Quantification of Rock Damage from Small Explosions and Its Effect on Shear-Wave Generation
Final rept. 15 May 2008-15 Jun 2009
WESTON GEOPHYSICAL LEXINGTON MA
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Weston Geophysical Corp., New England Research, Inc., and several geotechnical consultants conducted the VermontNew England Damage Experiment in central Vermont during July 2008. A series of five explosions using charges with yields of 135 and 270 lbs and three types of explosives were detonated in homogeneous, low fracture density granite. The goal of the experiment was to generate different amounts of rock damage around the source by using explosives with dramatically different velocities of detonation VOD, and then relate the shear wave generation to the amount of damage. We used pre- and post-shot core analysis to quantify significant differences in the damage induced by the explosions. Velocities are slower, permeabilities are higher, resistivities are lower, and porosities are higher in the damaged intervals. These results are consistent with a microcrack scale fracture population that is enhanced by the blasts. Over 140 seismic sensors were installed to record the blasts. Peak particle velocity PPV studies found that the fastest VOD explosive, Composition B, expended much of its energy at the source pulverizing the surrounding rock, while the middle VOD explosive, heavy ANFO, produced the largest PPV. Source scaling studies found the black powder shot produced seismic amplitudes up to an order of magnitude less than the ANFO and COMP B amplitudes above 5 Hz, but created Rayleigh waves similar in amplitude to those from the ANFO shot. The black powder shot produced larger Rayleigh and Love waves than the COMP B shot. The ANFO and COMP B shots generated similar amplitudes above 8 Hz, but the ANFO source Rayleigh waves were up to twice as large and the Love waves were up to three times as large as those from the COMP B shot. These results indicate that rock damage may be responsible for some of the differences in surface and shear wave generation.
- Safety Engineering
- Geology, Geochemistry and Mineralogy