This approach encouraged knowledge, experience and occasionally something a little unexpected,
After one of the overburden shots had been cleared and all the material had been dug out and after the area had been cleaned up it was noticed a large 500+ton rock hangup had occurred on the back wall of the overburden shot. This was a section of rock that had been included in the shot but for some reason the explosive had failed to move it so it hung as a large bulge of rock 15mtr long and four meters high and about a meter and a half thick, attached to the face, there were several original drill holes still visible passing completely through the back of the hang up, all empty and spaced 2.5 mtrs apart.
One of our training shot firers was given the task of calculating the required amount of explosive to enable this problem to be cut from the wall without damaging the wall face behind it.
The rock type known as Whyalla sandstone consisted of a mix of layers with numerous fault lines and variations in the actual hardness of the rock itself, this was a very difficult material to blast correctly and if not enough care was taken when loading the explosives, it would reward the shot firer by producing large 5, 10, or 15+ ton boulders with ease, making it difficult for the Front End Loaders to shift move or pick up and place into the back of the Dump trucks.
After careful calculations the suggested amount of explosive was loaded into a series of Bangalor torpedoes. A Bangalor torpedo is an explosive charge placed within one or several connected tubes, Originally used by combat engineers to clear obstacles that would otherwise require them to approach directly, possibly under fire. It is sometimes colloquially referred to as a "Bangalore mine", "banger" or simply "Bangalore" (wikipedia)
In this instance the explosive was placed in several lengths of 75mm PVC pipe. This allowed the explosive to be slid down into the existing holes which were open top and bottom.
The explosive in the form of a packaged sausage was tied with down line and after measuring the position with the aid of a tape measure lowered over the side of the face which then produced the measurement to lower the sausages down inside the PVC pipe to the required depth.
After tying the down lines together and clearing the pit, the time came to fire the charge, with the ultimate aim to sheer off in a clean cut and leave the face clear of the dangerous 500ton rocky hangup.
Warren Brown and a small group stood well clear on top of the pit watching as the charge was detonated.
A flash of det cord a milli second or two later the Bangalor Torpedo's all detonated sending rock and dust up and down the face behind the hangup, the sharp crack of the explosive reaching us a few seconds later.
As the dust cleared it became apparent the hangup had not shifted at all.
We waited, a minute and it became apparent nothing was going to happen.
Warren turned to the disappointed shot firer and began to explain that often even after doing all the calculations and planning it is always better to slightly overcharge the holes with explosive.
He then began a lecture on calculations for mass and quantities of explosive, when the overhang suddenly and quietly slid down the wall as one complete piece before noisily shattered into small pieces as it hit the floor ten meters below leaving behind a perfectly clean face looking as though it had been cut from the face with a knife while leaving half the drill hole still visible in the remaining face, making it a perfect result.
Warren shut up for a moment watching as the scene unfolded before him, Then, he went on, there is beginners luck to consider, he said finally.
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| Mt Gunson Mill stock pile and round Surge bin(silo) |
At start of the feed conveyor in the Ball Mill stood a surge bin, this was a tall silo like structure that stood twenty meters tall and about six in diameter, the top was capped with a conical shape roof of corrugated iron to help keep the crushed ore dry.
It's purpose was to collect the final crushed rock, the fines and feed it onto the feed conveyor at a rate to maintain the constant flow of feed into the Ball mill where it would be ground down to a fine slurry to enable the copper and other minerals to be separated from the waste rock.
Occasionally the contents would hang up inside the bin, a series of compressed air jets fitted at various levels and locations about the circumference of the bin could be activated to blast the material loose.
Sometimes the air was unable to shift this material, when this happened the internal volume of the surge bin was reduced making it difficult to maintain a constant feed flow.
The solution was to lower a shot firer in through the top of the bin on a bosuns chair with an auger where he would drill a series of holes all around and at different levels through out the hang up material and cut a 20 or 30 mm slice from one or two plugs of the sausage explosive and insert it into the augured holes taking care not to over prime the entire bin with explosives. The object was to shake and loosen the hang up.
After one Christmas I was returning to the mine after having a week off to discover the entire roof of of the surge bin completely missing.
One of our ex army shot firers who had been staying on site over the Christmas break had been asked if he could give the bin a bit of a shake up. Miss-understanding the instruction of using only 20 to 30mm slices of the 70mm sausage explosive he took it to mean 20 to 30% of the explosive in each hole. The result of the using five plugs of explosive in an overcharge quickly removed the entire roof in a spectacular fashion sending it flying hundreds of meters into the air where it rained sheets of twisted corrugated iron down all over the mill area.
Oh well,
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