Coal Waste Impoundments Risks, Responses, and Alternatives (2002) / Chapter Skim
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3 Planning Coal Slurry Refuse Impoundments
3 Planning Coal Slurry Refuse Impoundments
Pages 51-70
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From page 51... ...
The position of potential fluid pathways—such as coal seams, mine workings, and fractures relative to an impoundment is a significant factor in the design of new and modifications to existing coal refuse impoundment systems in the Appalachian region. Most impoundments in Appalachia utilize the natural topography to form the storage basin that will contain the slurry.
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From page 52... ...
The sections that follow describe current approaches to each of these topics. GENERAL IMPOUNDMENT SITING CRITERIA Site investigations require a preliminary examination for site selection followed by a detailed site study to develop safe and economical designs that satisfy regulatory requirements.
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From page 53... ...
However, despite this lack of regulation, standard engineering practice implies that some level of basin analysis should be included in design; the degree to which this is done appears to be site specific. The probability of exposing cracks, faults, coal seam outcrops, or other preferential pathways for water is greatly enhanced by clearing surface soils and exposing the basin foundation materials.
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From page 54... ...
Therefore, permeability of the original rock gradually becomes more important with depth (Hariow and LeCain, 1991~. Hydrogeologic studies of boreholes in coal-bearing strata in the Appalachian coalfields have consistently shown that fractured rock and coal seams have permeabilities nearly three orders of magnitude greater than that of adjacent, nonfractured bedrock (Harrow and LeCain, 1991; Kipp and Dinger, 1987; Minns; 1993; Wunsch, 1993~.
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From page 55... ...
The magnitude and extent to which subsidence will impact an area are governed by a variety of factors. For example, coal seam geometry, geology and hydrogeology, and mining history may directly affect the potential degree of subsidence from coal mining.
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From page 56... ...
For example, Kentucky guidelines require a barrier of 50 feet plus 1 foot for each foot of hydraulic head; West Virginia does not specify a minimum but requires mine maps and permits to note the outcrop barrier width (Table 3.1~. Geotechnical Characterization of Foundation and Construction Materials Assessment of the geotechnical properties of soil and rock materials forming the foundation for both the embankment and the impoundment basin is a critical part of the site characterization and design process (Vick, 1990~.
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From page 57... ...
~3) WV No standard minimum width is specified; how ever, the plans must note the outcrop barrier width and the maximum head.
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From page 58... ...
Using coarse refuse for embankment construction solves the disposal issue for the coarse waste, which would otherwise have to be disposed of elsewhere within the mine's permitted area. Coarse refuse is fairly homogeneous in particle size and strength characteristics over time and is therefore a comparatively predictable construction material for meeting the engineering design specifications for embankment materials.
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From page 59... ...
With this method, a starter dam is constructed using coarse refuse or locally available materials, and fine refuse is discharged hydraulically from the crest of the starter dam to form a beach. Coarse refuse is pushed out over the beach area of the impoundment and is compacted to form the foundation for a second embankment raise.
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From page 60... ...
. ~~ ~~ ~~,~,~,.~ ~~, a,`-,,, .,,, ~~ ~~,~,~,,~,~,,~,~,~.~W,,,,,,~,,.,,,,~ A, ,, ~ \ ~,~,~-~5,~;~ ~~,~ Am,, ~-,~,~,~-~ ~~-~ I,,- ~~ \, \ W~ ~~-~, ~~,~ ~~N,:_~\ \ \ ~~'~"~'~''~".'~ \ \ ~/6 _ [- '.,,.,__, \ \~/01,- \ ~~ 1~- ~ - ~ We,,,,; \ to, \ At ~ ~ ~0~\ ~0' ~ ' FIGURE 3.3 Schematic diagram of coal refuse impoundments: (a)
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From page 61... ...
~~ ~~ ~~ -ban~en' ~ of bet _~' ~ ~ ~ ~ i ~ ~1' ~ 1 I 1 ' ~ ~ ~ I I #'' '-''''-''I ~ ~ ~ ~ ~ _V...
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From page 62... ...
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From page 63... ...
Coal refuse impoundments are subjected to embankment stability evaluations under static conditions for the designed construction, operation, and closure conditions. The evaluation of embankment slope stability results in a calculated factor of safety, which MSHA requires the impoundment designer to provide, along win methods used to obtain.
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From page 65... ...
65 ~ o ca l, o an · c)
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From page 66... ...
In areas overlying underground workings, the increase in stress may cause significant differential settlement near the workings themselves or a zone of subsidence above the workings. The impoundment system's hydraulic performance is generally based on the comparative permeabilities of the coarse refuse embankment, the stored fine slurry, and the foundation materials beneath the entire impoundment.
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From page 67... ...
However, where geologic anomalies or coal seam outcrops occur, advantages can be gained by managing slurry deposition around He basin perimeter. Depositing slurry so that it forms a consolidated beach around the basin perimeter creates a control zone, which forces the separated water away from the basin foundation contact.
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From page 68... ...
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)
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From page 69... ...
This may require major regrading of solids or selective slurry discharge during the final stages of operation to create the desired draining surface. Cover materials such as coarse refuse or soils from surrounding locations are placed over the consolidated mine refuse mass and graded to the final closure configuration.
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From page 70... ...
The relative elevation of local drainage and slurry height, with respect to underground mines, can be critical. Existing impoundments with abovedrainage mine workings, where the outcrop slurry elevation does not exceed the level of the coal mine workings, can incorporate mitigating measures for these workings in their design relatively easily (see Chapter 6~.
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