Engineering & Mining Journal

JAN 2019

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Page 30 of 59

BLASTING JANUARY 2019 • E&MJ 29 burden to a major effect. They will instead interact with each other to a large degree, producing fines between boreholes and boulders in the burden of the blast. Should the spacing be too large for the timing of the blast, the boreholes will not interact with each other in the burden and instead large boulders will be produced between boreholes. In both of these situations backbreak will occur and a sawtooth effect will be visible on the subsequent face. Two old school design approaches ex- ist, one which is directly based on pow- der factor and the second which has its origins in powder factor. Both of these methods are still widely used today; it will be discussed as to why they are inappro- priate and are leading mines to a large in- crease in unneeded operational expenses. The first that will be discussed is the tra- ditional powder factor design approach. The powder factor design approach begins with the blaster choosing a goal powder factor. This typically comes from 'experience' and is oftentimes a guess at a level that the blaster is familiar with. A first pass may be a powder factor of 1.0 lb/ yd 3 . The blaster will then take into account the burden which is appropriate for the diameter of the explosive product and the bench height to design a spacing which will achieve this powder factor. For the example pattern, given in Figure 1, a 40-ft bench would yield a spacing of 13 ft. In many sit- uations, this is where the design will end. A good blaster may look at the results of a blast and say, well that material is too coarse and numerous boulders are in the burden of the muck pile perhaps a higher powder factor is needed. The blaster may then decide on a powder factor of 1.1 lb/ yd 3 and this would reduce the spacing from about 13 ft to approximately 12 ft. The next blast is fired; the material is still coarse and boulders are still prevalent. The blaster then reduces the spacing to 11 ft to achieve a powder factor of 1.2 lb/yd 3 ; the rock is still coarse and somehow more boul- ders exist than before. It is then assumed that the problem is not in the blasting, but in the geology and good fragmentation cannot be achieved. The actual problem, however, was that the 13 ft spacing was already too small and further reductions increased the problem. The problem was the approach to the blast design. The second old approach to blast de- sign was also based on powder factor. Specific patterns were created to achieve powder factors for certain scenarios. This would be a pattern such as the equilateral triangle pattern, where the blaster would lay out blastholes with a set burden in an equilateral triangle shape, giving a spac- ing of about 1.15 times the burden. This approach sets the spacing at a specific number and the powder factor slightly var- ies based on the bench height. Under this design approach the powder factor has a direct relationship with the bench height. The way this design is used in prac- tice today is that a blaster will set the spacing to some number that they are familiar with for that explosive diameter. In Figure 2, three different spacings are shown. In this case the powder factor is variable. Most blasters understand that firing a bench with a height that is less than 2.5 times the burden will produce poor results. This design approach then leads blasters to believe that the most economical blasts are those where the bench height is near that 2.5 times the burden, because that is the lowest powder factor on the graph. This design approach leads to large increases in cost to mines because the actual effects and mechan- ics behind a blast are not fully understood and in turn performance is poor. Today it is understood that both pow- der factor and set spacing patterns are poor design tools that misguide blasters and engineers. They also do not take into consideration the actual effects of chang- ing variables on a blast and rarely take into account final performance of a blast. When these tools do not work, most blast- ers and blasting companies will blame the geology of the site because these tools work elsewhere. These tools may work elsewhere within a certain range of Figure 2—Old design approach where ppacing is selected and powder factor varies (set patterns are used). Figure 3—Bench height to powder factor with modern spacing design.

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