Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 39

FIGURE 3.2 States of stress over a mined area. A generic diagram that
encompasses active and abandoned mines, and longwall and room-and-pillar
mines. D=depth to seam; t=mined coal thickness; p=mined area (longwall or
pillared) within coal seam. After Shadbolt, 1977.
The magnitude and extent to which subsidence will impact an area are
governed by a variety of factors. For example, coal seam geometry, geology
and hydrogeology, and mining history may directly affect the potential degree
of subsidence from coal mining. The amount of coal removed from the seam,
the depth of the mine, and the orientation of the void, respectively, determine
the degree, timing, and geometry of subsidence features. The mining methods
used, rate of face advances, degree of extraction, and presence or absence of
mined areas can also have an effect. Finally, factors such as in-situ stresses,
topography, and time elapsed since the area was mined can also influence
subsidence.
Site-specific geologic information, including the availability of mine maps
and associated geotechnical information, is critical to evaluating the potential
for subsidence. These characteristics apply to both active and abandoned mines.
An understanding of these phenomena is also required for planning mine
layouts near or under previously existing impoundments.