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MAY 2017 • E&MJ; 45 www.e-mj.com OPERATING STRATEGIES with the charge placed at the bottom of the borehole. With proper stemming, the powder factor for this would be approxi- mately 100 g/yd 3 . This method of boulder breakage will result in signifi cantly less air overpressure than the surface charge method, with reductions of more than 6 dB possible. It can lead to increased fl y- rock compared to the surface charge but the boulder will break more fi nely. A modifi cation to this method would be to add water and completely fi ll the drill- hole. 'Water stemming' will then change the charge weight to around 8 g/yd 3 and the charge should be hung one-half the length of the drillhole. This method can greatly reduce the amount of air overpres- sure (200% to 400% from a normal bore- hole charge), minimize fl yrock, and break the boulder into four or more pieces. When boulder busting, a key parameter is the width to height ratio of the boulder. All the previous design criteria assume a "square" boulder, with width and height about equal. If the width of the boulder is greater than the height of the boulder, then the charge must be scaled to be larg- er than that for a square. This can be done by taking the ratio of the width to height, and multiplying that ratio by the charge. Stemming Charges Dealing with boulders by breaking them during the blast, and avoiding secondary blasting, provides many advantages in- cluding reduced cost, increased produc- tion, longer throw of material and reduced equipment needs. This can be achieved by putting an explosive charge of smaller di- ameter into the stemming material. While this will not cause the top burden to break like the middle or bottom burden, it will cause the top burden to break to a much greater degree than without the charge. In the past, the Livingston cratering technique was used to estimate charge depth of burial; however, this would lead to blow-out of the stemming and min- imal fragmentation away from the bore- hole. This is because the charge would crater and eject the stemming above the borehole. If it was fi red before the main charge, then the entire shot performance would be degraded. This method of breaking the top burden would also have minimal, if any, effect a few borehole di- ameters away from the charge. Designing a proper explosive charge and depth of burial requires an under- standing of bench blasting principles and rock breakage mechanisms. In or- der to maximize the amount of material broken and the distance material is bro- ken from the hole, the bench must be placed into the bending mode of fl exural failure. This can be achieved by placing the stem charge approximately 50% to 60% of the depth of the stemming. This charge will generally be a small, cartridge explosive product that has a diameter less than that of the borehole. The dia- meter of the charge is normally around 50% of the borehole diameter and the length of the charge should be around four to six times the diameter. The stem charge should be initiated at the same time as the main charge to prevent blow-out of stem- ming or throwing of the stem charge. This is accomplished with the use of detonat- ing cord from the top of the main charge to the stem charge. This simple method can then be used to easily achieve better breakage of the top burden and minimize or eliminate secondary blasting on-site. To refi ne this method further, the au- thors have developed a simple ratio that can be used in different situations. This is assuming that the amount of stemming above the charge is reasonable and the rest of the blast is reasonably designed. A good way to tell if the stemming is ad- equate is to video record the blast and analyze whether the stemming is eject- ed from the borehole.If ejected, it is not properly designed. The ratio for designing stem charges is below: Where: d h = Diameter of drillhole (inches) d st = Diameter of stem charge (inches) T = Total stemming depth (feet) T st = Stem charge depth (feet) Calvin Konya is the founder of Precision Blasting Services (PBS) and Anthony Konya serves as project engineer for PBS. www.idc-pbs.com The water stemming approach for boulder busting generally breaks a boulder into four or more fragments.