Geology Reference
In-Depth Information
our ability to find adequate subsurface space for injec-
tion of captured CO2. Assuming we will be able to
extract energy from coal in an environmentally respon-
sible way and given that other fossil fuels have passed
their peaks in the United States, coal mining will
increase in importance.
The coal is mined in surface and underground
operations and both processes have environmental
impacts such as acid mine drainage, acid soils and waste
piles, and disrupted groundwater systems. In the case of
surface mining, the landscape and surface drainage is
completely disrupted and in the past, erosion and
stream sedimentation, landslides, and revegetation of
the landscape presented problems. Fortunately, the
Surface Mining Control and Reclamation Act of 1977
reduced the impact, but problems still exist, particularly
in mountain top removal. In the case of underground
mining of coal, surface subsidence is a process that con-
tinues to cause problems. Understanding the subsidence
process and risk in coal mining is the objective of this
part of the exercise.
The
room and pillar
method of underground
coal mining results in voids (rooms and tunnels) sepa-
rated by pillars that support overlying rock (sand-
stone, siltstone, and shale) and regolith. In this mining
process ~50% or more of the coal is removed from the
section of the coal seam being mined. Over a period of
time (often years to many decades) after mining, when
the overlying rock sags and breaks into the rooms, tun-
nels, and shafts, surface subsidence occurs and forms
small
circular depressions or collapse sinkholes
(over
shallow depth mines, usually within 100 feet or less of
the surface) or larger
subsidence troughs.
Factors in
the form, size, and amount of subsidence include void
size, depth of mining, strength of rocks (including
fractures and joints), age of timbers, soundness of the
coal pillars, and weakening of the coal pillars due to
fluctuations in groundwater (exposure to air promotes
oxidation and a lowered water table reduces the buoy-
ant force). The extent of missing or "pulled" pillars (at
the very end of mining) also reduces roof support. In
longwall mining,
a more recent mining technique not
considered here, almost 95% of the coal is removed in
the mining process, which results in a more uniform
lowering of the mined-out region as mining pro-
gresses. It can still have an impact on structures at the
surface.
Surface mining,
in which the overlying
regolith and bedrock are removed to expose coal, sub-
sidence of structures is not a problem (but settlement
and landslides may occur in the reclaimed landscape).
Part D of this subsidence exercise is based on
reports from Illinois and Colorado, but the examples
here would apply to most of the country where coal
is mined by the room-and-pillar method. To find the
general areas in the United States where subsidence
from coal mining might be a problem, see the Coal
Fields of the Conterminous United States map (USGS
Open-File Report OF 96-92). To locate actual mined
areas in these coal regions, one should check state
natural resources agencies or geological surveys for
extent of mining maps and more detailed mine maps.
Extent-of-mining maps
show the mined areas
on topographic maps. More
detailed coal mine maps
show the actual rooms, pillars, tunnels, and adits
(nearly horizontal passages into a mine); however,
these maps might not be completely accurate or easily
available. Any building that is close to a mine could be
at risk in some areas, even if it is shown on a map to be
somewhat beyond the mined area. For building-site
selection purposes, drilling can reveal the existence
and condition of rooms, pillars, previous failures, and
any later filling of voids. It might not be possible to
protect some existing buildings from collapse due to
the difficulty of predicting where subsidence will
occur and the cost of drilling and any void filling that
is needed (Turney, 1985).
Roads, buildings, and utilities are subject to dam-
ages from subsidence. Concrete or asphalt roads may
crack or sag, or completely collapse into circular pits.
Building damage shows up as a sagging roofline, cracks
in bricks, separation of steps, cracks in drywall, dis-
torted windows, sticky doors, floor sags, and cracking
noises in the home. Utilities also provide clues to subsi-
dence, with cracked water and sewer pipes, water and
natural gas leaks, and dirty tap water.
QUESTIONS (9, PART D)
1.
a.
Longwall coal mining is a more modern mining tech-
nique used in some areas, but the older type of under-
ground coal mining used in many parts of the country is
known as:
b.
Make a labeled sketch below showing a map view of a
subsurface coal mine in which about 50% of the coal has
been removed by the room and pillar method (see the
lowest part of the cross section in Figure 9.8 for a view of
such a mine).
2. a.
What are the two subsidence landforms common
above these coal mines?
b. Which of these two subsidence landforms would likely
develop over shallow-depth mines and might have a con-
nection between the surface and the mined-out void?
3.
List at least four factors that control subsidence over coal
mines.
