ABSTRACT
Single-row bench blasts at a reduced scale in the field were used to study the effects of delay time on fragmentation. Nine shots with a spacing of 21 in. and a burden of 15 in.(S/B : 1.4) and nine shots with a spacing of 30 in. and a burden of 15 in. (S/B : 2.0) were completely screened to determine particle size distributions. Delay intervals between holes ranged from 0 to 45 msec, corresponding to effective delay intervals ranging from 0 to 36 msec per foot of burden. Each shot was instrument- ed with strain and pressure gages to measure in situ blast dynamics and to evaluate interactions between blastholes. Finer overall fragmentation was produced for shots with delay intervals between 1 and 17 msec per foot of burden. Only for shots within this optimum delay range was it observed that the strains induced by stress waves constructively interacted with strains induced by gas pressure from an earlier detonated hole. Coarsest fragmentation resulted at delay intervals less than 1 msec/ft, where stress waves from each hole were observed to interact destructively, and at delay intervals greater than 24 msec/ft, where no interaction between holes was observed, indicating a condition that can be considered as the firing of ingle hole shots independently.
INTRODUCTION
Manufacturers have recently improved the precision of pyrotechnic caps, and benefits of improved fragmentation have been reported (1, 2). The Bureau of Mines is currently conducting research on blast induced fragmentation of rock. Tests thus far have been performed in the laboratory and at reduced scale in the field and have mostly been concerned with the effect of delay time on fragmentation and the interaction between shot-holes. Although the research program will eventually include full-scale blasts, initial testing in the laboratory provided an effective means for establishing a methodology of controlled experimentation. The tests at reduced scale in the field per- mitted the screening of the entire muckpile to develop fragmentation assessment techniques and results to optimize the expensive full-scale field tests. This report discusses the reduced-scale field tests and results. The reduced-scale field tests were conducted at the University of Missouri's Experimental Mine in Rolla. This site was chosen for its accessibility and geology and for the cooperation available from the University. Furthermore, the results of previous research conducted at the mine on blast design and fragmentation were reported in several theses (3-5). Although these studies investigated various design factors affecting fragmentation, such as coupling, initiation sequence, primer location, and air gap, there were tests that provided a comparison to the Bureau test results.
SITE
The 45-in. bench of dolomite in which the experiment was conducted is part of the Jefferson City Formation. The rock is of irregular grain size, 10% calcite, and thick-bedded with a specific gravity of 2.65 and longitudinal and shear velocities of 14,800 and 8,100 fps, respectively (5). Velocities of the dolomite were measured in situ using accelerometers located behind the blastholes and determined to be 14,700 fps (longitudinal) and 8,100 fps (shear).
