Effects of Accurate Delays on Fragmentation for Single-row Blasting in a 6.7-M(22-ft) Bench.
Paper in Proceedings of Second Internatl Symposiumon Rock Fragmentation by Blasting Keystone c :14 pages
The recently introduced improved-accuracy pyrotechnic caps have the potential for enabling blasters to design blasts with precision timing to tailor fragmentation to specific rock and site requirements. The Bureau of Mines conducted an experiment where six single-row blasts were shot in a 6.7-M(22-ft) bench of horizontally bedded limestone to study the effects of precise delays on fragmentation. Precise delays ranging from 2 to 48 ms between holes, equivalent to 1.1 to 26 ms/m(0.33 to 8.0 Ms/ft) of burden, were achieved using a sequential blasting machine with seismic caps. The shots were instrumented with dynamic pressure gauges and six- component strain gauges and filmed with high-speed cameras. Fiber optics were used to confirm firing times and measure detonation velocities. The blasted rock was sized using grizzlies and vibrating shakers. All the broken material from four shots and about one-third and one-half of the material from the remaining two shots was screened. The effects of preexisting fractures are discussed. Results are compared to those of recent Bureau tests at a reduced scale in a massive dolomite where at least a 40-pct increase in the average size resulted from the delay of 1.1 Ms/m(0.33 Ms/ft) compared to delays of 3.3 to 26 ms/m (1 to 8 ms/ft). Strain and pressure data suggest the improvement in fragmentation results from an interaction of blasthole stress wave fracture mechanisms with the previously detonated blastholes' late failure processes. The effect is predicted to last for delays as long as 56 ms/m (17 ms/ft) of burden. While cutoffs
Paper in Proceedings of Second Internatl Symposiumon Rock Fragmentation by Blasting, Keystone,c