ROCK BREAKAGE BY EXPLOSIVES

Explosive charges should provide a peak compressive strain at the blasthole wall that fails to cause crushing. Where higher peaks are generated, some strain wave energy is wasted in pulverising an annular section of rock immediately around the blasthole. The outgoing tangential tensile wave produces a highly-symmetrical radial crack pattern all the way around the blasthole. The number of radial cracks decreases at increasing distances from the blasthole. Any pre-existing cracks arrest the propagation of radial cracks prematurely. Symmetrical crack growth continues until the reflected radial tensile wave (which originates at an effective free face) interacts with the crack tips; this reflected wave then tends to open those radial cracks with which the wave front subtends a small acute angle. The explosion gases then stream into forward-looking cracks (and especially those favoured by the reflective wave), wedging them open and extending them towards the free face(s). Once the burden rock has been at least partially detached from the mass, the very high rate of decay of strain causes release-of-load fractures along cylindrical (or conical) shells around the blasthole. The reflected tensile wave is usually too weak to cause spalling at an external free face; it can, however, open up and extend pre-existing cracks and fissures. Internal spalling at open joints within the rock can be significant. Shear failure occurs where the pushing effect of the gases causes relative movement of adjacent sections of rock along blast-induced cracks and/or fissures. After radial cracking has been completed, the various adjacent wedge-shaped elements of the burden rock undergo bending, and fracturing by flexural rupture occurs in planes normal to the blasthole axis. Breakage by in-flight collisions takes place for certain blast geometries and/or initiation sequences. © 1979.