Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Planning Coal Slurry Refuse Impoundments This chapter summarizes the current engineering elements of the design, operation, and reclamation of coal refuse impoundment systems. The chapter addresses these elements in relation to the Appalachian coal region, where impoundments are often located in steep valleys. It is not a comprehensive overview of all engineering design options, but a description of the major elements currently implemented in the planning and design of coal slurry refuse impoundments. The engineering design process is specific to the conditions of each particular site: What works in one circumstance may not work in another. The position of potential fluid pathwayssuch 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. This is often accomplished by constructing an embankment at the mouth of a small valley or watershed to complete the basin structure used for storage. The stream channel at the base of the basin selected for the impoundment defines the approximate level of local drainage. Coal seams and mines that do not crop out above the level of the stream channel are termed below-drainage, whereas, those that crop out along the valley wall above the stream channel are termed above-drainage (Figure 3.1~. In some locations, both above- and below-drainage coal seams (and mines) may be present. The relative elevations of local drainage and slurry height can be critical. For example, during the filling of the impoundment, if the slurry elevation exceeds the level of an above-drainage coal mine, the mine could become submerged and provide a hydraulic conduit for the slurry. In addition, below- drainage workings can be in hydraulic connection with slurry through less direct means, such as fractures. Both above- and below-drainage mine workings require specific engineering considerations. 51
52 COAL WASTEIMPOUNDMENTS Hillside 1 Level of slurry ~ / Preexishng stream drainage Below-drainage mine .s ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ / Above-drainape mine ~ ~ ~ I / Outcrop barriers Above-drainane mine FIGURE 3.1 Cross section of schematic coal waste impoundment, depicting underground mine workings above and below drainage with respect to the impoundment; outcrop barriers are indicated. For impoundments with both above- and below-drainage mine workings, assessment of the strength and permeability of the material between the impoundment and the underlying mines is important. This assessment includes the distance and geologic conditions between the impoundment and the underlying mines, the potential for hydraulic connection, and the potential for collapse of the underground workings. At sites where above-drainage seams have already been covered or inundated by slurry, this assessment is more complex. Finally, for impoundments that will cross above-drainage coal seams or workings in the future, it is essential to explore thoroughly the site along the coal seam outcrop and to plug or seal off the contacts with the coal seam (see Chapters 5 and 6~. Proper engineering is critical at all stages of impoundment life: site assessment, construction, operation, monitoring, and closure. 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. These investigations are primarily focused on siting the embankment by assessing the factors influencing embankment foundation strength, water seepage through the embankment foundation and
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS 53 the bottom of the impoundment, and the quality and quantity of foundation materials. Specifically, these factors include the geology and hydrogeology of the site, the potential for subsidence, and the geotechnical characteristics of foundation materials. While the geotechnical characterization of the embankment foundation is well developed in the coal industry, the level of geotechnical investigation of the basin seems to exhibit much less consistency. Basin investigation can evaluate potential weaknesses along which fluids could travel. Examples include rock fractures, subsidence zones, and permeable strata. Although not common, some coal-bearing formations contain limestone, and there is the potential for karst development and fluid migration along solutionally-en- larged fractures or conduits. The committee could not identify a consistent set of criteria or a guidance document for conducting a site investigation of the impoundment basin. Regulations to this effect are usually promulgated by the states and vary by state. 