On the fracture process in blasting

In order to clarify the respective roles of stress wave and gas pressure in the fragmentation of an underground blast the fracture process in the zone immediately around the borehole was studied by separating the two principal blast forces analytically and experimentally. In model tests the explosion wave was simulated by the pulse generated by an underwater spark discharge, and the expanding combustion products by pressurized oil. The wave-generated radial fractures around the cavity were investigated in detail and the diameter of the fractured zone was found to approach six hole diameters for a spherical charge and nine hole diameters for a cylindrical charge. The stress field generated by the pressurized gas in this star-cracked cavity was shown to be identical to that of a pressurized and uncracked ‘equivalent cavity’ whose diameter is equal to that of the fractured zone. A very large region is therefore stressed by the gas, and consequently considerable crack extension can be expected. The influence of boundary conditions, i.e. preexisting fractures, static stress field, and close free surface, on the wave and gas-generated fracture pattern, was investigated in detail. Preexisting fractures grow to larger lengths than new ones and cause a fracture-free zone in their immediate vicinity. Cracks grow preferably in the direction of maximum principal stress of superimposed stress fields. Cracks pointing towards the free surface are longer than the ones pointing away from it. The gas pressure was shown to play an important role in blasting, but it was also shown to be only effective if the cavity and free surface have been preconditioned by the stress wave.