Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 51

materials in impoundment basins has revealed that the strength characteristics
of the stored solids tend to increase with depth because the pore water entrained
in fine refuse decreases with depth (Schiffman et al., 1988; Thacker, 2000;
Thacker et al., 1988). Common practice relies on the self-sealing of the basin
with slurry compaction and consolidation rather than the construction of liners.
SLURRY AND WATER MANAGEMENT
In most refuse impoundment systems, the slurry is discharged into the
basin area through a pipeline system along the embankment crest. Slurry is
either released into the basin from a single-point discharge that is moved
periodically across the embankment face, or it is discharged from multiple
points along the embankment. Both methods of discharge create a beach effect,
with coarser material close to the discharge point, and finer slurry and water
usually farther from the discharge point at the upstream end of the
impoundment. The water is removed from this area through a decanting or
pumping system.
The consolidated beach that forms against the embankment will enhance
its stability because the coarser material tends to drain more easily and is the
more competent component of the slurry. Most coal operators limit the location
of slurry discharge to the embankment face so as to improve the stability of the
embankment. However, where geologic anomalies or coal seam outcrops occur,
advantages can be gained by managing slurry deposition around the basin
perimeter. Depositing slurry so that it forms a consolidated beach around the
basin perimeter creates a control zone, which forces the separated water away
from the basin foundation contact. This reduces the potential for hydraulic
communication between the fully saturated pool area and an exposed coal seam,
crack, or opening in the basin area.
Operation of a coal waste slurry impoundment requires management of the
slurry liquid without discharge into the surrounding environment, as well as
accommodation of natural precipitation and evaporation. Effective water
management for mining facilities (Hutchinson and Ellison, 1992; Vick, 1990)
includes water balance, water reclamation, and seepage and underdrainage
control.
Water balance is determined by measuring the inflow and outflow of the
impoundment system on an annual to daily basis. The water balance can be
used to evaluate whether there is a long-term net gain or loss of water under
anticipated climatic conditions and to assess the effects of particular short-term
extreme precipitation events The balance is also used to ensure that the