It was great to see the pictures of those Tasmainian miners walk out of the mine. The diagram comes from the BBC and shows how the rescue team bored a hole underneath the space the miners were in.
According to some reports the rock was so hard it couldn't be drilled. So, instead, the rescuers used a technique called PCF or pentrating cone fracture. It's a lot like a highly controlled, highly directional explosion.
I did a little research on PCF and this is what I found:
The Pentrating Cone Fracture technique uses a cartridge that consists of a hollow plastic tube that holds propellant. It is initially open at one end allowing the tube to be filled with propellant. The open end is then closed with a small cap. The other end of the tube is machined into a wedge to lock into the stemming and seal the hole when the cartridge is initiated.
When ignited, the PCF charge fractures rock or concrete by the introduction of a pulse of high pressure gas at the base of a drill-hole. The generated gas penetrates into small microfractures and fissures in the rock. These microfractures and fissures are forced to expand and propagate into tensile cracks causing the rock to fail.
A cartridge filled with a specially formulated propellant produces gas by deflagrating when the propellant is ignited. Because the gas is confined down the hole in a very small volume, very high gas pressure is generated when the propellant is ignited.
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