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true level of blast‐induced fragmentation. (Figure 3) As displayed by the tables below, the merged fragmenta‐ tion data showed the stemming plug post‐blast muck piles were composed of a higher degree of fragmented rock with more uniform size distribution. The difference in average mean size was 15 percent smaller by using the stemming plug. Surface swell above the opening hole indicated that the stemming plugs contained the expanded gases an aver‐ age of 17 percent longer than non‐plugged holes. "These findings suggest that the longer the energy is contained, the better the rock fragmentation," explained McClure. Mining Procedures At an Alaskan gold mine, consultants analyzed two sepa‐ rate field applications of the Rocklock stemming plug. In phase one, field technicians evaluated a 156‐hole pro‐ duction blast in which 48 holes near the center of the blast were loaded with 21 ft. of ANFO, the stemming plug and 1.25‐ to 1.75‐in. sized stemming. The other 108 holes received ANFO and crushed rock stemming. The 6.75‐in. boreholes were drilled to a depth of 33 ft. Technicians filmed the blast using a standard digital video camera from a vantage point above the blast site. Video analysis of the blast revealed that the portion of the blast loaded with stemming plugs contained the gas energy longer than the holes without the plugs. "The area of the bench containing the hole plugs exhibited a reduced amount of stemming ejection," said Bartley. Vertical swell of the muck pile from the plugged 48‐hole section was noticeably higher than the non‐plugged areas. "This suggests that the rock in this area received a more sustained gas pressure pulse," added Bartley. Testing phase two at the mine consisted of four single‐ hole test blasts, designed to compare and quantify the retention times of boreholes loaded with and without stemming plugs. Two boreholes were loaded with only crushed rock stemming, while the other two were fired using a combination of stemming and the stemming plug. The 6.75‐in. blast holes were drilled to a depth of 33 ft. and loaded with 21 ft. of ANFO. Detonation signals posi‐ tioned at the collar of each hole enabled consultants to determine the firing time of the booster. Both a digital video recorder and high‐speed digital video camera were used to film each blast. Analysis of the high‐speed video revealed the time (ΔT) between the initiation of the explosive column and the initial first movement of the overburden. Retention times for the series of test blasts were as follows. (Figure 4) FIGURE 4 Increased retention time suggests that the stemming zone of the hole could be reduced. The analysis revealed that the holes using the stemming plugs increased the retention time from 4 to 8 ms. The increased retention time suggests that the stemming zone of the hole could be reduced. "This would enhance the energy dis‐ tribution and reduce the amount of oversize typically generated in the overburden zone of the blast," con‐ cluded McClure. E Rick Zettler is president of Z-Comm, a company specializing in construction- and aggregate-equipment marketing, public relations and freelance writing. He can be reached at zcomm@mchsi.com. FIGURE 3 The merged analysis findings were representative of the true level of blast-induced fragmentation. 29 ROCKproducts • OCTOBER 2011 www.rockproducts.com