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. Because the topography of the Appalachian coal region is characterized by steep surfaces covered with dense vegetation, the side slopes of the basin area are often difficult to access using conventional earth-moving equipment. As such, many basin investi- gations appear to be limited to visual reconnaissance of the basin side slopes, with clearing of vegetation and topsoil and stripping of soils limited to He accessible areas in the valley bottoms only. Site Geology and Hydrogeology Geologic site investigations should include the depth, thickness, continuity, and composition of each significant geologic layer and an evaluation of regional and local fractures, faults, and lineaments. Rock and fracture fluid pathways can become significant zones of weakness for both the impoundment basin and the embankment. Basic regional geologic infor- mation in the form of maps and stratigraphic data (e.g., coal seam identi- fication) can usually be obtained from the state geological survey. The principal methods used to determine detailed bedrock and soil stratigraphy are drilling and sampling. Geophysical methods, such as seismic refraction and electrical resistivity, may complement drilling and sampling (see Chapter
54 COAL WASTEIMPOUNDMENTS 5~. These methods can prove invaluable for determining bedrock surface and water table profile. However, they require careful interpretation in conjunction with other geologic information, particularly that obtained from boreholes. In the Appalachian Plateau, the underlying bedrock is strongly fractured, and nearly all of the rock types found in coal basins have very low primary porosity (Stach, 1982; Weinheimer, 1983~. Continuous regional joints and fracture systems are superimposed on the relatively shallow near-surface fracture systems, resulting in a complex array of fractures that dominate the shallow flow of groundwater (Minns, 1993~. The majority of water in coal- bearing rocks is transmitted in secondary permeability features, such as fractures, joints, bedding planes, and coal cleats. The Win rind of fractured bedrock that extends from the bedrock surface down to a depth of between 80 and 200 feet, has been termed the near-surface fracture zone (Kipp et al., 1983; Wyrick and Borchers, 1981~. The role of secondary permeability decreases with depth due to decreasing fracture width, length, and intercon- nectedness. 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~. .. Subsidence Mine subsidence is defined as the ground movement that occurs when the overlying strata collapse into the mine openings (Brauner, 1973; Gray et al., 1974; Shadbolt, 1977; Singh, 1992~. These can be either active or abandoned mines. These collapses can create zones of weakness or fluid flow, which are important considerations in impoundment design. On the surface the distur- bance is manifested as cracks depending on the depth-to-mined-height ratio and other factors. The layers above the caved-in zone are subjected to com- pressive and tensile stresses (Figure 3.2~. Tensile stress in the vertical direction generally gives rise to bed separation, whereas, in the horizontal direction it may open joints in the rock formations. Surface subsidence entails both vertical and lateral movements of four types: cracks (tension or shears, buckling (due to compression or shears, pits (also locally termed potholes, sinkholes, chimneys, crownholes, or pipes), and troughs (or sags).
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS D Vertical compression - Compression Coal Floor heave _ Neutral Original ~ ground surface ~~,er`S\on Compression Vertical extension Vertical compression . ~ Extension - , PI ~ Zone of incomplete convergence 55 FIGURE 3.2 States of stress over a mined area. A generic diagram that encompasses active and abandoned mines, and longwall and room-and-pillar mines. D = depth to seam; t = mined coal thickness; p = mined area (longwall or pillared) within coal seam. After Shadbolt, 1977. 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. The amount of coal removed from the seam, the depth of the mine, and the orientation of the void, respectively, determine the degree, timing, and geometry of subsidence features. The mining methods used, rate of face advances, degree of extraction, and presence or absence of mined areas can also have an effect. Finally, factors such as in-situ stresses, topography, and time elapsed since the area was mined can also influence subsidence. Site-specific geologic information, including the availability of mine maps and associated geotechnical information, is critical to evaluating the potential for subsidence. These characteristics apply to both active and abandoned mines. An understanding of these phenomena is also required for planning mine layouts near or under previously existing impoundments.
