South Crofty Mine Underground 13: This page is continuing the images of drive development.
This mainly covers the use and priming of the explosives before the blasting of the face.
Nick Le Boutillier kindly wrote a few lines about blasting in South Crofty.
The most commonly used explosive was Powergel, this was a plastic explosive that came in foot-long 3/4 inch plastic-wrapped ‘sausages’. This was used at the end of drill holes to break out to the hole sockets. Also by moonlighting geologists as ‘slap dab’ to ‘blow shit up’, which was enormous fun.
The main explosive in use was anfo, which is ammonium nitrate mixed with diesel oil. This was pink and had the consistency and form of small polystyrene balls. It was blown by compressed air into the drill holes, the detonators were placed in the Powergel and the wires run out of the holes. Also these were sometimes tamped by a bit of clay to close the hole.
Trammers would sometimes use Coretex, which was a nitrate-based explosive that looked like white electrical cable. This could be wrapped around troublesome rocks and was very effective. However it was not popular, because chemicals from the explosive were readily absorbed by the skin and could easily lead to very severe blinding headaches and nausea.
On another day, the next two images are again of Adrian Mugford in another part of the mine. Once again drilling for the next blast. Present day Health and Safety laws restrict the use of these rock drills because of the extreme vibration the operative felt.
The rest of the images on this page were taken on the NPZ (North Pool Zones) crosscut on 400 Fathom Level South Crofty Mine. This was on another day with Nick Le Boutillier. This set clearly shows the ANFO explosive being blown into the drilled holes by compressed air. It was rare to hang around and have the chance to take pictures. I certainly made the most of the opportunity.
Neil Hodges whom I met several times underground has written a piece about using explosives at South Crofty.
My first experience with explosives was at Wheal Jane back in the mid 80’s. Then, the cartridge shaped dynamite was NitroGlycerine based. If you got some on your hands and rubbed your forehead inadvertently it would give you a thumping headache! This was mainly used for rounds drilled in development ends which were often wet, because this explosive was not too badly affected by water.
At Crofty which was a much drier mine Anfo was most commonly used , this is a mixture of Ammonium Nitrate(fertiliser) and Fuel Oil(diesel). The cartridge type dynamite had changed to an emulsion paste in a plastic sausage. The trade name of this was Powergel E80.
One cartridge of Powergel would be used as a primer if there was a chance of some damp at the back of the hole. The primer contained the detonator, so it was important that the blast was initiated well. The rest of the hole would be filled with Anfo. This was blown into the hole by compressed air. If, which was more likely the ground was dry, the detonator could be placed at the back of the hole and then loaded with only Anfo and no primer.
Anfo’s big drawback was that water would decompose it very quickly preventing detonation. In this eventuality all the holes in the round would have to be flushed out and recharged. Worse still, a partial detonation of some of the holes could occur in the round, leaving a shattered mess at the face.