412 Safe Blasting Formula from Bulletin 656

When seismographs are not used to monitor ground vibrations from a blast, Bulletin 656 recommends the use of the "safe blasting formula."

where D = safe blasting distance from structure to blast hole, K = scaling factor, and W = charge weight per delay (in equivalent dynamite).

According to Bulletin 656, to insure that 2.0 in/sec is not exceeded, a scaling factor K = 50 ft/lb1/2 is recommended.

As was noted before, it is the "S" waves, or surface shear waves that transmit the most blast energy through the ground and cause the most damage. These waves can be imagined as concentric rings traveling along the surface of the earth, similar to surface waves in a pond. Because of this, the intensity of the peak velocity ground vibration diminishes with the square of the distance. In other words, if the peak velocity is no more than 2.0 in/sec at 100 feet, it will be no more than 0.50 in/sec at 200 feet.

where I = peak velocity intensity of vibration, k = arbitrary constant, and r = distance from blast hole to location of structure.

Using Equations (x) and (xi), it is possible to determine from the blasting logs whether the blasting was harmful to nearby structures even if seismo-graphic monitoring was not done.

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