56 COAL WASTEIMPOUNDMEN7S Coal Barriers Another potential ground weakness resulting from mining may occur near above-drainage workings. Coal pillars are sized to protect mine openings from overburden stresses. Individual pillars protect the adjacent openings, and large "barrier" pillars are left in place to protect groups of entries or to assume abutment loads from pillar mining. Boundary or perimeter pillars are left in place to isolate the mine workings from the effects of stress associated with adjacent mines, and from the effects of water or gases in adjacent mines. In mines that extend close to the outcrop, a bounda~perimeter barrier or outcrop coal barrier is left in place to contain any water that collects in the mine in the future. Similarly, when mining close to the outcrop, a solid coal barrier is left in place to contain any such water. If the outcrop barrier width is insufficient, a blow-out or outcrop barrier failure can occur. A blow-out failure occurs when water pressure within the mine forces the outcrop barrier outward, or when deterioration of the floor enables the outcrop barrier to slide. Blow-ins could occur where the pressure of water or slurry impounded above the outcrop barrier forces the barrier into the mine. A breakthrough is a general term that describes the catastrophic failure of outcrop barriers, resulting from water or slurry entering the mine through fractures, joints, "mud seams," or lineaments in the overburden strata. If the mine is active, a breakthrough could endanger miners. Water or slurry in a mine creates the potential for stream pollution and consequent endangerment of public safety. Currently, no federal (but some state) regulations govern the width of outcrop barrier that should be left. 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~. Embankment foundations receive special consideration in MSHA reviews since they must support loading, control seepage, and satisfy structural and water management issues. In addition, seepage from the impoundment often is controlled by the permeability of the underlying natural rock or soil strata.
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS TABLE 3.1 Coal Outcrop Bander Minimum Width, by Selected States 57 State Minimum Width in Feet Regulations AL No standard minimum width is specified. No standard minimum width is specified. Alabama Administrative Code, Ch. 880-X-81 Illinois Administrative Code, Title 62 IN No standard minimum width is specified. Indiana Code, Title 14 1~( Minimum width is to be determined on a case Kentucky by case basis but will be at least W = 50 feet Administrative + H. unless it can be shown that less will be Regulations, Title effective. An exception may be made if no 405, Subtitle 18:010.6 water accumulation is expected. MD Minimum width is to be 50 feet or W = 20 + Code of Maryland, 4T + 0.1 D (whichever is greater). If barrier will Title 26, Subtitle be affected by hydrostatic head, the barrier 20.13.1 1 will be a foot wide for each foot to hydrostatic head. OH No standard minimum width is specified. Ohio Administrative Code, 1501:13 PA No standard minimum width is specified, but Pennsylvania Code, the necessity for a coal barrier to prevent Title 25, Chapter water release is noted. 89.54 (b) TN* No standard minimum width is specified. 30 CFR, Part 942 VA Minimum width will be determined either as a Virginia site specific design or by W = 50 + H. An Administrative Code, exception may be made if the barrier will not Title 4, Agency 25, be subject to hydrostatic head. Chapter 130-817.41 (a)(i)~3) WV No standard minimum width is specified; how- ever, the plans must note the outcrop barrier width and the maximum head. In addition, mine maps must designate the location and thickness of outcrop bamers. West Virginia Surface Mining Reclamation Regulations, Title 38- Series 2-Section 3.13 (a) and (c) and Section 3.4 0.2.H). NOTE: W = minimum width (feet); H = maximum hydrostatic head (feet); T = thickness of extraction (feet); D = depth of overburden (feet) * Tennessee does not have primacy.
