Page 269 - Civil Engineering Formulas

SOIL AND EARTHWORK FORMULAS 203

Stemming = 10 ft

Nearest free Top of explosive column
face
5
2 to 1
10
5
FIGURE 8.2 To compute the stemming for a side hill cut, use
a right triangle, as shown here. (Hemphill-Blasting operations
McGraw-Hill.)

Air Concussion
Air concussion, or air blast, is a pressure wave traveling through the air; it is
generally not a problem in construction blasting. The type of damage created by
air concussion is broken windows. However, it must be realized that a properly
2
2
set window glass can generally tolerate pressures of up to 2 lb/in (13.8 kN/m ),
whereas a wind of 100 mi/h (161 km/h) produces a pressure of only 0.35 lb/in 2
2
(2.4 kN/m ).
Air concussion is caused by the movement of a pressure wave generally
caused by one or more of three things: a direct surface energy release, a release
of inadequately confined gases, and a shock wave from a large free face (see
Fig. 8.3). Direct surface energy release is caused by detonation of an explosion
on the surface.
Air concussion is also referred to as “overpressure,” that is, the air pressure
over and above normal air pressure. The difference between noise and concus-
sion is the frequency: air blast frequencies below 20 hertz (Hz), or cycles per
second, are concussions, because they are inaudible to human ears: whereas
any air blast above 20 Hz is noise, because it is audible.
Overpressure can be measured in two ways: in pounds of pressure per
2
square inch (lb/in ) or in decibels (dB). Decibels are a measurement or expres-
sion of the relative difference of power of sound waves. Pounds per square inch
and decibels can be made relative by the following equation:
p
overpressure (dB) 20 log (8.68)
p 0
where overpressure (dB) overpressure in decibels
log the common logarithm
p overpressure in lb/in 2
–9
p 3 10 lb/in 2
0