Environmental Engineering Reference
In-Depth Information
Pillar Punching
A common cause of mine collapse appears to be the punching of the pillar into either the
roof or the floor stratum. Associated with coal beds, clay shale strata are often left exposed
in the mine roof or floor. Under conditions of high humidity or a flooded floor in a closed
mine, the clay shales soften and lose their supporting capacity. The pillar fails by punch-
ing into the weakened shales, and the roof load is transferred to adjacent pillars, which in
turn fail, resulting in a lateral progression of failures. If the progression involves a suffi-
ciently large area, surface subsidence can result, depending upon the type and thickness
of the overlying materials.
Earthquake Forces
In January 1966, during the construction of a large single-story building in Belleville,
Illinois, settlements began to occur under a section of the building, causing cracking. It
was determined that the settlements may have started in late October or early November
1965. An earthquake was reported in Belleville on October 20, 1965. The site was located
over an old mine in a coal seam 6 to 8 ft thick at a depth of 130 ft, which was closed ini-
tially in about 1935, then reworked from 1940 to 1943. Mansur and Skouby (1970) consid-
ered that building settlements were the result of pillar collapse and mine closure initiated
by the earthquake. Some investigators, however, consider that the collapsing mine was the
shock recorded in Belleville.
Subsidence over Abandoned Mines
Two types of subsidence occurring above abandoned mines have been classified; sinkholes
(pits) and troughs (sags). The subsidence form usually relates to mine depth and geologic
conditions.
Sinkholes (Pits)
A sinkhole is a depression on the ground surface resulting from the collapse of overbur-
den into a limited mine opening, such as a room or entry. They usually develop where the
cover over the mine is less than 50 to 100 ft. Competent strata above the mine will limit
sinkhole development, but sinkholes developed over a mine in Illinois where the over-
burden was 164 ft deep.
Troughs
Where a pillar or pillars fail by crushing or punching into the mine roof, troughs develop.
Subsidence troughs usually resemble those that form above active mines but often do not
conform to the mine boundaries. Trough diameters above abandoned mines in the
Northern Appalachian Coal Field commonly measure 1.5 to 2.5 times the overburden
thickness.
10.3.4
Strength Properties of Coal
General
Pillar capacity analysis requires data on the strength properties of coal. A wide range of
values has been obtained by investigators either by testing specimens in the laboratory,
or by back-analysis in which the strength required to support an existing roof is calcu-
lated for conditions where failure has not occurred. For the determination of the stability
of a working mine, the strength of fresh rock specimens governs, whereas for problems
