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Blast Initiation Delay and Vibration Frequency: A Surface Coal Mine Case History.

Paper in Proceedings of Second Internat'l Symposium on Rock Fragmentation by Blasting :8 pages
Vibration wave amplitudes and frequency from surface mine blasting are important influences on the response and potential cracking of nearby structures. The U.S. Bureau of Mines studied blasting vibrations in a midwestern coal mine that occasionally produces 4-hz surface waves in its production blasting and has had numerous complaints from neighbors. The mine and nearby town are underlain by abandoned underground coal mines at depths of 30 to 110 m (100 to 360 ft) below the surface. Blast vibration measurements at the site and analysis of mine and regulatory agency records indicated that the propagating medium was primarily responsible for the vibration wave characteristics, including low frequencies, long durations, and lower-than-normal attenuations of amplitude with distance. The observed low-frequency waves were consistent with predictive theoretical models of surface wave generation using the depths to the old mines. Blast designs also contributed to the vibrations problem. The complex multidelayed blasts generated vibration amplitudes up to three times those of same-weight-per-delay single charges. Because of these low-frequency, long-period waves, the widely adopted 8-ms minimum charge separation criterion may not apply at this site. Knowing how and when initiation timing can influence vibration character would provide the industry with a valuable tool for controlling and reducing vibrations.
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OP 65-89
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Paper in Proceedings of Second Internat'l Symposium on Rock Fragmentation by Blasting