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FEATURE rock face, and, in a chevron patter, the holes were indi‐ vidually detonated. The holes with stemming plugs were first loaded with 13.5 in. of crushed stoned stemming, followed by a 6‐in. diameter plug and then filled the rest of the way with stemming. The rest of the holes in the pattern where loaded with crushed stone stemming only. The consultants performed a post‐blast analysis using a 1,000‐frame‐per‐second high‐speed digital video cam‐ era. A portion of the holes in the center of the blast were identified with signal indicators, which flashed when the hole was fired. The precise firing time of the detonation signals were recorded by the high‐speed video. In addition to showing actual firing times, analysis of the high‐speed video provided visual references of gas re‐ tention time, stemming ejection times, the apparent ini‐ tial burden movement vertically and overall vertical throw of the blast event. The below table identifies the above times relative to the starting detonator. The time between the signal flash and the first visible movement of the surface as stemming ejection or surface swell is referred to as Retention Time or Tmin. (Figure 1) Analysis of the time between the signal flash and first visible sign of surface movement showed that the holes with the stemming plug exhibited a 12.75 ms gas reten‐ tion time, 216 percent longer than those with stemming only. Fragmentation data were also gathered and analyzed using a digital analysis system. The merged fragmenta‐ tion data revealed that the stemming plugs produced a slightly higher degree of rock fragmentation with more uniform size distribution. The stemming plugs delivered a 2 percent reduction in the average mean size of rock from 4.88 to 4.79 in. and an 8 percent decrease in the D90 (90 percent passing) screen size from 9.14 to 8.41 in. "Improved f ragmenta‐ tion and uniformity of aggregate material yields quicker excavation and hauling times, more efficient crusher throughput and lower crushing costs," said McClure. FIGURE 2 Testing included a series of five analyzed production blasts. Similar results were found at Pennsylvania Site 2. Test‐ ing included a series of five analyzed production blasts. Blast numbers one and five included only stemming ma‐ terial to establish a baseline, while blasts two through four incorporated stemming plugs. (Figure 2) For this series of tests, five rows of 6‐in. diameter bore‐ holes were drilled on a 12‐ by 12‐ft. parallel pattern to a bench depth of 68 ft. To ensure proper toe burden di‐ mensions, set‐back markers were placed prior to the detonation of each blast. For comparative data integrity, the five production blasts were symmetrical to one an‐ other in terms of geometry and loading parameters. The holes were loaded with a 40 percent bulk emulsion blend and detonated with electronic detonators. The front row holes of each blast were stemmed with 12 to 15 ft. of drill cutting, while the rest of the holes were stemmed with 5 ft. of cuttings. Again, high‐speed video was taken of the blast to deter‐ mine the amount of time elapsed between the explosive detonation and the vertical heave and gas venting above the borehole. Digital still im‐ ages were taken during ex‐ cavation procedures at set locations throughout the re‐ sulting muck piles to ensure the merged analysis findings were representative of the FIGURE 1 The table identifies the above times relative to the starting detonator. 28 ROCKproducts • OCTOBER 2011 www.rockproducts.com