The U.S. Bureau of Mines performed a comparative study of nine sites at eight surface coal mines to determine if the presence of near- surface underground abandoned workings resulted in the generation of adverse long-duration, low-frequency blast vibrations. Six of the nine sites had underlying workings, and two had thick layers of low- velocity unconsolidated surface material. Extended seismic arrays were used to identify the vibration characteristics within a few tens of feet of the blasts and also as modified by the propagating media at distances over 1 mile. Production and specially fired single-charge blasts allowed the determination of natural ground frequency and also the influence of the initiation delay timing. Vibration amplitudes from the production blasts at all sites exceeded historical norms, particularly at the greater distances. This contrasts with the near-normal results from single-charge blasts. Apparently, between-hole time delays were insufficient to separate vibrations from adjacent charges for the relatively low- frequency waves present. This suggests that the widely-used design criteria of 8 ms minimum delay separation may not be sufficiently long for sites with abnormal low frequencies. Both undermined and sites with thick unconsolidated overburden produced low-frequency surface waves. A similar paper by d. E. Siskind appears as op 232- 90.
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