Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 38

These methods can prove invaluable for determining bedrock surface and water
table profile. However, they require careful interpretation in conjunction with
other geologic information, particularly that obtained from boreholes.
In the Appalachian Plateau, the underlying bedrock is strongly fractured,
and nearly all of the rock types found in coal basins have very low primary
porosity (Stach, 1982; Weinheimer, 1983). Continuous regional joints and
fracture systems are superimposed on the relatively shallow near-surface
fracture systems, resulting in a complex array of fractures that dominate the
shallow flow of groundwater (Minns, 1993). The majority of water in coal-
bearing rocks is transmitted in secondary permeability features, such as
fractures, joints, bedding planes, and coal cleats. The thin rind of fractured
bedrock that extends from the bedrock surface down to a depth of between 80
and 200 feet, has been termed the near-surface fracture zone (Kipp et al., 1983;
Wyrick and Borchers, 1981). The role of secondary permeability decreases with
depth due to decreasing fracture width, length, and interconnectedness.
Therefore, permeability of the original rock gradually becomes more important
with depth (Harlow and LeCain, 1991). Hydrogeologic studies of boreholes in
coal-bearing strata in the Appalachian coalfields have consistently shown that
fractured rock and coal seams have permeabilities nearly three orders of
magnitude greater than that of adjacent, nonfractured bedrock (Harlow and
LeCain, 1991; Kipp and Dinger, 1987; Minns; 1993; Wunsch, 1993).
Subsidence
Mine subsidence is defined as the ground movement that occurs when the
overlying strata collapse into the mine openings (Brauner, 1973; Gray et al.,
1974; Shadbolt, 1977; Singh, 1992). These can be either active or abandoned
mines. These collapses can create zones of weakness or fluid flow, which are
important considerations in impoundment design. On the surface the
disturbance is manifested as cracks depending on the depth-to-mined-height
ratio and other factors. The layers above the caved-in zone are subjected to
compressive and tensile stresses ( Figure 3.2 ). Tensile stress in the vertical
direction generally gives rise to bed separation, whereas, in the horizontal
direction it may open joints in the rock formations. Surface subsidence entails
both vertical and lateral movements of four types: cracks (tension or shear),
buckling (due to compression or shear), pits (also locally termed potholes,
sinkholes, chimneys, crownholes, or pipes), and troughs (or sags).