How deep a coal seam lies decides whether miners dig a pit or a tunnel
Coal lying less than about 61 metres down is usually stripped from the surface, while seams below 90 metres generally mean going underground. Thickness can override depth, though: Germany's Tagebau Hambach works an open pit reaching seams as deep as 460 metres, and some western American seams 20 metres thick justify digging from above.
Surface mining starts by blasting the rock and soil on top, called overburden, then clearing it with draglines, shovels and trucks or giant bucket-wheel excavators. In strip mining the first strip's spoil is dumped outside the planned area, and each later strip's spoil fills the hole left by the one before. It recovers more of a deposit than tunnelling and supplies around 80 percent of Australian output and about 67 percent in the United States, against roughly 40 percent worldwide.
Hilly ground calls for other tactics. Contour mining follows a seam around a hillside, often leaving an untouched ridge 5 to 6 metres wide at the outer edge to keep refilled slopes from slumping. When the tonnage of overburden per tonne of coal gets too high to pay, augers can bore sideways into the exposed wall instead. Mountaintop removal slices off an entire summit in parallel cuts and dumps the waste into neighbouring valleys, a hotly disputed practice because it buries streams and flattens hills into plateaus.
Underground methods produce about 60 percent of the world's coal. In room and pillar mining, crews cut chambers while leaving columns of coal to hold up the roof, then, in retreat mining, remove those pillars too. Hydraulic roof supports that resemble huge tables on jack legs shield workers until they withdraw, after which the roof usually caves in. Longwall machines drag two spinning drums studded with carbide bits along the seam.
In 2023 coal mining employed over 2.7 million people, 2.2 million of them in Asia. Its record of polluted water, acid drainage and degraded land has made coal among the first fossil fuels to be phased out, though China, Indonesia, India and Australia have yet to reach peak production.
Source: Coal mining
