CAVITIES AND GAS PENETRATIONS FROM BLASTS IN STRESSED ROCK WITH FLOODED JOINTS

Columnar, 13.6-13,000 kg charges of water gel explosive containing 30% aluminum were detonated in well-stemmed vertical holes in jointed and water-flooded shale under known tectonic stresses at depths of 23-137 m. Cavity volumes were measured and, for the shallower shots, permeabilities also. For shots at 23 m depth, a zone of uniformly increased permeability extended from 30 out to 123 cylindrical charge radii and merged into an outer zone of reduced permeability restorable by flushing fines from the formation. Similar results were obtained in other experiments, to be reported elsewhere, wherein patterns of single and paired charges at different depths also showed a wide zone of uniform fragmentation. These findings seem to fit the hypothesis that a pattern of gas-pressured radial fractures grew outward through the rock, preceded by shear deformation of outlying joints with concomitant sliding, abrasion of asperities to produce fines, and dilation. Following this, uniform fragmentation resulted from point load failures within that part of the dilated structure of blocks overrun and therefore compressed by the gas-pressured fractures. Calculations assuming cool gas-pressured fractures with about 1 10 the calculated cavity volume gave uniformly fragmented zones of the observed size. Explosion cavities at less than a critical depth gradually decreased in size with increasing depth and tectonic stress. They had about 64% of the calculated volume, suggesting temperature gradients equivalent to a hot gas-pressured fracture volume 4.5 times that of the cool gas-pressured fracture volume. When tectonic stresses exceeded a critical level approximately that required to cause compressive failure in the skin of an unsupported vertical hole, measured cavity volumes diverged from calculated ones and approached zero. The tectonic principal stresses, measured by hydraulic fracturing at 26-133 m depths at two locations 1.4 km apart showed high shear stresses have a linear relationship with the compressive stresses. The stresses in this vicinity may lie on the long-term failure envelope of the massif. © 1979.