Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 105

clearing of the basin area may be warranted to uncover geologic anomalies that
might otherwise go undetected. While the steps to be taken in the selection of
mitigative measures are the same as noted above, many of the measures become
more difficult to implement ( Sidebar 6.1 ). Hence reliability associated with
assessment and intervention in this case is reduced. This can be overcome, to
some degree, by increased reliance on monitoring.
The assessment of below-drainage workings constitutes a subset of the
second case above. Here the integrity of the cover must be evaluated by some
combination of drilling, geophysics, fracture analysis, and subsidence
assessment. A distinction should be made between active and inactive
workings, because the safety of miners is at stake in active workings. Here, if
basin sealing cannot be relied upon, and alternative methods of slurry
management cannot be practiced, as may be in steep topography, then the size
of the impoundment may have to be restricted.
Coal seams above the maximum permitted slurry elevation may also be
susceptible to blow-out. In this case, the assessment requires an understanding
of the hydrogeological circumstances and whether they are favorable for blow-
out. If so, mitigation is appropriate and some combination of drainage, sealing,
and barrier construction, together with monitoring, is needed.
The committee concludes that selecting the appropriate mitigative
measures relies strongly on reliable basin characterization. The committee
recommends that MSHA and OSM develop and promulgate guidelines for
the site evaluation, design, construction, and operation of basins. They
should be comparable in scope to the guidelines used in embankment design.
If slurry from an impoundment leaks into active or abandoned mine
workings, or may do so, bulkheads or seals may be constructed to preclude the
water from escaping into the outside environment (Chekan, 1985). As discussed
above, many mitigative measures can be designed using established procedures;
bulkheads designed to support high hydrostatic pressure present a different kind
of problem. The committee recommends that MSHA review its current
practice and develop guidelines for the design of bulkheads intended to
withstand hydraulic heads associated with slurry impoundments. The
bulkhead should be constructed of material that can withstand water action
without deterioration in the presence of the various chemicals in the
impoundment water. Further, the bulkhead should be suitably anchored in
competent, unfractured strata. If such an area is not available, pressure grouting
may be needed. Deterioration of the anchoring strata can be a major structural
problem where the bulkhead is keyed into