Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 110

to be installed to evaluate performance must be identified, and the plan to be
executed in the event that performance is not met should be spelled out.
The committee believes there is a limit to risk tolerance, for both existing
and new impoundments. When risk is high, and when mitigation, either through
more reliable characterization or barrier constructing is impossible, of limited
precedent, or so expensive that it is infeasible, then a substantial change in
operation of the impoundment is warranted. This may range from minimizing
slurry fluidity to ceasing operations. If an impoundment fails risk-assessment
criteria and if risk cannot be mitigated it should be phased out or alternatives
considered.
MONITORING
Monitoring is an integral part of the observational method used in
geotechnical engineering. In the design and construction of coal waste
impoundments, monitoring is critical since the construction process continues
for the life of the facility. During the life of the impoundment, which may span
decades or more, conditions may change. For example, the nature and
characteristics of the refuse may differ because of the areal variations in the
geology of the coal seam, mining of different coal seams, alterations in mining
or preparation methods, or rate of waste generation. A well-planned monitoring
program can help to detect when major changes are occurring so that design
modifications can be implemented in a timely manner. The savings that may be
realized by not designing for the most conservative scenario can justify the
scale of the monitoring program. Monitoring can help ascertain when repairs,
improvements, or other upkeep is needed.
Monitoring procedures and instrumentation are well documented (ASCE,
1999; Dunnicliff, 1988) and need not be repeated here. Monitoring
instrumentation requires that appropriate target sites be identified and
accessible. Monitoring of potential failure modes of embankments typically
measures pore pressures, surface and internal deformations, hydraulic
parameters, and vibrations, especially if blasting is being conducted nearby.
Occasionally, temperature and rock stress or soil pressure, especially if high
horizontal stresses exist in the area, should be measured. Hydrogeologic
monitoring and downstream flow and quality measurements may give evidence
that would provide warning of impending basin failures.
It cannot be overemphasized that instrumentation should be used as a
complement to visual observation and not as a substitute. Instruments cannot
adequately establish the extent of vegetation and undergrowth or its removal,