Page 60 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 55

from the standpoint of percent extraction, it causes differential settlement of the
overlying rocks that can make an area at least temporarily unusable for support
of structures.

Abandoned Coal Mines
Most underground mines are below the natural groundwater table, so when
pumping stops they become flooded with water. This is advantageous for several
reasons: (1) the weight of a submerged rock mass is reduced by about one-half,
which can almost make up for the stress concentration from room-and-pillar
mining and helps to explain why more mines have not caved in; (2) submergence
protects support timbers from decay; and (3) submergence reduces the likelihood
of spontaneous underground burning. These advantages are compromised by
lowering the groundwater table or if the mine is drained, which can lead to
spontaneous combustion in the mine that is very difficult to control.

Flooding of a coal mine unfortunately does not completely prevent underground
burning because of air pockets and roof collapses, and a pervasive coal-smoke
odor often can be detected in mined-out areas. In extreme cases underground fires
warm the earth up to the ground surface, causing snow melt and contributing
noxious-fumes to the atmosphere. Because of the trapped heat a fire quickly
rekindles as soon as any air becomes available. Dry ice may be used to cool a mine
fire and put it out with CO 2 . Another approach that may be more permanent is to
pump in water and fly ash, in effect recycling the unburnable part of the coal
back into the mine.

Difficulties Locating Collapsed Mine Tunnels
Collapse of a mine does not occur all at once, but is localized where roof
support is weakest. Because room-and-pillar mining follows a rectangular pattern,
a hint to a collapsing mine can be a series of shallow ground depressions following
a straight or intersecting lines.

Even where collapse features are evident at the ground surface, finding or isolat-
ing the cause can be difficult because rubble from a caving mine roof occupies
more space than the rock did prior to caving. Caving therefore proceeds upward
by a process called ‘‘stoping’’ (rhymes with groping). As stoping proceeds upward
the cross-sectional area of a tunnel is reduced and can disappear, so collapse
will never reach the ground surface. It therefore is the shallow mines that are
of the greatest concern.

Example 3.1
Assume that after roof rock falls into a mine tunnel the rock contains 10 percent voids
compared to zero percent before caving. The mine tunnel is 2 m (6 ft) high and 50 m (150 ft)
deep. Will stoping extend to the ground surface?

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