Rock Products

JAN 2019

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Page 42 of 101 ROCK products • January 2019 • 39 Scenario One: Borehole Diameter = 3.5 in. Burden = 7.5 ft. Spacing = 10.25 ft. Bench Height = 30 ft. Scenario Two: Borehole Diameter = 7 in. Burden = 15 ft. Spacing = 21 ft. Bench Height = 30 ft. Table 1 - Percent Passing For Increasing Borehole Sizes Percent Passing Scenario One Scenario Two 10 percent 3.8 in. 3.9 in. 20 percent 5.8 in. 7.5 in. 50 percent 10 in. 16 in. 80 percent 18 in. 31.5 in. 95 percent 26 in. 56 in. As can be seen from Table 1, by doubling the borehole size and scaling the blast dimensions, or the other controllable variables, proportionally the fragmentation size significantly increases. This could have dramatic considerations for a mine that has oversize limits such as a primary crusher. Take for a moment a mine that has a 36-in. maximum size crusher, in Scenario One this mine would hardly ever have oversize. However, with Scenario Two the mine would have about 15 percent of material left as oversize which would require either rock picking or secondary blasting. This would dramatically increase the cost of the operation over just drill- ing additional boreholes at a smaller diameter. Stiffness Ratio One of the major problems with increasing the borehole diameter is that it will change the mechanics of the blast due to the stiffness of the blast. When a bench is long, compared to the burden, the blast has horizontal displacement and properly bends out toward the free face. When a bench is short, compared to the burden, the blast has a significant amount of vertical displacement and is cat- egorized as a violent blast. These blasts are not random but are decisions that are made in the blast design process, the long bench is breaking with the borehole effect and will have good throw, good fragmentation and lower ground vibration. The blast that is breaking through a vertical component is breaking through a cratering mechanism and will have worse fragmentation, bad throw, and much higher ground vibration. The way this is quantified is from the stiffness ratio and with proper design the stiffness ratio will be optimized to give proper breakage mechanisms to ensure proper fragmenta- tion sizing is achieved. The stiffness ratio is the bench height (L) divided by the true burden (B). The table below (Table 2) discusses the effects of the stiffness ratio: Table 2 - Stiffness Ratio Relationships Stiffness Ratio Fragmentation Throw Ground Vibration 1 Bad None Very High 2 Poor Minimal High 3 Good Good Good 4 Excellent Excellent Minimal Once a benches stiffness ratio has increased to four or greater, another additional benefit is that the borehole spacing can be expanded further. This is due to the relationships between bore - hole interaction and bench height. In fact, as the stiffness ratio increases from 1 to 4, the spacing is increased as well. This not only leads to consistent powder factor as bench height increases, but it is also much more economical to have tall benches. While no additional benefits to cost, fragmentation, throw or environmental factors exist for increasing the stiffness ratio above four, it can be done and the bench suffers no problems. Next, three further scenarios will be examined this time keeping the borehole diameter the same and changing only the bench height to show the difference in stiffness ratios: Scenario One: Borehole Diameter = 3.5 in. Burden = 7.5 ft. Spacing = 10.25 ft. Bench Height = 15 ft. Table 3 - Stiffness Ratio Adjustments Percent Passing Scenario One (SR = 2) Scenario Two (SR = 3) Scenario Three (SR = 4) 10 percent 3 in. 3.8 in. 3.8 in. 20 percent 5 in. 5.5in. 5.8 in. 50 percent 11 in. 11 in. 10 in. 80 percent 22 in. 19.5 in. 18 in. 95 percent 35 in. 30 in. 26 in.

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