Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 43

IMPOUNDMENT DESIGN AND CONSTRUCTION
Planning for a coal slurry impoundment includes the design of two major
elements, the embankment and the basin. Information gathered from the site
investigation plays an integral part in this process. The regulations MSHA
established require detailed investigation of the stability of the embankment but
are less specific concerning the basin structure. In the subsections that follow,
typical design procedures for each are described.
Embankment
MSHA classifies three types of impoundments: cross-valley, diked, and
incised ( Figure 3.3 ). For coal slurry impoundments in the Appalachian coal
region, the cross-valley impoundment is predominant (ICOLD, 1996; MAC,
1998; U.S. Army Corps of Engineers, 1994; Vick, 1990). The cross-valley
impoundment consists of an embankment constructed across a valley, with
slurry discharged within the valley upstream of the embankment. The operation
of a refuse impoundment normally includes staged construction of the
embankment, generally by upstream or downstream methods.
The oldest and most commonly used method of embankment construction
is the upstream method ( Figure 3.4 ), where the embankment centerline is
moved upstream with sequential raises. This method is used if suitable materials
are available to build a starter dam, if the seismic hazard is low, and if the
embankment can be raised in a stable manner. With this method, a starter dam
is constructed using coarse refuse or locally available materials, and fine refuse
is discharged hydraulically from the crest of the starter dam to form a beach.
Coarse refuse is pushed out over the beach area of the impoundment and is
compacted to form the foundation for a second embankment raise. Construction
continues in this manner as the embankment increases in height. One
disadvantage of this type of embankment construction is that the sections of the
embankment constructed at later stages lie above finer-grained material
discharged during the preceding stages. Under static loading conditions, the
ultimate embankment height will depend on the strength of the consolidated
fine refuse within the zone of shearing, the steepness of the downstream slope
of the embankment, and the location of the phreatic surface within the
embankment. Under seismic loading, the stability of the embankment depends
on the potential of the consolidated fine refuse to liquefy. Upstream
construction lends itself to concurrent reclamation on the downstream face of
the embankment, because the downstream face is usually designed at the final
reclaimed configuration of the embankment.