When White Flame blasts on the mountain above Varney Grade School, there’s often a palpable shudder at Judy Justice’s home, about half a mile southwest of the mine. At Jackie Keck’s house, which is on the road up to the mine on the southeast side, things shake on the wall when the afternoon blast goes off. He’s been considering trying to videotape the movement. Keck did some blasting while in Vietnam and other stints in the military. He thinks the solution would be to do a series of smaller blasts, like sections, instead of one big blast. That way there would be smaller amounts of compression to dissipate.
Several people have filed complaints, and Justice kept a detailed log, which often says whether the blast was light or hard. As a condition of its permit, White Flame also had to seismograph the blasts, so there is an extensive record of air blasts and frequencies.
The problematic blasts often seemed the ones with air blasts above 115 dB. Harold Ward, one of the DEP inspectors for the mine, said that over the past few years they have found complaints start when blasts go over 116 dB.
Justice believes her home may be more susceptible to the low frequency ground vibrations because it is newer and built on solid rock. Its natural frequency could be closer to that of the blasts. Indeed, the frequencies of the blasts are generally in the 4 to 11 Hz range.
WHAT THE DATA SHOWS
We examined 134 blasts, of which 63 caused problems.
Air blasts were one factor. Of the 25 problematic blasts for which there was a seismograph reading, 20 were 115 dB or greater. Of the 71 that did not generate complaints, only 16 were large enough to trigger the seismograph. Of those only five had readings of 115 dB or larger.
Amount per delay was also a problem. Of the 42 blasts of more than 700 lbs./delay, 27 generated a complaint.
The frequency readings are quite revealing. Thirty-five of the problem blasts generated a frequency reading. And 28 of those were within the 4 to 11 Hz range that OSM has found to amplify the vibrations of a house. Only 16 of the non-problem blasts generated readings, and only 4 of those were with the susceptible range.
Continue to in-depth analyses of blasting problems in nine communities:
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