When the hole becomes too deep for the miner to simply shovel the dirt, known as mullock or overburden, up and out of the shaft it has to be hauled out and this can be done with –
Hand windlass - a hand operated winch used to lift buckets of mullock out of the shaft. There are not many people using these on the fields anymore.
Power winch - operates on the same principles as the windlass but is powered by compressed air or a power generator.
Yorke hoist – a hoist mounted on a vertical pipe, stayed by wires from the top, enabling free movement of the hoist mechanism around the mine shaft. Using a hoist is generally a two person job. A bucket of mullock from deep inside a mine shaft can be hoisted to the surface, swung away from the shaft and emptied by an operator at the surface. The entrance of the shaft, known as the shaft collar, can be reinforced by boxing with galvanised iron or wooden frames to prevent mullock falling down into the shaft potentially injuring the miner below. In some horizontal tunnels leading off from the main shaft, the miner underground may fill the buckets at the face of the tunnel where they are working, and slide it to the vertical shaft along lengths of water pipe on the floor lubricated by sump oil. Hand carts can be used to move buckets of mullock along horizontal tunnels to the vertical shaft also. Once at the shaft a safety hook is used to prevent the bucket dislodging from the winch cable as it is raised to the surface. Once emptied the whole process is repeated again.
Vehicle mounted winch – these days most miners have a portable winch mounted on their work vehicles. They are used to lower persons and equipment down mine shafts. The one I made, pictured below, is charged by a solar panel with dual batteries and can fold away for travelling long distances.
Ute Mounted Winch System
The most common method of sinking a shaft is by the use of –
Calweld drill – The California Welding Co. in the USA produced large diameter drill rigs, which comprise of a truck with a separate engine on the back running a hydraulic system for the rig assembly at the back of the truck. It comprises of a large cylindrical steel bucket with a diameter of around 1 metre. The bucket has a hinged base with two slots fitted with cutting teeth. The base is locked, the bucket is lowered down the hole and filled by a rotary action. When full, the bucket is brought to the surface and pulled away approximately 5 metres from the drill hole by a boom and pulley system. The hinged base is then opened to dump the mullock out of the bucket to the ground. As each bucket is emptied, miners search for signs of opal. Calweld holes can be reamed out to approximately 2 metres diameter to allow tunnelling machines and other machinery down the shaft. When several of these drill rigs were brought to the opal fields in the 1970’s they became the preferred method of shaft sinking and are still used today.
Prospecting Drill – comprising of a setup similar to that of a Calweld drilling rig, an auger of approximately 9 inches in diameter, drills a hole to the desired level. The material bought up by the auger is checked by the driller for signs of potch, precious opal or signs of different layers of sandstone or clay which in turn can hold precious opal. A prospecting drill can also be used to sink ventilation shafts.
9" Prospecting Drill
Ventilation Shafts – the extent of mining in the clay levels of a mine was often limited because of difficulty in providing ventilation to remove hot air and dust at the end of a long tunnel, especially after explosives were used. After the introduction of auger drills for prospecting in the 1980’s miners could quickly drill ventilation shafts down to the longest tunnels to allow more air to circulate.
Following the opal level - driving on the level or (vertical) means following along the opal bearing ground. The potentially opal rich layer can run horizontally or vertically. It can then be removed with mechanical means or by hand using a pick or a rotary hammer drill etc. I usually chase this along using a Blacklight. It works by using a Ultra Violet lamp or LED. when the UV light hits the face of the mine any opal including potch opal (trace opal) lights up or glows making it easier to follow the vein until it hopefully turns into precious opal gemstones. Once precious opal is found it's usually worked with a hand pick or a screw driver in preference to a jackpick to avoid damaging any opal.
Using a UV light to chase opal
Using a hand pick is always used once an opal pocket is found
The mullock from underground mining is removed in a number of ways, some of them peculiar to the opal fields.
