PERMFILE67045

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Chapter 26
Original Face
pick In Motion

II ~ '-Z_
D=distance between holes.
T = tlelay time between holes.
C=sountl velocity In air.
Figure 26-J. This drawing shows elmulated Waveforms I, II antl Ill Imm a bench b4st
where rate of progression along the face Ie leas than the velocity of sountl In the air.
the time between the individual pulses is equal to the time delay
between holes. In the direction of waveform III the pulses are com-,
pressed because of the Doppler effect, while in the direction of
waveform I the pulses are spread out for the same reason.
When D/'I' is equal to C, a strong air blast is produced in line with
the face as the contributions from all holes aze additive. When the
holes aze fired instantaneously (T = 0 and D/T is infinite), a strong air
blast is formed directly in front of the face as illustrated in Figure 26I
(a). In a similar manner superposition occurs in a particulaz direction
whenever DPI' exceeds C.
Another way that beam forming may occur is shown in Figure 26-K.
In this case detonating cord is deployed so that the air blast from the
diagonal trunk lines may superimpose in the direction of point B.
With 17 millisecond del a}' connectors and a spacing and burden of 26
feet, the effective velocity of blast progression from A to B is 1,082
feet-par-second. This matches the velocity of sound in air at about 30
degrees F.
Beam forming is not restricted to charge arrays, but may occur in a
single charge when the length-to-diameter ratio is large. For example,
the air blast produced from a length of detonating cord is stronger otT
the side than off the end of the cord. Also, beam forming can occur in
bench blasting if the length of the charge is large compared with
the burden.
Atmospheric Effects. Atmospheric conditions may affect the inten-
sity of noise from a blast at a distance. These conditions determine the
speed of sound in air at various altitudes and directions. The speed of
sound in turn is determined primarily by two factors: (1) temperature;
and (2) wind speed. At sea level the approximate speed of sound in air
can be determined from the equation:
C = 1,052 + 1.1T + 1.SV
Vibration and Air Blast
Where C is the speed of sound in air in feet-par-second,
T is the temperature in degrees F,
and V is Lhe wind speed in miles-per-hour.
Assuming the wind speed is zero, the sound speed varies from 1,052
to 1,151 feet-par-second as the temperature changes from 0 to 90
degrees F.
Normally, the air temperature decreases as the altitude from the
earth's surface increases. This change is known as the adiabatic lapse
rate and amounts to 3.5 degrees-F-par-1,000-feet. If the temperature
remains constant with altitude, an isotherm exists, while an inversion
exists if the air becomes warmer as the altitude increases.
Isothermal air and zero wind velocity would produce isovelocity
conditions which would result in the straight ray paths and spherical
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438 ~ 439
rigure 26-K. Beam /orming with tletoneting cord end ertay may occur because effective
velocity of array from A to B Is 1,062 feet-per-aewnd, which is clone to the velocity
of sountl In air. Moles are 26 feet apart end "x'e" Intlicate 17 MS Daley Connectors.