2008-10-07_GENERAL DOCUMENTS - M2005071 (2)

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Approximate correction of this amplitude for spherical divergence and inelastic attenuation at a frequency of 6.5 Hz predicts that the amplitudes on site at a distance of 20 feet from the ripper would exceed 0.15 in/s and could be as high as 0.3 in/s. While extrapolation of these measurements back to the source is only approximate, my data are consistent with the much higher peak particle velocities observed at the quarry site. The new study shows that ground motions produced by the mining equipment reached 70% of that required to crack drywall in a home on the site (0.34 in/s). Vibrations capable of cracking drywall in a house are huge, and far exceed the amplitudes necessary to produce rockfall. These data verify that ripping of bedrock is a very significant source of seismic energy, as experts in rockfall risk have stated in the literature, and as I presented at the 2006 hearing. Dr. E. Hoek states in his book "Practical Rock Engineering" that the best way to reduce the risk of rockfall is to eliminate blasting and ripping of bedrock. The increase in vibration amplitudes by a factor of 10 times to 30 times over Mr. Revey's previous measurements obviously had no impact upon the DMLR assessment of rockfall risk at this site since the mining permit was issued without any further discussion. As in the previous Revey report, the magnitudes of ground motion are compared with those required to crack drywall in homes, and the conclusions then claim that rockfall risk is nonexistent. Yet the connection between cracking plaster and rockfall risk is never established! What is the conversion factor or mapping function that converts drywall cracking into rockfall risk? Why is cracking of drywall the only relevant parameter in rockfall risk? What ground motions are required for rockfalls to occur? How does the rockfall risk change as a function of ground motion? Why is the nature of fracturing in the cliff not relevant to assessing rockfall risk? None of these fundamental questions have been answered, and there is, therefore, no logical connection between the data presented and the conclusion drawn. The giant leap from the fact that measured ground motions will probably not crack plaster or drywall, to the conclusion that rockfall risk is nonexistent is completely illogical and unjustified! It is disturbing enough that a contractor would make such illogical conclusions, but it is especially disturbing to me that Mr. Sorenson with the DMG accepted such unfounded conclusions uncritically, and has overlooked the logical and technical errors in Mr. Revey's work. Mr. Sorenson and Mr. Revey have extremely limited experience in this matter, yet have placed themselves above recognized experts in the field by disregarding rockfall hazards simply because there is no blasting. Returning to the larger picture, our concerns for rockfall involved three sources; one is vibration induced rockfalls from ripping of bedrock close to the fractured cliff, the second is from rocks rolling off the 35 degree slope and cliff during ripping of the top of the ridge, and the third is from hydrostatic pressure in joints resulting from infiltration of