Environmental Engineering Reference
In-Depth Information
FIGURE 10.9
Plan of coal mine room and pillar layout,
Westmorland County, Pennsylvania. (From
Gray, R.E. and Meyers, J.F.,
Proc. ASCE J. Soil
Mech. Found. Eng. Div.,
96, 1970. With
permission.)
“Robbing” occurred subsequent to first mining and consisted of removing the top and
bottom benches of coal in thick veins and trimming coal from pillar sides.
Collapse occurs with time after mine closure. The coal is often associated with beds of
clay shale, which soften and lose strength under sealed, humid mine conditions.
Eventually failure of the pillars occurs, the roof collapses, and the load transfer to adjacent
pillars causes them to collapse. If the mined area is large enough and the roof thin enough,
subsidence of the surface results.
Old mines may contain pillars of adequate size and condition to provide roof support
or robbed pillars, weakened and in danger of collapse. On the other hand, collapse may
have already occurred.
Modern Operations
Two major coal seams of the Pennsylvanian underlie the Pittsburgh area, each having an
average thickness of 6 ft: the Pittsburgh coal and the Upper Freeport coal. The Pittsburgh
seam is shallow, generally within 200 ft of the surface, and has essentially been worked
out. In 1970, the Upper Freeport seam was being worked to the north and east of the city
where it lay at depths of 300 to 600 ft (Gray and Meyers, 1970).
In the new mines complete extraction is normally achieved. A system of entries and
cross-entries is driven initially to the farthest reaches of the mine before extensive mining.
Rooms are driven off the entries to the end of the mine, and when this is reached, a sec-
ond, or retreat phase, is undertaken. Starting at the end of the mine, pillars are removed
and the roof is permitted to fall.
In active mines where surface development is desired either no extraction or partial
extraction proceeds within a zone beneath a structure determined by the “angle of the
draw” (Gray and Meyers, 1970). The area of either no extraction or partial extraction
is determined by taking an area 5 m or more in width around the proposed structure
and projecting it downward at an angle of 15 to 25
from the vertical to the level of
the mine (angle of the draw). With partial extraction, where 50% of coal pillars remain
in place there is a small risk of surface subsidence, whereas with no extraction there
should be no risk.
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