Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 99

being mined that contain increasing amounts of clay partings. Since the
clay particles typically slow the rate of consolidation and reduce shear
strength properties of mill tailings and fine refuse (Vick, 1990), the
consolidation and shear strength characteristics of the fine refuse should
include the anticipated amount of clay that would be present.
• Seismic evaluation. Coal refuse impoundments are designed for stability
under anticipated static and seismic conditions during the critical phases of
construction and operation. For closed refuse impoundments, these stability
conditions still apply, but the period for acceptable performance is
indefinite. This means that for evaluation of seismic conditions (as well as
precipitation events), events representing long-term recurrence intervals are
required (such as 500-year to maximum credible earthquake loading
conditions). These long-term seismic loading conditions, coupled with
whether the refuse impoundment remains saturated, should be included in
the evaluation of the post-closure performance of slurry impoundments.
Seepage and Piping
Seepage through embankments can lead to failure by internal erosion, the
process commonly known as piping. In design and construction of water-
storage embankments, the possibility of seepage-caused piping is commonly
prevented by installation of filters or drains. Although the use of internal filters
or drains in embankments is not common practice in Appalachia, drainage
through the embankment is an important consideration in the design and
construction of the structure. The coarse coal refuse filters the fines while
allowing clear seepage to flow. The drains that are included provide a
secondary line of defense for the control of seepage. The lack of an internal
filter is acceptable, but it places an extra burden on high-quality compaction
control and the use of subaerial beaching at various locations around the
perimeter of the impoundment (the beach area created near the discharge point
of the slurry transfer pipe) as an additional line of defense. Beaching should
also be considered as a line of defense around the basin.