These include -
Hand windlass - (see above description)
Power winch - (see above description)
Yorke hoist – (see above description)
Automatic bucket tipper – or self-unloader allows the entire operation to be controlled by one miner from underground. A full bucket of mullock is strapped to a cradle at the base of the shaft. The miner pulls a lever which engages the winch motor. The bucket is hauled to the surface along two parallel rails which when are above the ground curve downwards to allow the bucket of mullock to be emptied about 5 metres from the shaft. At the point of tipping, an automatic reversing switch is tripped sending the now empty bucket back down the shaft to repeat the process when the miner is ready.
Automatic bucket tipper dumping into a mullock heap
Blower – a truck mounted machine which is similar to a vacuum cleaner. A big fan or blower, driven by a separate diesel engine on the back of the truck, sucks mullock out of the shaft through a series of large metal pipes, the mullock is then collected in a hopper (large metal bin) on the surface, which is emptied either automatically or when requested.
Blowers working on the 6 mile field Coober Pedy
Bogger – a small, air powered, front end loader. After blasting underground, the bogger is used to scoop up the scattered mullock with its small bucket and then tips it into a small holding bin. This is then emptied into a sump where a bucket elevator takes it to the surface. A sump is a dumping area dug out below the tunnel floor.
Bucket elevator – consists of a series of buckets on an endless chain which continually come out of the shaft full, are emptied out on the surface, and go back down the shaft empty to repeat the cycle again. It starts in the bottom of a sump where loads of mullock are dumped from the tunnels face by a bogger.
Backhoes – used for digging trenches where opal is close to the surface, or for working in abandoned bulldozer cuts. The large bucket on the front can be used for moving mullock heaps or loading trucks and hoppers, etc.
Excavator – consisting of a boom, dipper (or stick), bucket or head attachment and a cabin which sits on top of a turret with tracks or wheels. Excavators are frequently used to dig trenches to get to the opal layer, and to work on the face of an open quarry.
Mate prospecting for opal on the Hitachi Excavator Coober Pedy
Bulldozers – operate more efficiently on combined claims where there is enough area for the disposal of overburden by pushing from one claim to the next, allowing maximum exposure of opal bearing ground. The Bulldozer operator rips up and pushes overburden from out of the open cut mine until they reach the level that they hope to find opal on, often with spotters watching for opal in case the opal level is shallower than expected. When opal is spotted, the ground is worked over by hand, first with a pick and shovel, then smaller hand tools can be used to carefully remove a precious opal seam.
Bulldozer pushing overburden
Quite a bit of precious opal may be crushed, broken or overlooked by careless operators. As a result, bulldozed mullock heaps are the preferred choice for noodling machines rather than the dumps resulting from underground mining. Noodling is often on a commission basis, with the noodling contractors employed by the claim owner who gives them a percentage of what they find.
Types of tunnelling machines –
Circular tunneling machine
This particular type (see picture) has a large rotating arm on the front with a cutting head attached, this makes a circular tunnel. The mullock is generally removed with a blower. I haven’t seen one of these in action so I don’t know much about them, but I’m guessing having a rounded floor underneath you would create its own problems.
Multiple discs with teeth
The tunnelling machine (see above picture) consists of a large drum with discs of teeth, mounted on two boom arms either side of the drum. The drum is rotated and lifted up and down with the arms. It cuts out a curved rectangle shape and is inches forward by the rest of the machine driven with small bulldozer like tracks. The mullock material is either removed with a blower or a conveyor belt type removal system. It is commonly used to make dugouts, which are underground dwellings commonly found in Coober Pedy.
Hydraulic tunneling machine with bogger
The picture of this last tunnelling machine (see picture above) is similar to a mini excavator consisting of a two-toothed pick and is hydraulically driven to pick and scrape away at the face of a mine. It has two large vertical hydraulic stays that jam up into the tunnel roof to stop the machine moving and to help support the roof while digging. It moves forward by digging the pick into the floor and dragging itself forward. The mullock is removed usually with a blower system.
Hopefully this has given you a brief overview of some of the mining techniques used on opal fields in Australia. If there is something specific about mining for opal that you would like to know, leave me a comment below and I will do my best to give you the information you request.
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