Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 50

unplanned explosions or pillar failures can produce vibrations, but are
generally of smaller magnitude and shorter duration than actual seismic events.
The duration of the seismic motion and the accelerations over this duration
influence the potential for liquefaction, displacement, and stability of the refuse
in the impoundment.
Another key factor in the static and seismic slope stability of the
embankment is control of seepage for hydraulic considerations (subsurface
water pressure, piping, and erosion), as well as downstream water quality
aspects. The embankment is designed to control water pressure on the upstream
side of the embankment and to prevent migration of fines and to minimize
water pressure and potential for piping on the downstream side of the
embankment. This can be done through filter and drain zones within the
embankment that collect and route water to the downstream toe of the
embankment. While basal drainage beneath the embankment is common in
upstream construction in Appalachia, internal filter and drain zones are not.
Management of excess water (by embankment freeboard, upstream
diversion, and controlled water removal) can prevent embankment overtopping.
Overtopping of an embankment occurs when the water inflow volume exceeds
the storage capacity of the impoundment or spillway. Overtopping is prevented
by providing sufficient freeboard or allowing for a sufficient spillway discharge
rate to either store or safely discharge the anticipated inflow to the
impoundment. To increase the stability of an embankment, inflow can be
reduced with a diversion channel to direct upstream runoff past the
impoundment.
Basin
Filling the basin with slurry increases stress on the basin floor and walls
and increases hydraulic pressure. In response to this increased stress,
compressible materials, such as unconsolidated surface soils, settle. In areas
overlying underground workings, the increase in stress may cause significant
differential settlement near the workings themselves or a zone of subsidence
above the workings.
The impoundment system's hydraulic performance is generally based on
the comparative permeabilities of the coarse refuse embankment, the stored fine
slurry, and the foundation materials beneath the entire impoundment. In the
Appalachian coal industry, the drainage of the refuse slurry water and the pore
water entrained in the fine refuse is primarily by upward water migration as the
slurry settles and consolidates. The collected water then evaporates or is
decanted for reuse in the preparation plant. Drilling into fine refuse