All other mining techniques in which coal is extracted from beneath the overburden by mechanical devices situated at the face of the cliff or highwall and extending laterally into the coal seam, such as extended depth, secondary recovery systems, are included in the definition of auger mining, which is the process of mining coal at a cliff or highwall by drilling holes into an exposed coal seam from the highwall and transporting the coal along an auger bit to the surface.
Coal is chopped and forced out of the hole by the auger’s revolving movement as it goes ahead and turns. The term comes from the operation, which is akin to a carpenter using an auger bit to bore a huge hole in wood.
Approximately one-third of the coal beneath the highwall is recovered through auger mining, regardless of the depth of penetration attained. Auger cutting heads can be drilled down more than 300 feet and have a diameter of up to 7 feet.
It is challenging to “pack up and leave” when the final pit limit is reached because of the exposed coal seam. Therefore, the business views auger mining as the “icing on the cake” in terms of secondary recovery techniques.
It’s essentially very cheap coal recovery. But it’s also a little contentious and troublesome. The poor recovery rate (less than 35%) indicates that much of the reserve has been sterilised, meaning that it is now virtually impossible for anyone to recover the remaining portion in the future. One major issue is that the holes in the hillside provide drainage holes for water that is tainted with acid.