Prospector Trails
Equipment/Underground · Coal
Most underground coal in the United States comes from longwalls, and they are widely used in Australia, China and Poland. A cutting machine called a shearer runs along a straight coal face hundreds of metres long, under a row of hydraulic shields that hold the roof. Each pass shaves off a slice of coal, the shields step forward, and the roof behind them is left to fall.
01 · Late 1600s
Longwall mining is thought to have started in Shropshire, England, towards the end of the 17th century. Instead of leaving pillars of coal to hold up the roof, miners worked along one long face, undercutting the coal so it fell, and propped the roof just behind them with timber, letting the old workings collapse.
It recovered far more of the seam, and it became known as the Shropshire method.
02 · 1939
For centuries the undercut was made with a pick, lying on one side. By the 1930s electric cutting machines, with a chain of teeth running round a long bar, sawed a slot under the seam instead. This one was working at the Old Ben No. 8 mine at West Frankfort, Illinois, in January 1939.
In the decades after the Second World War, powered roof supports and steel face conveyors turned the whole face into one machine.
Sources: Library of Congress; AusIMM.
03 · How it works
A longwall has three main parts. The shearer, with two drums studded with picks, rides along the face and cuts the coal. The armoured face conveyor, a heavy steel chain conveyor, runs the length of the face and carries the coal away. Behind it, a row of hydraulic shields holds up the roof over the crew and the machines.
This shearer and conveyor were at work in the Twentymile mine in Colorado.
04 · On the face
Each pass of the shearer takes a slice, the web, about a metre deep. Behind it, the shields step forward one at a time, pushing the conveyor against the new face. The roof behind the shields is left unsupported and falls into the empty space, called the goaf or gob.
The face walks steadily through the block of coal, a panel that can be kilometres long.
Source: Coal Age (2013).
05 · Scale
In 2013 a face about 1,050 ft, or 320 m, wide was described as the US industry standard, with proposals to go to about 490 m. A wider face means fewer moves of all that equipment, and more coal per move. Even the standard face is longer than Bagger 293, the 225 m excavator.
Source: Coal Age (2013).
06 · How much
The EIA calls longwall the most productive type of underground mining, with high recovery, but it only works in flat, thick, uniform seams. In 2024 longwalls produced 112.8 million short tons of the 206.2 million mined underground in the United States, 55%. Almost all the rest, 92.8 million, came from continuous miners in room-and-pillar mines.
Sources: EIA, Table 3; EIA.
07 · Above
Because the roof caves behind the face, the ground above a longwall sinks. Mines predict and monitor the subsidence. Under rivers it can crack the rock.
South of Sydney, the Metropolitan mine works coal beneath the catchment of the Woronora Reservoir. Cracks in a rock bar on the Waratah Rivulet let water drain from pools; the company grouted the cracks to restore flow and says impacts have been limited, while community groups have said the losses are greater.
Sources: NSW Chief Scientist; NSW Resources Regulator; Eco-Business (2014).
08 · 2018
Germany built much of the world's longwall equipment, and its Ruhr mines worked coal more than a kilometre down. On 21 December 2018 Prosper-Haniel in Bottrop, Germany's last hard coal mine, closed, ending centuries of hard coal mining there.
These shields stand in the German Mining Museum in Bochum. Longwalls keep working in China, Australia, the United States, Poland and elsewhere.
Source: Clean Energy Wire.
09 · Next
From the face, coal goes onto a stage loader and a belt conveyor that can run for kilometres to the shaft or the portal. From there it goes by belt, train and ship to its buyers.
More machines: underground mining equipment and the whole fleet.
Source: SME.
The full story
Longwall mining is an underground method for flat seams, mostly of coal, in which a long face is cut by a machine that travels back and forth along it, under a continuous row of moving roof supports. The whole block of coal, the panel, is removed, and the roof is allowed to cave in behind the supports. It recovers much more of the seam than leaving pillars, and it is the most productive way of mining coal underground.
