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OCR for page 51
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 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.
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
OCR for page 52
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
OCR for page 53
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
OCR for page 54
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).
OCR for page 55
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.
OCR for page 56
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.
OCR for page 57
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.
OCR for page 58
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.
OCR for page 59
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.
OCR for page 60
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- ~~ ~~~ ~~ ~~ ' ~ ~~ ~'-~
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~~ '' '' ~ '-_ ' ~~ '':' '' '' ' A'. '.. ~~ ~~ ~ ' ~ ~ ~2~.',~ ' ~ ,,, , _~\ 1 ~ \
W~.~:~:'.:~'-~'~'~'~ ~'~.~'~'~'~2~'~'W'~ _ _
: ~~ ~ ~~: -aft ~~ : ~~ ~~ ~~ , _. ,,- ~~. . ~~ ~~ ~~,~,~,.~ ~~, a,`-,,, .,,, ~~ ~~,~,~,,~,~,,~,~,~.~W,,,,,,~,,.,,,,~ A, ,, ~ \
~,~,~-~5,~;~ ~~,~ Am,, ~-,~,~,~-~ ~~-~ I,,- ~~ \, \
W~ ~~-~, ~~,~ ~~N,:_~\ \ \
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FIGURE 3.3 Schematic diagram of coal refuse impoundments: (a) cross-valley, (b) diked,
and (c) incised impoundments.
OCR for page 61
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OCR for page 62
62
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OCR for page 63
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
OCR for page 64
64
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OCR for page 65
65
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OCR for page 66
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
OCR for page 67
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
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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.
=
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PLANNING COAL SLURRY REFUSE IMPOUNDMENTS
69
Groundwater Levels Measured by monitoring wells. In addition,
the groundwater chemistry may also be monitored.
Surface Water Discharge—Measured by weirs, flumes, or flow
meters.
Subsurface Settlement, Readjustment, and Subsidence—Measured
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~.
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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.
Representative terms from entire chapter:
minimum width
