Coal Waste Impoundments Risks, Responses, and Alternatives (2002) / Chapter Skim
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6 Limiting Potential Failures
6 Limiting Potential Failures
Pages 111-130
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From page 111... ...
Particular emphasis is placed on basin failure modes and mitigation activities, because the largest remaining uncertainties for impoundments lie in the characterization of the basin area and in the mitigation of risks associated with the breakthrough potential (see Chapter 3~. Risk assessment of new and existing impoundments is the first step toward risk reduction.
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From page 112... ...
EMBANKMENT FAILURE MODES An embankment to contain coal mining waste is similar to an embankment to store and contain mine waste in other extractive industries, where they are usually called tailings dams. While the nomenclature is different in the coal business refuse impoundment versus tailings dam, fine refuse versus tailings, and coarse refuse versus waste rock the concept of impounding slurry behind an engineered embankment is the same.
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From page 113... ...
Worldwide experience with upstream-constructed tailings dams indicates that many, particularly those with wide subaerial beaches, have performed well in significant seismic events, when subjected to intense rainfall, and sometimes in spite of questionable operating practices. Davies and Martin (2000)
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From page 114... ...
Increasing clay content in fine refuse. Fine refuse impoundments contain increasing amounts of clay, because more coal seams are
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From page 115... ...
Coal refuse impoundments are designed for stability under anticipated static and seismic conditions during the critical phases of construction and operation. For closed refuse impoundments, these stability conditions still apply, but the period for acceptable performance is indefinite.
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From page 116... ...
The designed precipitation event used for a coal refuse impoundment will vary depending on the hazard classification of the facility, but in nearly all cases in the Appalachian region will be the probable maximum precipitation event. This criterion is conservative in that designers are obliged to provide sufficient storage in the facility to contain direct precipitation from the probable maximum precipitation event plus all other influent fluids from processing and runoff and still maintain 3 feet of freeboard.
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From page 117... ...
Basin failure above active or abandoned mine workings may involve one or more of the following modes: Leakage along naturally occurring joints and fractures. Joints and fractures may fill with ultra-fine material but do not necessarily develop resistance to the flow of fluids.
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From page 118... ...
The committee recommends that MSHA and OSM jointly pursue the issue of outcrop coal barrier width and overburden thickness and its competence and develop minimum standards for them. Mine workings below an impoundment should be avoided unless they can be confirmed to be deep enough and to contain an aquitard layer
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From page 119... ...
If specific data cannot be obtained where longwall or other hill extraction pillar recovery has occurred, the surface fractured zone may be considered to be from 50 to 200 feet thick (Kipp and Dinger, 1987; Singh and Kendorski, 1981~. The permeable zone immediately above the openings may be 60
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From page 120... ...
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.
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From page 121... ...
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.
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From page 122... ...
Mine workings were backfilled; a filter fabric was placed along the entire perimeter of the coal seam; cohesive soil fill was compacted in lifts to create a barrier, and coarse refuse was then used to backfill the remaining excavation; and French drains were installed around the entire perimeter of the facility to drain the coarse refuse perimeter embankment {B. Thacker, Geo/Environmental Associates, personal communication, 2001 )
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From page 123... ...
The second classification scheme, which deals with basin failures, comes closer to the standard definition of a risk assessment. That system includes an assessment of the proximity of underground workings and the potential for a failure that would lead to a release of water or slurry from the basin area into underground mine workings.
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From page 124... ...
Geotechnical engineering is used in the design and construction of waste impoundments. Given the inevitable uncertainties in site characterization, knowledge of material properties, and the need for use of idealized models to describe both physical and human behavior, risk is inherent.
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From page 125... ...
Such reviews often examine whether the project is being designed to appropriate standards, and the construction is being managed appropriately, and the committee suggests that coal companies consider whether similar reviews would add value and help manage risk in the design, construction, and operation of coal waste impoundments. To maximize the potential for risk reduction, the committee recommends that all impoundment designs be accompanied by a risk analysis utilizing qualitative methods.
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From page 126... ...
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.
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From page 127... ...
While these investigations and monitoring programs are not directly germane to limiting the potential for refuse impoundment breakthroughs and failures, they may act as an early warning system for mine operators to change their management of the impoundment system. Conjunctive research should also be conducted to demonstrate the use of continuous data loggers and real-time monitoring devices that can monitor and warn of changes in hydrologic, hydrogeologic, and geotechnical conditions around the impoundments.
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From page 128... ...
For example, specific electrical conductance, which is a measurement of capacity of a fluid to transmit an electrical current, is directly proportional to increased solids or salt content in the water. Water from waste impoundments or mine waste usually contains increased dissolved salts, such that significant changes in electric current can be a predictive tool for leakage.
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From page 129... ...
The committee recommends that MSHA and OSM consider requiring additional continuous monitoring in specific instances and evaluate automation of monitoring instrumentation. EMERGENCY PLANNING AND RISK COMMUNICATION All coal companies that operate a coal refuse impoundment system are required to develop emergency response and evacuation plans that describe what may happen and what should be done to limit the damage of any reasonably foreseeable event.
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From page 130... ...
The committee also concludes that geophysical techniques have been underutilizedt in the coal-mining industry and could benefit from additional research. The committee recommends that demonstration projects using modern geophysical techniques be funded, and that the results be widely conveyed to the mining industry and to government regulatory personnel through workshops and continuing education.
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