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Theory of Air Deck Blasting |
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The Russian Mel'nikov proposed the original air deck or air column blasting theory. Detonation of a typical, full column confined charge produces a single high amplitude stress wave which crushes the borehole wall and moves out into the surrounding rock producing a crack mechanism. In conjunction with the stress wave, high temperature gases assist in extending the crack formation and moving the rock mass. By incorporation an air gap (air deck) above or within the explosive column, shock wave reflections within the hole produce a secondary stress wave. This wave extends the crack formation before gas pressurization. The reduced borehole pressure caused by the air column reduces excessive crushing of the rock adjacent to the borehole wall but still is capable of extending the crack formation and moving the rock out away from the face. Tests run by Moxon, et al, have shown that substantial air deck volumes can be used before there is any reduction in fragmentation. The chart on this page is from his paper showing the air deck volume versus the change in fragmentation. As you can see, you can save as much as 35% of your explosives without much change in fragmentation.
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