Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 104

times the thickness of the extracted seam, if sufficient width of opening with
respect to seam depth has been excavated. It should be recognized, however,
that these values may have to be modified for known site conditions.
MITIGATIVE MEASURES
The ability to intervene in the event of undesirable performance and to
introduce mitigation measures is an integral aspect of eliminating basin failure.
The ease of doing so depends upon whether slurry elevation has already
exceeded the level of the above-drainage coal seams or associated workings.
If the basin is new or if the slurry elevation has not yet exceeded the level
of above-drainage coal seams, the quality of the existing outcrop barrier
between the basin and the coal seam should be evaluated. If the outcrop barrier
is inadequate, it should be enhanced. A number of enhancement measures are
available, including construction of additional barriers, installation of drains,
promotion of seals such as what might occur with the consolidation of the slurry
itself, and reinforcement of the existing barrier by grouting or other means. The
solids deposited at the bottom of the impoundment can, under some
circumstances, consolidate enough to create an impervious blanket of cohesive
material. This reliance on the impervious zone can be evaluated using well-
established procedures in geotechnical engineering.
In the case where the slurry elevation has not exceeded the level of the
above-drainage mine workings, the assessment of the outcrop barrier is
generally feasible. However, if the condition of the outcrop barrier cannot be
adequately assessed, the risk associated with developing a new facility or
continuing with a preexisting one increases. If the costs of mitigation or the
risks associated with the proposed operations are excessive, an alternative
disposal strategy, which might even entail an alternate site, becomes necessary.
When slurry elevation exceeds the level of the above-drainage workings, it
becomes more difficult to assess the quality of the existing barrier. This may
result in increased cost of investigation or greater uncertainty with the results or
both. For example, as discussed in Chapter 5 , the use of geophysical and remote
sensing techniques in and adjacent to the basin area may reveal anomalies
related to voids or geologic features such as fractures. These anomalies can
provide clues about the potential mechanical and hydrogeological conditions in
the proposed impoundment area. In the absence of geophysical, remote sensing,
or hydrogeological anomalies,