Geology Reference
In-Depth Information
October 1996 incident at Lone Mountain appears to have been induced by
subsidence (K.Mohn, Lone Mountain Processing, personal communication,
2001).
Sinkhole or pit subsidence. If the interburden between the pool floor and the
room-and-pillar workings is shallow (100 feet or less), sinkhole or pit
subsidence may occur. This could cause a sudden inflow of the
impoundment material into the mine openings. The November 1996
breakthrough into abandoned mine workings at Buchanan Mine was of
such origin (B.Thacker, Geo/Environmental Associates, personal
communication, 2001).
Catastrophic events. Heavy rainfall, debris flows, sudden snowmelt,
breakage of seals, or blow-outs in mines above the slurry level would
impose a severe load on the basin floor and could cause collapse of the
pillars in workings below, creating a connection. The failure in Martin
County in October 2000 occurred after heavy rain fell in a short period.
Piping. Piping due to erosion may occur once seepage or leakage has been
established.
If the width of the outcrop coal barrier or the overburden above the mined
area are of insufficient thickness or the surrounding strata are too weak,
impounded water could break into a mine and lead to a basin failure (see
Chapter 3 ). Conversely, an inundated mine under higher hydraulic head could
introduce a large volume of water into the impoundment. Should a blow-out
occur elsewhere in the watershed above pool level, it could cause an influx of
substantial amounts of water into the impoundment. This, in turn, may overtop
the embankment, damage the spillways, or induce a breakthrough in the pool
bottom as a result of the additional load. Synergy between geologic and
hydrogeologic conditions can compound the instabilities created by any failure
mode ( Figure 6.1 ).
Currently, no federal regulations address the width of outcrop barrier that
should be left during underground coal mining. Kentucky and Virginia
currently have standards for outcrop barriers, but allow variances where
conditions are appropriate. In some cases it is prudent to allow openings for
drainage. OSM has studied the problem of outcrop barriers but has yet to
release any conclusions to date. The committee recommends that MSHA and
OSM jointly pursue the issue of outcrop coal barrier width and
overburden thickness and its competence and develop minimum standards
for them.
Mine workings below an impoundment should be avoided unless they can
be confirmed to be deep enough and to contain an aquitard layer
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