Planning Coal Slurry Refuse Impoundments - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 52

amount of water reclaimed is maximized for reuse in the preparation plant (or
other beneficial use).
Collection of reclaimed water consists of returning water from the
impoundment back to the preparation plant or other use. Where possible, this
process is optimized to promote settling of suspended solids and to minimize
the amount of suspended solids in the reclaimed water.
Finally, control of seepage and underdrainage minimizes the amount of
water that leaves the impoundment through the foundation. The objective is to
collect and reuse the maximum amount of water.
IMPOUNDMENT SYSTEM MONITORING
Impoundment monitoring, which continues until closure, includes both
required inspections and the utilization of instrumentation to detect changes
within the system. After closure, both the instrumentation required and the
frequency of monitoring are reduced. Impoundment inspection usually includes
a visual examination of the entire impoundment area, including the
embankment, basin, and proximate surroundings. The embankment inspection
includes looking for cracks, seeps, slumping, or other unusual conditions.
Monitoring instrumentation is often used to measure water pressure within the
embankment (ASCE, 1999; Dunnicliff, 1988; Penman et al., 1999; U.S. Army
Corps of Engineers, 1995; U.S. Bureau of Reclamation, 1987; USCOLD, 1993).
The water level in the basin pool is measured and instrumentation data are
reviewed to assist in water management. In addition, other monitoring
equipment can be installed to detect movement within the embankment or
subsurface. Measurements are commonly made to detect the following effects:
• Surface Displacement—Measured most often by conventional surveying
equipment to detect vertical and horizontal displacement.
• Internal Movement—Measured by single- and multi-point extensometers,
continuous profile gauges, inclinometers, tilt-meters, transverse-acting
devices, and time domain reflectometers (to determine where beds
separate) to detect vertical and horizontal movement below the surface of
the impoundment slurry level.
• Pore Pressure—Measured by piezometers—including open, well-point, and
closed types—to detect the pressure exerted on the instrument. This
pressure is then balanced against an equivalent hydrostatic head or
equivalent fluid pressure.