PERMFILE67045 - Laserfiche WebLink

Home Browse Search

Chapter 26 velocity and frequency, ground motion of shorter duration is ge ally less disturbing to neighbors. • Eliminate or reduce hole-to-hole propagation between charg intended to detonate al different delay periods. Use explosiv such as water gels, which are much less sensitive to hole-to-hol6l propagation than dynamite. Hole-to-hole propagation occurs wheD the explosive charges or boreholes are only a few feet apart, ae ip' trenching, decked holes, or underwater excavating, or at larger? distances when blasting interbedded soft and hard layer ruck, suchF as coral or mud-seamed rock, that is saturated with water. • Use delay electric blasting caps or surface connectors to reduce the number of holes on a delay period when excessive vibration is caused by presplitting shuts. When using many regular, not millisecond, delay caps in large shttR~ or tunnel rounds, the usual limitations on chazge-weight-per-delay. period do not apply because of the appreciable variation in the firing times, as illustrated in the seismic recording in Figure 26-G. The first ~-- 0.1 sec. --~ Time Base Figure 26-G.7his Is a recording o/the verticalcomponentof ground motbn protlucad br 7th period "Acudet" Mark 5 Delay Electric electing Caps Ina 130-hole tunnel round. few periods, however, must be treated as millisecond delay caps be• cause of the smaller variation in firing times. Since the lower penode are normally used under conditions of high confinement, the special relationship shown in Figure 26-B should be used. AIR BLAST FROM EXPLOSIONS The air blast from an explosion is a compressional wave in air. It is produced either by the direct action of the explosion products from an unconfined explosive on the air or by the indirect action of a confining material subjected to explosive loading. Noise is the portion of the spectrum of the air blast lying in the audible range from 20-20,000 Hz, while concussion is the portion lying below 20 Hz. The peak overpressure of an unconfined charge of TNT as a function of scaled distance, R/W'^, is given in Figure 26-H. As with ground motion, the use of scaled distance (cube root scaling for air blast) permits the prediction of blast effects over a large range of charge weights and distances. The equation given in Figure 26-H is a simple power law equation, but it is applicable only at overpressures less than one psi (pound per square inch). This overpressure is approxi- Vibration and Air Blast 1.0 171 161 6 151 ~ .10 0 5 141 m a v Y O 1 .010 13 n 121 .001 111 10 loo loon Sealed Dlstenee Ibt ' 26•N. Air blest overpreawre as a function of tlistance and charye weight for the linetl and confined charges. P Is expressed in psi, R In leet, entl W in pounds. mately equal to that produced at the rock-air interface from a confined explosive charge at the normal burdens used in bench blasting. The normal range of overpressures for confined explosive chazges is 30-40 decibels less than for an unconfined charge of the same weight over the range of scaled distance from 10-30 feet/pound"'. For confined charges W is usually taken as the maximum charge-weight-per- delay-period, As an example in the use of Figure 26-H, if an unconfined, one- pound cast primer were fired in air at a distance of 40 feet, the scaled distance would be 40 feeUpound"' and the corresponding pressure from the graph would be one psi. Assuming isovelocity atmospheric wnditions (Figure 26-L), the same overpressure would result from the firing of 1,000 pounds of TNT at a distance of 400 feet. Measurement and Character of Air Blast. An accurate and complete measurement of air blast from explosive sources requires a sufficient band width to include all frequency components. Because of the large dimensions typically associated with the burdens on explosive R -t.x R m vat P = 92 ~ Wr r,,~'~ R In leet W In Ibe. G,y co 9~ ~~F H O e 0 C ~9 F. C o9 O °~ ~~1 y ~ ~~e 0 C,S. -30 tl0 REDUCTION '9 CF, -10 Oa REDUCTION • • 434 ' 435