Quantification of Rock Damage from Small Explosions and its Effect on Shear-Wave Generation: Phase I - Homogeneous Crystalline Rock

摘要

Weston Geophysical Corp., New England Research, Inc., and several geotechnical consultants conducted the New England Damage Experiment (NEDE) in central Vermont during July 2008. A series of five explosions using charges having 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), ranging from 0.5 to 8.1 km/sec, and then relate the shear wave generation to the amount of damage. Increased VOD causes increased borehole pressures, which exceed the rock's compressive breaking strength, and results in more extensive fracturing of the source rock. This zone of highly fractured rock prevents the explosive gases from being able to drive long fractures. In contrast, slower VOD explosives generate fewer fractures around the borehole, but the explosive gases are able to enter the cracks and drive long fractures. Surface observation of the test site confirmed this as no fractures were found for the fast VOD explosive, multiple small cracks were seen for the middle VOD explosive, and large fractures with surface displacement were found for the slowest VOD explosive. We have also used pre- and post-shot core analysis to quantify significant differences in the damage induced by the explosions. The extent of damage can be characterized by determining rock properties. Laboratory measurements of ultrasonic velocities, permeability, resistivity and porosity are sensitive to the microcrack density in the granite. 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.