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44 E&MJ; • MAY 2017 www.e-mj.com OPERATING STRATEGIES An explosive detonation releases a large amount of gas pressure in a borehole that, with proper design, is contained and forced to do maximum work horizon- tally away from the borehole. The main method to ensure the blast does not lift upward, causing minimal fragmentation and up to a 6 decibels (dB) increase in air overpressure, is through the use of proper stemming material and length. Stemming is a double-edged sword leading to better fragmentation of the face, but also with increased boulders from the top burden (see fi gure below) in the stemming zone of the blast. This can cause problems if the boulders are too large to be handled with the digging equipment or cannot be effectively cast with the overburden. There are many ways to break these boulders with explosives; however, it is always better to break the boulders in the blast or prevent these boulders from forming during the blasting process rather than having to break them in the muck pile after the blast. Secondary Blasting of Boulders Once boulders have formed, they can be set aside in the pit; however, this can result in lost production and rehandling costs throughout the life of mine. More often these boulders are broken up into smaller sized rocks that can be more eas- ily removed. This can be done with me- chanical or explosive means. In this arti- cle, explosive methods will be discussed. It is imperative to not only select the most economical form of breaking boul- ders, but also to ensure that regulations on environmental effects of blasting are not exceeded. Explosive fragmentation of boulders will require handling and possi- bly drilling, will generate air overpressure, and can result in fl ying debris. The surface-charge method of boul- der busting involves placing an explosive charge on top of the borehole. This gener- ally has poor breakage and requires up to 2 pounds (lb) or more of explosive per cubic yard (yd 3 ). This can generate signifi cant air overpressure affecting nearby structures and will produce some fl yrock around the boulder, breaking it into a variety of sizes. There are ways to improve the results of this method: One is in explosive place- ment and maximizing the contact area of the charge against the boulder. In the di- agram above, two examples are given of explosive placement on a boulder; the left charge has a much larger contact area and will result in signifi cantly better breakage than the right charge. This placement is typical for emulsions and dynamites; however, if using a cast booster to shoot, the right is a more typical placement. For this reason, when using a cast booster for boulder busting choose one with a very large diameter and short length. Another way to increase the breakage of a surface charge is to use a mudcap after placing the explosive charge, simply covering the charge with mud or clay to form a conical cap over the explosive. The cap helps to retain some of the explosive energy and direct it to breaking the boul- der. By using both improvement methods for surface charges the total powder fac- tor could possibly be brought down to 1 lb/yd 3 of boulder, depending on the shape of the boulder. Another method for breaking boulders is the drill and shoot method in which the boulder is generally drilled in approx- imately the middle with a borehole di- ameter slightly larger than the explosive charge. It should be drilled to one-half to two-thirds of the way through the boulder Breaking Boulders in Top Burden Stemming is widely used to ensure that a loaded borehole achieves optimum blast results, but the practice can leave large boulders unbroken in the upper layer of burden. The authors explore several proven methods for boulder-busting using explosives. By Anthony Konya and Dr. Calvin J. Konya Zones of burden through borehole view. Surface charge contact area maximization (left - proper, right - poor).