58 COAL WASTEIMPOUNDMENTS The geotechnical properties of embankment foundation materials are determined by physical tests to assess the compressive and shear strengths, bearing capacity, permeability, consolidation and cohesion, plasticity, and moisture content of the potential foundation materials. In particular, determining the permeability of the surficial materials that underlie Me impoundment is necessary for assessment of possible seepage from the pond; this is especially important if a mine underlies the impoundment. Geotechnical investigations also involve drilling in the "footprint" of the embankment. Drilling programs are designed to penetrate the foundation materials to an appropriate depth. Multiple holes are required along the embankment footprint to establish homogeneity or variability of the foundation materials throughout the structure. Materials recovered from the drill hole are tested to establish strength, consolidation, and permeability characteristics. Settlement of the embankment foundation is generally not a factor in embankment design where the foundation is sound rock and the embankment area has not been undermined. Although many embankments are constructed from noncohesive materials and are not subject to cracking, some compacted refuse materials exhibit brittle behavior and may crack as the foundation material settles. Particular attention is given to the foundation of both starter dams and embankments to avoid embankment cracking, which can be caused by differential settlement between a compressible valley bottom and rock abutments or outcrops. In some cases, the geotechnical properties of the basin foundation materials are also characterized. The basin area is not always drilled, but where subsidence of below-drainage mine workings is a possibility, operators sometimes drill and test as they do for the embankment foundation. Bore- holes in the basin area must be plugged to limit communication between the impoundment and the underlying strata. Finally, geotechnical characterization of the construction materials to be used for the embankment is also a routine part of site evaluation. Most coal operators utilize the coarse refuse generated by the coal preparation process to construct the embankment. Using coarse refuse for embankment construc- tion 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.
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS IMPOUNDMENT DESIGN AND CONSTRUCTION 59 Planning for a coal slurry impoun~nent includes the design of two major elements, the embankment and the basin. Information gathered from the site investigation plays an integral part in this process. The regulations MSHA established require detailed investigation of the stability of the embankment but are less specific concerning the basin structure. In the subsections that follow, typical design procedures for each are described. Embankment MSHA classifies three types of impoundments: cross-valley, diked, and incised (Figure 3.3~. For coal slurry impoundments in the Appalachian coal region, the cross-valley impoundment is predominant (ICOLD, 1996; MAC, 1998; U.S. Army Corps of Engineers, 1994; Vick, 1990~. The cross-valley impoundment consists of an embankment constructed across a valley, with slurry discharged within the valley upstream of the embankment. The operation of a refuse impoundment normally includes staged construction of the embankment, generally by upstream or downstream methods. The oldest and most commonly used method of embankment construc- tion is the upstream method (Figure 3.4), where the embankment centerline is moved upstream with sequential raises. This method is used if suitable materials are available to build a starter dam, if the seismic hazard is low, and if the embankment can be raised in a stable manner. 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. Construction continues in this manner as the embankment increases in height. One disadvantage of this type of embankment construction is that the sections of the embankment constructed at later stages lie above finer-grained material discharged during the preceding stages. Under static loading conditions, the ultimate embankment height will depend on the strength of the consolidated fine refuse within the zone of shearing, the steepness of the downstream slope of the embankment, and the location of the phreatic surface within the embankment. Under seismic loading, the stability of the embankment depends on the potential of the consolidated fine refuse to liquefy. Upstream construction lends itself to concurrent reclamation on the downstream face of the embankment, because the downstream face is usually designed at the final reclaimed configuration of the embankment.
60 \ ~ -'/~/~ COAL WASTEIMPOUNDMENTS Coal slurry refuse - Elevation 50' (a) Cross-valley refuse impoundment. 1 : :: . ,~ :: i: :: : : ~.~: ::! I; 'aft ~~ ~~ ~.~ I: ~ ~~'~ ~~ ~~ ~ :- 1: I. ~~ - i: ~ ~ ~~'~ ~~ ~~:~ : aft: ~~: ~~ ~~ .~:~':~.~:~-~,~ ~~W ~~ ~ ~~ ~ ~ ~ If. ~~.~ ~ ~ ~~'~ ~~ I'd. ,~ ~~ I:' ~ ~:~:~ . _.' ~~'~-~''~'~ 'A : ~ . ~ ' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ - l- ~~ ~~~ ~~ ~~ ' ~ ~~ ~'-~ W~'~'~'~'~'~'"~''~''':~ ' - 11 1 my. ~ \ \ ~~ '' '' ~ '-_ ' ~~ '':' '' '' ' A'. '.. ~~ ~~ ~ ' ~ ~ ~2~.',~ ' ~ ,,, , _~\ 1 ~ \ W~.~:~:'.:~'-~'~'~'~ ~'~.~'~'~'~2~'~'W'~ _ _ : ~~ ~ ~~: -aft ~~ : ~~ ~~ ~~ , _. ,,- ~~. . ~~ ~~ ~~,~,~,.~ ~~, 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) cross-valley, (b) diked, and (c) incised impoundments.
