Rock Products

JAN 2019

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38 • ROCK products • January 2019 A n often-overlooked topic in drilling and blasting is the selection of the borehole diameter. Often this is decided on an economic basis where it is cheaper to reduce drilling by using larger boreholes with the understanding that an extremely large borehole can be problematic. However, the selection of the proper borehole size is the first decision that is made and will directly impact all other blasting decisions. The outcome of this decision will also heavily influence the fragmentation, throw and environmental considerations of a blast. Assuming bulk explosives are being used, this single decision may be the most important decision a mine can make for its drilling and blasting program; however, this is typically decided by a guess and check method to see if the diameter makes sense for the operating conditions. In many situations, quarries believe they must use a smaller diameter borehole and large metal/nonmetal mines that utilize large shovels to move material should use a larger borehole. This is not always the case and in fact the fragmen- tation is not directly tied to the borehole diameter. In blasting two types of variables exist, uncontrollable vari- ables and controllable variables. Uncontrollable variables are variables such as rock type, structural geology and govern- ment regulations. Controllable variables are those that are directly under the control of the operator, such as borehole diameter, burden and stemming. These sets of variables then decide the outcomes of the blast, such as the fragmentation and throw. The first controllable variable that is to be decided at a site is the borehole diameter. This borehole diameter will directly decide the burden, and the remainder of the blast variables are then decided based on the burden. The borehole diameter will then influence all other parts of the blast and must be of careful consideration, especially when one desires optimal fragmentation. Borehole Diameter The authors have consulted for numerous operations where the current borehole that is being used is presenting prob- lems for a mine, yet they are attempting to implement a larger diameter borehole. These mines believe that by imple- menting a larger borehole they will be able to alleviate the current problems. However, this is the exact opposite of what will occur because when a larger borehole is implemented, and the blast is scaled accordingly, the problems that the mine currently faces are exponentially increased. Additionally, some operations think that the problems will remain the same if the powder factor remains the same. This is also not true as powder factor is not an appropriate design tool and as a larger borehole is used the actual mechanics of breakage will change. The use of a larger blasthole with the same scaling parame- ters will result in courser, less uniform fragmentation; worse throw of material; larger ground vibration; larger air over- pressure; more flyrock; and increased backbreak. This can be thought of as a scaling effect, when a larger borehole is used the burden is scaled accordingly. Just as the burden is scaled and increased the outcomes of the blast are scaled and increased in a negative manner. Let us look at two scenarios where the fragmentation has been projected based on two different borehole diameters where all parameters of the blast, except for the bench height, were scaled for different borehole diameters. Drilling Selection of Borehole Diameter for Optimizing Fragmentation The Selection of the Proper Borehole Size Is the First Decision That Is Made and Will Directly Impact All Other Blasting Decisions. By Anthony Konya and Calvin J. Konya

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