Collie coal was mined from underground mines until 1994. The coalface was drilled and blasted before the coal was picked up using skid steer loaders, and loaded onto a conveyor belt to be taken out of the mine. Towards the end of the underground mining era, mining machines were introduced which cut and loaded the coal simultaneously.
- Can be a quicker way of extracting coal.
- Can reach deeper seams not economically viable to open cut.
- Level of coal recovery not as high as open cut.
- Coal at Collie too soft and permeable for this type of mining.
Underground mine development was always by the “bord and pillar” method, the bords being the underground roadways from which the coal was extracted and the pillars being the coal in between, which had to be left to support the overlying rocks. Typically the bords were about 5.5 metres wide and the pillars about 15 metres square.
In overseas mines where this system was developed, it was generally possible to mine most of the pillar coal as well, by developing “panels” of 50 to 100 pillars before starting to mine the outer pillars and retreating back to the main driveages, allowing the roof to collapse in a controlled fashion.
The Collie Basin sediments are too soft and too permeable for this phase to proceed. Cracking of the overlying rocks would result in uncontrollable inrushes of groundwater, such as what caused Griffin’s Hebe underground mine to be abandoned in 1965.
The maximum percentage of coal that could be extracted was determined by the ratio of the areas of bords and pillars; for the example above it was only 46%. In practice it was generally much less, due to:
- Support pillars being left between working panels, and underneath surface features that had to be protected from subsidence (eg bridges, railways and the river).
- Inability to mine the full thickness of thicker seams.
- Inability to mine thin seams at all.
- Some seams being rendered unmineable by subsidence caused by workings in lower seams. Normally upper seams are mined first, but better seams lower down can be very tempting.
Bord and pillar mining has been adapted and made more efficient in Australia using the Wongawilli method. Where conditions permit, recoveries can be as high as 90%.
Longwall mining is another method of underground coal mining in which two long parallel bords are driven laterally from the mines main headings and connected by a shorter crosscut, to define a block of coal typically more than 200 metres wide and a kilometre long.
“Longwall” mining uses a coal shearer, armoured conveyor and row of hydraulic roof supports in the crosscut. The shearer travels back and forth in the crosscut, cutting a thin slice of coal from the face on each pass.
Coal is conveyed across the face by the armoured conveyor, broken by a feeder/breaker and then carried to the surface by a series of conventional conveyor belts. The whole face assembly retreats back toward the main drivages, again allowing the roof to collapse (behind the hydraulic supports) in a controlled fashion. Longwall mining isn’t currently used in the Collie Coalfield because of the soft, saturated sediments.