Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 47

The downstream method consists of construction such that the centerline
of the embankment is moved downstream with subsequent raises ( Figure 3.5 ).
For this method, the embankment is not underlain by previously discharged fine
refuse, and it is generally more stable than an upstream embankment. However,
downstream construction requires increasing volumes of embankment material
with subsequent embankment raises. Sedimentation ponds, pump stations, and
other facilities downstream of the embankment may conflict with future
structural fill using downstream construction methods. A drawback of
downstream construction is that the embankment cannot be reclaimed until
closure, since the embankment face is continually being added to as the
structure moves downstream. For these reasons, the downstream method is less
attractive than the upstream method for construction of coal slurry refuse
impoundments.
The centerline and modified centerline methods of embankment
construction are a compromise between the upstream and downstream methods,
in that the crest of the embankment is raised vertically, or nearly vertically,
instead of being displaced upstream or downstream ( Figure 3.6 ). This method is
generally between the upstream and downstream methods in terms of stability
and required volume of embankment material.
Embankment stability in coal slurry waste impoundments is evaluated with
the same techniques and criteria used for water-storage dams (U.S. Army Corps
of Engineers, 1982; Wilson and Marsal, 1979). Much like a water-storage dam
or reservoir, the embankment of a coal slurry impoundment retains a saturated
material having low shear strength (D'Appolonia Consulting Engineers, 1975).
MSHA regulations are based on the stability of the embankment, foundation,
and abutment; control of seepage for hydraulic considerations; and management
of excess water to control embankment overtopping.
Coal refuse impoundments are subjected to embankment stability
evaluations under static conditions for the designed construction, operation, and
closure conditions. The evaluation of embankment slope stability results in a
calculated factor of safety, which MSHA requires the impoundment designer to
provide, along with methods used to obtain. The factor of safety is typically 1.5,
but is depends on site conditions (30 CFR 77 § 216-2 (a)(13), similar to those
used for water-storage dams.
Evaluation of seismic stability of the embankment (ICOLD, 1989a; Seed,
1979) is based on the potential seismicity of the site and the expected response
of the embankment to seismic vibration. Seismic activity can also affect the
stability of pillars and the strata overlying the coal seam. While seismic hazards
are generally low in Appalachia, they are not absent (Frankel et al., 1996).
Mining-related activities such as underground blasting or