Three basic mechanisms contribute to rock breakage with charges confined in boreholes. The first and least significant mechanism of breakage is caused by the shock wave, at most, the shock wave causes microfractures to form on the borehole walls and initiates microfractures at discontinuities in the burden. This transient pressure pulse quickly diminishes with distance from the borehole and since the velocity of the pulse is approximately 2.5 to 5 times the maximum crack the pulse quickly outruns the fracture propagation.
The two major mechanisms of rock breakage results from the sustained gas pressure in the borehole, When the solid explosive is transformed into a gas during the detonation process, the borehole acts like a cylindrical pressure vessel. Failures in pressure vessels, such as water pipes or hydraulic lines, offer an analogy to this mechanism of rock breakage. When the pressure vessel is over pressurized, the pressure exerted perpendicular to the confining vessel’s walls will cause a fracture to occur at the weakest point in the pressure vessel. In the case of frozen water pipes, a longitudinal split occurs parallel to the axis of the pipe.
The same phenomenon occurs in other cylindrical pressure Vessels due to the generation of hoop stresses. If a borehole is considered a pressure vessel, one would expect fractures to orient themselves parallel to the axis of the borehole. The major difference between pressurizing a borehole and pressurizing a water pipe is rate of loading. A borehole is over pressurized almost instantaneously and therefore does not fail at one weakest point along the borehole wall. Instead, it will simultaneously fail in many locations. Each resulting fracture will be oriented parallel to the axis of the borehole. Failure bk this mechanism has been recognized for many years and is commonly called radial cracking
Reference
U.S. Department of Transportation, « Rock Blasting and Overbreak Control».
