Investigation of Blasting Vibrations Above Abandoned Underground Coal Mines

摘要

The Bureau of Mines performed a comparative study of nine Indiana surface coal mine sites at eight different 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 mine workings and two had thick layers of low-velocity unconsolidated surface material. Vibration amplitudes from the production blasts at all sites exceeded historical norms, particularly at the greater distances. This contrasts with the near-normal, single-charge blasts. Apparently, between-hole time delays were insufficient to separate vibrations from adjacent charges for the low-frequency waves present at the sites. Single-charge tests showed that the propagating media produced low-frequency, ground-roll-type surface waves at nearly all sites studied. Large blasts at such sites could produce a heightened and unacceptable risk of vibration-induced cracks in nearby structures.