Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 37

the bottom of the impoundment, and the quality and quantity of foundation
materials. Specifically, these factors include the geology and hydrogeology of
the site, the potential for subsidence, and the geotechnical characteristics of
foundation materials.
While the geotechnical characterization of the embankment foundation is
well developed in the coal industry, the level of geotechnical investigation of
the basin seems to exhibit much less consistency. Basin investigation can
evaluate potential weaknesses along which fluids could travel. Examples
include rock fractures, subsidence zones, and permeable strata. Although not
common, some coal-bearing formations contain limestone, and there is the
potential for karst development and fluid migration along solutionally-enlarged
fractures or conduits. The committee could not identify a consistent set of
criteria or a guidance document for conducting a site investigation of the
impoundment basin. Regulations to this effect are usually promulgated by the
states and vary by state. However, despite this lack of regulation, standard
engineering practice implies that some level of basin analysis should be
included in design; the degree to which this is done appears to be site specific.
The probability of exposing cracks, faults, coal seam outcrops, or other
preferential pathways for water is greatly enhanced by clearing surface soils and
exposing the basin foundation materials. Because the topography of the
Appalachian coal region is characterized by steep surfaces covered with dense
vegetation, the side slopes of the basin area are often difficult to access using
conventional earth-moving equipment. As such, many basin investigations
appear to be limited to visual reconnaissance of the basin side slopes, with
clearing of vegetation and topsoil and stripping of soils limited to the accessible
areas in the valley bottoms only.
Site Geology and Hydrogeology
Geologic site investigations should include the depth, thickness,
continuity, and composition of each significant geologic layer and an evaluation
of regional and local fractures, faults, and lineaments. Rock and fracture fluid
pathways can become significant zones of weakness for both the impoundment
basin and the embankment. Basic regional geologic information in the form of
maps and stratigraphic data (e.g., coal seam identification) can usually be
obtained from the state geological survey. The principal methods used to
determine detailed bedrock and soil stratigraphy are drilling and sampling.
Geophysical methods, such as seismic refraction and electrical resistivity, may
complement drilling and sampling (see Chapter 5 ).