~~ ~~ ~~ - -ban~en' ~ of bet _~' ~ ~ ~ ~ i ~ ~1' ~ 1 I 1 ' ~ ~ ~ I I #'' '-''''-''I ~ ~ ~ ~ ~ _V... ...................................... V ~ ~ ~ I ~ _ Ha/ Ed ~~ (b) Diked reuse i~oundmenL o_ ~_~ -_ sin He ~~ s~n of Beam remold ~ =~= Mae As (c) Incised reuse impoundment ~7 ^~e of amok/ far em~enf magi/ \ an/ ~~ ~~ / \ )=f of salt Mae / Beans
62 o - ._ - ~ a' ~ _ in Q ,~ Ct5 ~ 9~ ._ LL ~ o - ._ CO a) - Q G) O ~ C' 9 En 1\ 1 1 o Q ,3 'I o S U) U. 00 - .= - e~ o ._ ._ - ~S - ~: C) V] C'
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS 63 The downstream method consists of construction such that the centerline of the embankment is moved downstream with subsequent raises (Figure 3.5~. For this method, the embarkment is not underlain by previously discharged fine refuse, and it is generally more stable than an upstream embankment. However, downstream construction requires increasing volumes of embankment material with subsequent embankment raises. Sedimentation ponds, pump stations, and other facilities downstream of the embankment may conflict with future structural fill using downstream construction methods. A drawback of downstream construction is that the embankment cannot be reclaimed until closure, since the embankment face is continually being added to as He structure moves downstream. For these reasons, the downstream method is less attractive than the upstream method for construc- tion of coal slurry refuse impoundments. The centerline and modified centerline methods of embankment con- struction are a compromise between the upstream and downstream methods, in that the crest of the embankment is raised vertically, or nearly vertically, instead of being displaced upstream or downstream (Figure 3.6~. This method is generally between the upstream and downstream methods in terms of stability and required volume of embankment material. Embankment stability in coal slurry waste impoundments is evaluated with the same techniques and criteria used for water-storage dams (U.S. Anny Corps of Engineers, 1982; Wilson and Marsal, 1979~. Much like a water-storage dam or reservoir, the embankment of a coal slurry impound- ment retains a saturated material having low shear strength (D'Appolonia Consulting Engineers, 1975~. MSHA regulations are based on the stability of the embankment, foundation, and abutment; control of seepage for hydraulic considerations; and management of excess water to control embankment overtopping. 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. The factor of safety is typically l.5,.but is depends on site conditions (30 CFR 77 § 216-2 (a)~13), similar to those used for water-storage dams. Evaluation of seismic stability of the embankment (ICOLD, 1989a; Seed, 1979) is based on the potential seismicity of the site and the expected response of the embankment to seismic vibration. Seismic activity can also affect the stability of pillars and the strata overlying the coal seam. While seismic hazards are generally low in Appalachia, Hey are not absent (Frar~kel et al., 1996~. Mining-related activities such as underground blasting or
64 ~ o _ ._ W .~ a)= C 0 in w ._ ~ ~~ 3 w C15 ~ =~ E ° ._ ~ lo ~ \ o Q . - 50 92 en o U. - ._ Ct o ED A: ._ U. ·Cd ._ CO o Cat ._ C, ._ C) U. = At _I
65 ~ - o ca l, o an · - c) cSs a, so i: - . ct He o · · cd · - cD At rid 5 C) -
66 COAL WASTEIMPOUNDMENTS unplanned explosions or pillar failures can produce vibrations, but are generally of smaller magnitude and shorter duration than actual seismic events. The duration of the seismic motion and the accelerations over this duration influence the potential for liquefaction, displacement, and stability of the refuse in the impoundment. Another key factor in the static and seismic slope stability of the embankment is control of seepage for hydraulic considerations (subsurface water pressure, piping, and erosion), as well as downstream water quality aspects. The embankment is designed to control water pressure on the upstream side of the embankment and to prevent migration of fines and to minimize water pressure and potential for piping on the downstream side of the embankment. This can be done through filter and drain zones within the embankment that collect and route water to the downstream toe of the embankment. While basal drainage beneath the embankment is common in upstream construction in Appalachia, internal filter and drain zones are not. Management of excess water (by embankment freeboard, upstream diversion, and controlled water removal) can prevent