Some coal is also cut with ploughs, which are dragged along the face instead of a shearer, in thin seams.
In room-and-pillar mining, continuous miners cut rooms and leave pillars of coal to hold up the roof. It is more flexible and cheaper to start, but leaves much of the coal behind. Longwall panels are first outlined by gate roads cut with continuous miners, then extracted by the longwall. In the United States in 2024, longwalls produced 55% of underground coal (EIA).
Extraction of a panel causes the ground above to subside, typically by a large fraction of the thickness of the seam mined, in a trough over the panel. Effects on buildings, roads, streams and groundwater are predicted before mining and monitored during and after. Regulators set limits in mining approvals, for example around reservoirs and rivers.
Makers include Komatsu (Joy), Caterpillar, Eickhoff and ZMJ, with other makers in China and Europe. Names here are facts about who makes what, not a ranking or advice.
More machines: all mining equipment and underground mining equipment. Places: mining in Australia, the United States, China and Germany.
Two parallel tunnels, called gate roads, are driven into a block of coal, then joined by a third to make the face. Along the face stand a row of hydraulic shields that hold up the roof, and in front of them an armoured face conveyor. A shearer, with spinning drums studded with picks, rides along the conveyor and cuts a slice of coal from the face. The shields then step forward one by one, pushing the conveyor with them, and the roof behind collapses into the empty space, called the goaf or gob.
In room-and-pillar mining, rooms are cut into the coal and pillars of coal are left to hold up the roof, so a lot of coal stays in the ground. In longwall mining, the whole panel is removed and the roof is allowed to cave behind the shields, so recovery is much higher. Longwall needs flat, thick, uniform seams and a large investment, while room-and-pillar with continuous miners is more flexible.
In the United States in 2024, longwalls produced 112.8 million short tons, or 55% of the 206.2 million short tons mined underground, according to the EIA. Longwalls also produce much of the underground coal in Australia and China.
Yes. When the roof caves behind the shields, the ground above sinks, usually by a fraction of the seam thickness and over months. Mines predict and monitor it. Under rivers and swamps, cracking can let surface water drain away; at the Metropolitan mine south of Sydney, for example, cracks in a rock bar on the Waratah Rivulet let water drain from pools, and the company grouted cracks to restore flow.
The main makers of longwall shearers, shields and face conveyors include Komatsu (which owns Joy), Caterpillar, Eickhoff and the Chinese maker ZMJ, with others in China and Europe. This is a list of facts, not a ranking or advice.
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Pictures in this story: Bochum1805, Flickr (CC BY-SA 2.0); Farm Security Administration, Library of Congress, no known restrictions (Public domain); Peabody Energy, Inc., Wikimedia Commons (CC BY-SA 4.0); Bochum1805, Flickr (CC BY-SA 2.0); Lock the Gate Alliance, Flickr (CC BY 2.0); Till Niermann, Wikimedia Commons (CC BY-SA 3.0). Cover photos from Wikimedia Commons: Bucket wheel excavators by bluecloud9, CC BY-SA 2.0; Draglines by Roy Luck, CC BY 2.0; Haul trucks by Lechhabmed, CC BY-SA 4.0; Autonomous trucks by RobSimmons223311, CC BY-SA 4.0; Drilling and blasting by Siim Roov, CC BY-SA 4.0; Crushers by photosmith2011, Flickr, CC BY-SA 2.0; Ball mills and SAG mills by photosmith2011, Flickr, CC BY-SA 2.0; Froth flotation and tailings by Nina111955, CC BY-SA 4.0; Ore sorting by Tomra Sorting GmbH, CC BY-SA 4.0; Underground mining equipment by Kaupo Kikkas, CC BY-SA 4.0; Conveyors, rail and ports by Kabelleger / David Gubler, CC BY-SA 4.0; Who supplies the mines by Alexandre Prevot, CC BY-SA 2.0.
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Cover photos from Wikimedia Commons: Super Pit, Kalgoorlie by Kgbo, CC BY-SA 4.0.
