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Chapter 26
t00mr
e. Recording taken 1750 ft. In front of face In 3 row blast
In a granite quarry. Peak air blest was 122 dBL.
OOms
b. Recording taken 650 It. behind face In 3 row blast
in a granite quarry. Peak air blast level was 129 dBL.
100mr
e. Recording taken 720 ft. In front of face in double row
blast In a limestone quarry. Peak air blast level was 133 dBL.
i I I 1 I I I I I I I I I I
~--+i 0.1 sec.
Figure 26-1. Blast recortlings taken from bench Dlasts In granite entl limestone quarties.
charges, the air blast usually contains a considerable amount of
energy at frequencies below 20 Hz. These low frequencies can on
occasion damage structures directly, but more commonly can excite
higher frequency vibrations, which are perceived as noise, in win-
dows, doors, and brit-a-brat.
436
Vibration and Air Blast
Noise is generally measured with A or C weighting scales. Exten-
sive experience has shown that hearing loss in humans from continu-
ous and impulsive noise can be correlated with measurements taken
on the A or C scales. However, these weighting scales strongly dis-
criminate against low-frequency noise. For example, a 50-decibel dis-
crimination occurs at 20 Hz on the A scale and 8 decibels on the C
scale. Consequently, these weighting scales should not be used to
measure air blast from confined explosive charges. For this purpose
linear or unweighted instrumentation is needed.
Figure 26-1 shows typical blast recordings taken with wide band
instrumentation (0.1-1.000 Hzl from bench blasts in granite and
limestone quarries. The recording in Figure 26-I (al was taken in
front of a three-row, 6,700-pound, 45-hole, flat V blast in a granite
quarry having a 38-foot face. The initial large peak was caused by the
simultaneous firing of four opening holes. The air blast from the other
holes in the front row was much smaller. There was no indication of
air blast associated with the firing of holes in the second and third
rows.
Figure 26-I (bl shows a recording taken 650 feet behind athree-row,
B;L00-pound, 54-hole, flat V blast on a 40-foot face in the same granite
quarry as the recording in Figure 26-I (a). The relatively smooth
waveform shows the absence of the high frequency energy present in
front-of the face. Consequently, the blast sounds quieter behind the
face. The wiggles on the record beginning before the arrival of the air
blast were caused by the sensitivity of the gauge to ground vibration
and by the compressional waves in air caused by the vertical compo-
nent of the ground motion. The large drop below the base line follow-
ing the positive phase may be partially instrumental.
The recording in Figure 26-I (c} was taken 720 feet in front of a
two-row, 3,700-pound, 28-hole blast using second-hole initiation in a
limestone quarry having a 34-foot face. The pulses from each of the 14
holes in the front mw are readily discernible. Again, there is no
indication of air blast from the firing of the holes in the second row.
This blast was very noisy because of the presence of sharp pulses rich
in high-frequency components.
•
Directional Effects. When a large number of neighboring explosive •
charges aze detonated with small time delays between them, the air
blast pulses from the individual charges may superimpose in a given
direction and produce a strong air blast.
Figure 26-J shows simulated waveforms from a bench blast where
the rate of progression along the face is less than the velocity of sound
in air. In this case beam forming in all directions is theoretically
avoided although in practice D/T should not be allowed to exceed
about 2/3 C because of the finite width of the individual pulses.
Waveform II in Figure 26-J is radiated at right angles to the Cace, and
437