Coal Age

JAN-FEB 2017

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40 January-February 2017 operating ideas Increase Blast Burdens with Bottom Charges A more energetic bottom could offer the same or better results by anthony konya and calvin j. konya, ph.d. A mine is typically faced with the problem of trying to decrease drill and blast costs while maintaining the same or better frag- mentation. This can be accomplished with continuous improvement, redesigning the physical dimensions or the timing, or hir- ing a consultant to recommend new meth- ods of blasting. While these methods can all be effective, one simple way to do this is by increasing the burden of the blast, which reduces both drilling and blasting costs. Then the question is asked, won't this bring about poor blasting results? It most definitely will if the burden is solely increased, however, using a more energet- ic bottom charge, the burden can be effec- tively increased with the same or better blasting results. The following methods are one way of achieving a larger burden for casting of overburden materials. Breaking the Burden The burden of a blast is defined as the perpendicular distance from the explo- sive charge to the nearest free face at the time of firing. While this has significance in terms of timing, the article will look at how the burden is specifically broken. When a blast is detonated, the burden breaks as a result of the enormous gas pressures (not shock waves) yet has distinct differences in the regions of breakage. The first region will be considered the top burden. This is where the stemming material is placed to confine the gas pres- sure. This area typically exhibits the least amount of fragmentation and results in large amounts of boulders. With minimal to no explosive load in this area, the bur- den in the collar of the hole is often less than the design burden because of over- break and breaks differently than along the explosive because there is no explosive in the collar. The second region will be considered the middle burden. This is everything in between the bottom of the stemming to where the bottom burden begins. This area typically exhibits the greatest amount of movement and fragmentation. The third region is the bottom burden, which is the area below the middle burden to the grade or bottom of the bench. This area can frequently leave toe above grade at the bottom of the blast. Typically, a primer is placed in this area to increase the likelihood of pulling the toe and breaking to the desired grade. In addition, benches are typically subdrilled as much as 30% of the burden to increase the breakage and movement of this bottom region. Have you ever stopped to consider why we subdrill to 30% of the burden? This subdrilling rarely affects the middle or upper portions of the bench, but one knows from a practical standpoint that it does help decrease the toe and increase the movement of the bench. Have you ever instead of subdrilling tried to de- crease the burden? This would add sig- nificant cost to any operation, but would achieve similar results. This is because the bottom of the bench, or the bottom burden, is what most of the blast design is based on. The first sign of excessive bur- den will be at the toe. Bottom Charge In most blast designs, it is assumed a con- tinuous charge will be used throughout the borehole and the entire design evalu- ated based on whether toe is pulled or not. The blaster wants to be sure that the bot- tom of the bench is broken and pulls, and that a toe is not left behind. In this view, one normally designs an entire blast based on the bottom of the blast; however, the bottom of the blast rarely has an effect on the middle or upper area. This is where the bottom charge concept begins to apply, that a stronger bottom charge would allow for the expansion of the burden, and this Figure 1—Zones of burden through borehole view. Figure 2—Middle of bench bulging and moving. Figure 3—Bottom charge configuration.

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