embankment overtopping. Overtopping of an embankment occurs when the water inflow volume exceeds the storage capacity of the impoundment or spillway. Overtopping is prevented by providing sufficient freeboard or allowing for a sufficient spillway discharge rate to either store or safely discharge the anticipated inflow to the impoundment. To increase the stability of an embankment, inflow can be reduced with a diversion channel to direct upstream runoff past the impoundment. Basin Filling the basin with slurry increases stress on the basin floor and walls and increases hydraulic pressure. In response to this increased stress, com- pressible materials, such as unconsolidated surface soils, settle. 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. In the Appalachian coal industry, the drainage of the refuse slurry water and the pore water entrained in the fine refuse is primarily by upward water migration as the slurry settles and consolidates. lathe collected water then evaporates or is decanted for reuse in the preparation plant. Drilling into fine refuse
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS 67 materials in impoundment basins has revealed that the strength characteristics of the stored solids tend to increase with depth because the pore water entrained in fine refuse decreases with depth (Schiffinan et al., 1988; Thacker, 2000; Thacker et al., 1988~. Common practice relies on the self- sealing of the basin with slurry compaction and consolidation rather than the construction of liners. SLURRY AND WATER MANAGEMENT In most refuse impoundment systems, the slurry is discharged into the basin area through a pipeline system along the embankment crest. Slurry is either released into the basin from a single-point discharge that is moved periodically across the embankment face, or it is discharged from multiple points along the embarkment. Both methods of discharge create a beach effect, with coarser material close to the discharge point, and finer slurry and water usually farther from the discharge point at the upstream end of the impoundment. The water is removed from this area through a decanting or pumping system. The consolidated beach that forms against the embankment will enhance its stability because the coarser material tends to drain more easily and is the more competent component of the slurry. Most coal operators limit the location of slurry discharge to the embankment face so as to improve the stability of He embankment. 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. This reduces the potential for hydraulic communication between the fully saturated pool area and an exposed coal seam, crack, or opening in the basin area. Operation of a coal waste slurry impoundment requires management of the slurry liquid without discharge into the surrounding environment, as well as accommodation of natural precipitation and evaporation. Effective water management for mining facilities (Hutchinson and Ellison, 1992; Vick, 1990) includes water balance, water reclamation, and seepage and underdrainage control. Water balance is determined by measuring the inflow and outflow of the impoundment system on an annual to daily basis. The water balance can be used to evaluate whether there is a long-term net gain or loss of water under anticipated climatic conditions and to assess the effects of particular short- term extreme precipitation events The balance is also used to ensure that the
68 COAL WASTEIMPOUNDMENTS amount of water reclaimed is maximized for reuse in the preparation plant (or other beneficial use). Collection of reclaimed water consists of returning water from the impoundment back to the preparation plant or other use. Where possible, this process is optimized to promote settling of suspended solids and to minimize the amount of suspended solids in the reclaimed water. Finally, control of seepage and underdrainage minimizes the amount of water that leaves the impoundment through the foundation. The objective is to collect and reuse the maximum amount of water. IMPOUNDMENT SYSTEM MONITORING Impoundment monitoring, which continues until closure, includes both required inspections and the utilization of instrumentation to detect changes within the system. After closure, both the instrumentation required and the frequency of monitoring are reduced. Impoundment inspection usually includes a visual examination of the entire impoundment area, including the embankment, basin, and proximate surroundings. The embankment inspec- tion includes looking for cracks, seeps, slumping, or other unusual conditions. Monitoring instrumentation is often used to measure water pressure within the embankment (ASCE, 1999; Dunnicliff, 1988; Penman et al., 1999; U.S. Army Corps of Engineers, 1995; U.S. Bureau of Reclamation, 1987; USCOLD, 1993~. The water level in the basin pool is measured and instrumentation data are reviewed to assist in water management. In addition, other monitoring equipment can be installed to detect movement within the embankment or subsurface. 