Engineering & Mining Journal

SEP 2017

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Page 42 of 99

SEPTEMBER 2017 • E&MJ 41 BLASTING Managing Boulders Typically the V-Cut will eject a sizeable triangular wedge shaped boulder out from the blast, which is usually found posi- tioned in the center of room well out from the face and often intact. The rest of the holes throw a well fragmented muckpile slightly forwards due to the angled nature of the majority of the holes. Two methods exist for management of boulders from the center of a V-Cut and are critical if the initial V will have holes spaced apart for prevention of precompression and dead- pressing. The fi rst newer and preferred method is the use of buster holes, which are paral- lel holes that are put into the fi rst V cut to partial depth. These holes are normally fi red on the fi rst delay and can each be delayed from one another if vibration is a concern, as they generally will have larger vibration based on scaled dis- tance (pound per delay) than a normal hole would. These buster holes will radially crack the center portion of the face to reduce the amount and size of boulders after the shot. The second and tra- ditional method to break these middle boulders is through the use of Baby V's, which tend to produce less vibration than the buster charges. The Baby V is a smaller V that is placed in the middle of the main V and does not extend to full depth, but is fi red before the main V. This Baby V will then break out an area in the face in front of the main V, removing boulders that would be created in that zone. The Baby V can also allow for a farther face advance and lower vibration levels when properly de- signed into the main round. However, this method involves more holes and delays than the former and as such is the more expensive and complicated option. These boulder-busting techniques can also be used with some advanced vibra- tion control techniques such as signature hole and sacrifi cial hole techniques, as described by (Lusk, et al., 2006) to sig- nifi cantly reduce vibration levels if blast- ing near structures. Figure 2—Modern V-Cut design (Worsey, 2015). Figure 3—V-Cut design at Springfi eld Underground, with Ikon logger assigned delays (Lusk, et al., 2006).

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