Engineering & Mining Journal

APR 2018

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Page 50 of 91

BLASTING APRIL 2018 • E&MJ 49 one will be able to see this will cause a well-distributed curve to sway to large boulder and very fine material. Now take the opposite case where the two-adjacent borehole are 18 in. farther apart than designed. The spacing be- tween these two borehole is now 19 ft instead of 16 ft. This will also leave a jagged face and cause a large number of boulders between holes as the two bore- hole will not interact properly. What about a reduced burden of 18 in.? This will cause a scattered muckpile of finely broken rock, causing a reduced fill factor and increase in muck-haul costs, and an increase in blasting costs. However, this will cause a reduction of crushing and processing costs. What about an increased burden of 18 in.? This will cause a higher muck pile with larger boulders that will probably have the same fill factor for a shovel as the normal bur- den, but an increase in the crushing and processing costs. How can the drill location be moni- tored then? Multiple systems are available and are detailed in the table below with the system, accuracy and training time. A measuring tape can provide infor- mation on how far holes are apart, but serve almost no purpose in alignment of the holes, which is also of concern. Borehole Depth The next important part of checking the drilling system is to ensure that the drill has reached proper depth, and the proper depth is open at the time of load- ing. For example, the drill in this example should drill the full 40-ft bench plus an additional 4 ft (1.2 m) of subdrill. If the drill only drills a 40-ft hole, and no major bedding plane exists at this location, it is common practice to expect a toe after the shot that is a few feet thick in that spot. Now that would be the same assump- tion if the hole had 4 ft of material fall back into it, or would it? For this reason, it is important to not only document how far of a depth the drill has reached, but to measure the borehole immediately before loading and ensure that the full 44 ft of borehole is open. This is what is often missed, and is a simple addition in the process. How can one ensure that the borehole is always open? Predict what the typical backfill of a borehole would be between drilling and loading (assume for this ex- ample it is 2 ft), then have the driller drill that much additional depth. In this situation, that would now be drilling a 46-ft borehole. What if the blaster comes back and finds that it has not backfilled at all and is open to 46 ft? Simply fill in the bottom 2 ft with drill cuttings to bring the borehole to 44 ft. Borehole Inclination The final important aspect that must be checked is the actual path of the bore- hole. This is one of the most important aspects of the initial parts of the Six Sig- ma process. While there is importance in where the blasthole is placed and how deep it is, modern instrumentation now allows for very rapid data collection on the entire path of the borehole. This now enables an engineer to address all the concerns previously mentioned in the "Borehole Location" section throughout the entire depth of the borehole. Howev- er, this is an extremely complex process. Therefore, an engineer will typically take the bottom location of the borehole and use this in the analysis. The trace of the borehole will then be taken by the blaster, along with a 3-D profile of the face, then vary the loading procedures throughout the borehole to ensure the same amount of energy per burden distance is used. In this way, the engineer is not overwhelmed with data and can still use this informa- tion in analysis. As computer systems improve and the software with these borehole trackers improves, the potential exists for using these in a more sophisti- cated solution. Explosive Testing In addition to this, the explosive quality needs to be tested on each blast. The first part of this is the testing of the percent- age of fuel oil in the ANFO mixture. ANFO should ideally hold 5.7% fuel oil in the mixture, however, 6% ± 0.5% is an ac- ceptable limit. Traditionally, through the author's testing worldwide, the current Figure 3— Drilling errors make up a majority of the original problems with the blasting process and must be monitored and minimized.

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