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 exten- someters, continuous profile gauges, inclinometers, tilt-meters, transverse-acting devices, and time domain reflectometers (to determine where beds separate) to detect vertical and horizontal movement below the surface of the impoundment slurry level. Pore Pressure Measured by piezometers including open, well- point, and closed types to detect the pressure exerted on the instrument. This pressure is then balanced against an equivalent hydrostatic head or equivalent fluid pressure. =
PLANNING COAL SLURRY REFUSE IMPOUNDMENTS 69 Groundwater Levels Measured by monitoring wells. In addition, the groundwater chemistry may also be monitored. Surface Water DischargeMeasured by weirs, flumes, or flow meters. Subsurface Settlement, Readjustment, and SubsidenceMeasured by borehole extensometers, time domain reflectrometers, accelero- meter arrays, and piezometers to detect changes in hydrostatic pressure. Visual observations are also used to identify subsidence by propagation of fractures to the ground's surface. CLOSURE AND RECLAMATION The process of changing from an active or inactive impoundment to an abandoned impoundment is referred to as closure and reclamation (Sweigard, 1992~. Three major elements comprise reclamation: surface grading, closure water management, and long-term stability. Surface grading reconfigures the final impoundment surface so it sheds runoff and will not erode or form pools of water. 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. The regraded surface is then covered with topsoil or an approved substitute material and revegetated. Closely tied with surface regrading is closure water management. This involves: removal of the residual ponded slurry water (by reuse or evaporation) before surface regrading; control of adverse geochemical reactions, sometimes using chemical or other additives, and capping; and management of runoff to control erosion and sediment before and during establishment of vegetation. Long-term stability is maintained as the impoundment is transformed from an operating facility with active observation and monitoring, to a closed facility with less frequent observation and monitoring. This means that the slope stability and erosional stability of the closed and reclaimed facilities must not deteriorate over time. The reclamation surety for slurry impoundments is typically released in phases. As an operator completes a discrete phase of the reclamation obligation, such as earthmoving or revegetation, portions of the surety are released. When the final phase is released, the company has no further liability for the reclaimed impoundment, unless the regulatory authority can show that the operator misrepresented submitted material facts (L. Adams, Kentucky Department of Surface Mining, Reclamation, and Enforcement, personal communication, 2001~. -
70 COAL WASTEIMPOUNDMENTS SUMMARY The principles that govern regulation of the design of structures that were promulgated in response to the Buffalo Creek disaster are well understood and fully documented. While continued" vigilance concerning design, construction, and operation of impoundments is clearly warranted (Chapter 6), the committee concludes that uncertainties remain in the characterization of the basin area and, therefore, in the mitigation of risks associated" with the breakthrough potential. The potential for underground coal mine workings to be in close proximity to an impoundment is a factor in the design of new and in modifications to existing coal waste impoundments in Appalachia. The relative elevation of local drainage and slurry height, with respect to underground mines, can be critical. Existing impoundments with above- drainage 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~. Above- drainage coal mine workings in existing impoun~nents, where the slurry elevation exceeds the level of the mine workings are the most challenging in the design and operation of a facility.