Longwall mining, first introduced in the U.S. in the 1960s, is the newest of three types of underground
stoping methods and accounts for more than 20% of underground
coal production. The other two common underground mining methods are known as
room and pillar mining, and
hydraulic mining.
Longwall mining is the method to be used when the
coal seams are at great depths underground. The roof of the mine is kept from collapsing by
self-advancing hydraulic systems, and using a
longwall miner -- the entire seam of coal is removed leaving no
pillars for support. This involves the total removal of large blocks of coal causing the
overburden to collapse in a planned manner. The
seam panels may measure 1,200 feet wide and be over two miles long. This method of mining is five times more profitable and 200 times safer than room and pillar mining (which leaves part of the
coal seam to support the land and structures above it). Prior to the 1960s longwall mining advanced from multiple directions from the
central shaft.
The two variations of longwall mining are
slicing and
caving. The slicing method extracts full sections of a thick seam in two or three slices and is considered less productive and not nearly as safe as caving. In the caving method -- a rectangular shape single panel in the lower portion of a thick coal seam is extracted, the top coal is allowed to cave in the normal manner. The
tail-canopy can be moved up and down to allow coal in the
goaf area to spill onto the
face conveyor. The caving method's advantage is that it can extract a greater seam of coal within the panel than with slicing -- thus the production costs are up to 40% lower than the slicing variation.
Longwall mining known as '
full extraction mining' nets more coal with fewer workers and far faster than traditional techniques, however it can cause extensive damage to the area above ground. Known as '
subsidence' -- the land above the mine may sink, or rise causing roads to split, gaslines and waterlines to break and buildings to crack. In addition, this technique can diminish and pollute above ground water wells, and dry up the streams in the surrounding area. So clearly this is not the most popular method of mining amongst the property owners living nearby.
To view a longwall shearer drum see:
http://www.advancedcuttingsystems.net/photo.htm