediums with different impedances will have differ- ent coefficients of transmission for the shock wave. When the explosive fires, it is in direct contact with air, therefore the shock wave of the explosive would have to move into the air. The problem is air has a dramatically lower acoustical impedance than the explosive, so more than 99% of the shockwave is lost al- ready going from the explosive into the air. Let's assume that the less than 1% remainder could break rock, if it reached the free face. After it goes into the air, it then has to go into the rock, again having a reduction in total shock wave pressure by about 99%. This leaves al- most no shock-wave pressure starting in the rock. Additionally, the shock-wave pressure also reduces exponentially as it travels through the rock. In the end of this example, less than 0.01% of the to- tal shock-wave pressure would reach the free face, which would put it well under the tensile strength of the rock. What does this mean? If the shockwave theo- ry was correct a decoupled charge could never be used in a borehole. Let's look at another example, in the past, it was stated that when presplitting the collision of shock waves between bore- holes is what would cause the presplit. This was put into the DuPont Blasting Handbook, which was then bought by the International Society of Explosive Engi- neers, and even in the 17th edition of the handbook, still exists today. DuPont knew this was wrong in the 1970s and readily admitted to it, seeking to find authors to rewrite the section on presplitting. How do we know this to be false? A good starting point is cap scatter. Throughout history (with the exception of very modern electronic caps), caps typically fired with- in 10% of their rated firing time, meaning a cap rated to fire at 100 milliseconds (ms) would fire between 90 ms and 110 ms. Now when these caps where used with presplitting, the scatter time would cause each borehole to fire at a different moment in time. What if these boreholes fired just one millisecond apart — in this time, the shockwave from the first bore- hole would have moved about 150 feet, leading to no interaction of shock waves between boreholes. According to the the- ory, this would cause no presplit to form, yet presplitting has been done for decades and continues to be done with nonelec- tric caps. In addition to this, the authors have tested numerous methods of pre- splitting, including Precision Presplitting (should not work under this theory do to decoupling) and presplitting with propel- lants (no shock wave created) and all have worked perfectly, in many cases better than with high explosives. Finally, let's look at a very real example of how the impedance theory and velocity matching miserably failed in full-scale. Back in the 1960s, ANFO was becoming a big deal as it was extremely cheap and, by using coal fines and fertilizer grade prill, mines could make their own blasting agents. ANFO was invented by Bob Akre in Indiana and quickly made its way to the Minnesota Taconite (iron ore) Range. Taconite is one of the densest and highest velocity rocks on Earth. According to the velocity matching theory, ANFO would not be able to break it and an expensive slurry or dynamite would have to be used. This was one example given by explosive com- panies — except the taconite miners were laughing at the explosive industry as they shot millions of pounds of ANFO each day and it broke their taconites perfectly. So why did these theories persist? Well it was one of the best sales tools that had been discovered in the explosive industry. However, in the long run, tech- nical information beat down the sales — after costing mines millions of dollars. Today, almost no one discusses velocity matching and those who do are the last die-hard proponents that made their ca- reers using it as a sales tool. Today, the selling of more expensive products is left for other methods. Instead, a new sales tool has emerged that distorts technical data as a way to sell much more expen- sive electronic caps. Electronic Caps Now to preface this, both authors recom- mend and believe electronic caps have many benefits from the reduced cap scat- ter. In certain vibration situations, they are extremely good tools that can give a site a lot of options with blasting pro- grams. However, the following points are not true about electronic caps: • By reducing cap scatter, fragmentation will improve. This is only partially true because while accuracy is important, selecting the proper time delays for hole firing is the major import factor. According to the velocity matching theory, ANFO would not be able to break it and an expensive slurry or dynamite would have to be used. ...miners were laughing at the explosive industry as they shot millions of pounds of ANFO each day and it broke their taconite perfectly.

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