Ground Vibration and Airblast Control
When an explosive charge is detonated, a large amount of energy is released. Correctly designed blasts use the majority of the energy to fragment and displace the rockmass. Improperly designed blasts waste some of the available explosive energy and produce excessive ground or air vibrations.
The key factor that controls the amount and type of blast vibration produced is explosive energy confinement. If the energy is over-confined then excessive ground vibrations can occur. On the other hand if the explosive energy is under-confined then excessive airblast levels may be produced.
One method used to control ground vibrations is to restrict charge weights based on the following scaled distance equation:
The scaled distance equation takes into account the distance from the blast to the point of concern and the amount of charge weight detonated during any 8 ms interval. The United States Office of Surface Mining (OSM) has the following scaled distance restrictions for coal mines (CFR 30 715.19, 816.67, 817.67) when a seismograph is not used to monitor the blast:
0 - 300 ft away - minimum allowable SD is 50
301 - 5000 ft away - minimum allowable SD is 55
over 5000 ft away - minimum allowable SD is 65
It should be noted that the scaled distance equation does not take explosive energy confinement into account and can be very restrictive for close-in blasting.
When ground vibrations are monitored with seismographs the ground motion is typically measured in terms of displacement, peak particle velocity and frequency. Particle displacement is the distance the particles move back and forth as the vibration wave goes by. The peak particle velocity refers to the maximum speed that ground particle moves and is typically measured in millimeters or inches per second. The frequency is the number of times the particle oscillates per second. Most regulatory authorities dictate specific allowable peak particle velocity levels. The OSM for example has the following peak particle restrictions for coal mines:
0 - 300 ft away - max. allowable PPV is 1.25 ips
301 - 5000 ft away - max. allowable PPV is 1.00 ips
over 5000 ft away - max. allowable PPV is .75 ips
When frequency is taken into consideration for vibration restriction the United States Bureau of Mines "Z" curve (from USBM RI8507) is typically used.
One way to predict ground vibration is to use the following formula:
V = K x SD ^ A
Where:
V = ground vibration or peak particle velocity (inches / sec)
K = site vibration modifying factor (such as confinement)
The K factor will typically vary with confinement as follows:
Under confined, K = 24
Normal confinement, K = 160
Over confined, K = 600
SD = scaled distance (distance away / (max. charge weight)^.5)
A = constant related to the vibration attenuation characteristics of the site, always a negative number (typically -1.6)
The following vibration attenuation curves are based on the analysis of thousands of vibration records.
VA Curve:
Most surface blasting will fall between the two limits shown above.
Excessive airblast levels are an indication of poor explosive confinement. Unlike typical ground vibrations, airblast levels often do not decay linearly with distance. This is due to the focusing effects of atmospheric conditions such as wind and inversion levels. The primary way to reduce airblast is to increase the confinement of the explosive charges and to orientate free faces away from the point of concern. Typical airblast restrictions are as follows:
Maximum Allowable Air Overpressure With Monitors Having Various Frequency Response Capabilities0.1 Hz or lower flat response / 134 dB peak
2.0 Hz or lower flat response / 133 dB peak
6.0 Hz or lower flat response / 129 dB peak
C Weighted slow response / 129 dB peak