Coal Mining
How can subsidence induced damage be minimised to surface structures in a longwall coal mining area?
Longwall mining involves the complete extraction of coal in panels 200 to 1,200 feet (60 to 360 meters) wide. Typically over the panel the ground subsides by about half the thickness of coal extracted. Thus if a 13 foot (4 meter) coal seam is mined, the ground will subside by about 6½ feet (2 meters).
The damage is usually not caused by the fact that the ground has sunk, but by differential settling. This happens temporarily as the wave of sinking passes through the area, and permanently around the edges of the panel where coal has been left as pillars.
Minimizing damage can be done in several ways:
► design the panels so that all sensitive structures are in the center of panels.
► alternatively leave coal pillars under sensitive buildings. This can be difficult.
► back fill the longwall to replace the coal – typically this reduces the sinking by a further 25% of the total – so the example given above would see 3¼ feet or 1 meter of subsidence. It is very expensive.
► ensure that all pipes which cross the area are flexible enough to withstand the strain. Leaking pipes can cause extensive structural damage.
► ensure buildings are flexible. Wood or reinforced concrete buildings will generally take the strain. Just as in earthquakes, unreinforced masonry, cement or stone buildings are most at risk.
► Inspect buildings thoroughly before mining. Pre-mining inspections typically show 80% of buildings have existing structural damage from other causes, which the building owner is often not aware of. Subsidence can exacerbate existing problems, so it actually makes sense to repair and strengthen buildings before mining.
►When mining deeper areas, because the subsidence zone is wider at the surface, subsidence from different panels can overlap. It is possible to time the mining of successive panels to reduce surface strains.
► buildings on concrete pads that have been tilted by differential subsidence along panel edges can be level led, but this is expensive.
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