Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 40

Coal Barriers
Another potential ground weakness resulting from mining may occur near
above-drainage workings. Coal pillars are sized to protect mine openings from
overburden stresses. Individual pillars protect the adjacent openings, and large
“barrier” pillars are left in place to protect groups of entries or to assume
abutment loads from pillar mining. Boundary or perimeter pillars are left in
place to isolate the mine workings from the effects of stress associated with
adjacent mines, and from the effects of water or gases in adjacent mines. In
mines that extend close to the outcrop, a boundary—perimeter barrier or
outcrop coal barrier—is left in place to contain any water that collects in the
mine in the future. Similarly, when mining close to the outcrop, a solid coal
barrier is left in place to contain any such water. If the outcrop barrier width is
insufficient, a blow-out or outcrop barrier failure can occur. A blow-out failure
occurs when water pressure within the mine forces the outcrop barrier outward,
or when deterioration of the floor enables the outcrop barrier to slide. Blow-ins
could occur where the pressure of water or slurry impounded above the outcrop
barrier forces the barrier into the mine. A breakthrough is a general term that
describes the catastrophic failure of outcrop barriers, resulting from water or
slurry entering the mine through fractures, joints, “mud seams,” or lineaments
in the overburden strata. If the mine is active, a breakthrough could endanger
miners. Water or slurry in a mine creates the potential for stream pollution and
consequent endangerment of public safety.
Currently, no federal (but some state) regulations govern the width of
outcrop barrier that should be left. For example, Kentucky guidelines require a
barrier of 50 feet plus 1 foot for each foot of hydraulic head; West Virginia does
not specify a minimum but requires mine maps and permits to note the outcrop
barrier width ( Table 3.1 ).
Geotechnical Characterization of Foundation and
Construction Materials
Assessment of the geotechnical properties of soil and rock materials
forming the foundation for both the embankment and the impoundment basin is
a critical part of the site characterization and design process (Vick, 1990).
Embankment foundations receive special consideration in MSHA reviews since
they must support loading, control seepage, and satisfy structural and water
management issues. In addition, seepage from the impoundment often is
controlled by the permeability of the underlying natural rock or soil strata.