EPA600/
2-86-026
EVALUATION OF THE APPLICABILITY OF SUBSIDENCE
MODELS TO HAZARDOUS WASTE SITES
by
PEI Associates, Inc.
11499 Chester Road
Cincinnati , Ohio 45246-0100
Contract No. 68-02-3963
Task Officer
Ronald D. Hill
Land Pollution Control Division
Hazardous Waste Engineering Research Laboratory
Cincinnati , Ohio 45268
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
-------
DISCLAIMER
The information in this document has been funded wholly or in part by the
United States Environmental Protection Agency under Contract No. 68-02-3963 to
PEI Associates, Inc. It has been subject to the Agency's peer and administra-
tive review, and it has been approved for publication as an EPA document.
Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
11
I
-------
FOREWORD
Today's rapidly developing and changiny technologies and industrial prod-
ucts and practices frequently carry with them the increased generation of solid
and hazardous wastes. These materials, if improperly dealt with, can threaten
both public health and the environment. Abandoned waste sites and accidental
releases of toxic and hazardous substances to the environment also have impor-
tant environmental and public health implications. The Hazardous Waste
Engineering Research Laboratory assists in providing an authoritative and
defensible engineering basis for assessing and solving these problems. Its
products support the policies, programs, and regulations of the Environmental
Protection Agency, the permitting and other responsibilities of State and local
governments, and the needs of both large and small business in handling their
wastes responsibly and economically.
This report reviews the available information on subsidence phenomenon and
available predictive models. It will be useful for designers of hazardous
waste facilities, Federal and State hazardous waste permit reviewers, and plan-
ners of hazardous waste remedial actions.
For further information, please contact the Land Pollution Control
Division of the Hazardous Waste Engineering Research Laboratory.
William A. Cawley, Acting Director
Hazardous Waste Engineering Research Laboratory
i n
-------
ABSTRACT
EPA has discovered a number of uncontrolled hazardous waste sites in close
proximity to abandoned underground mines. Further, several Resource Conserva-
tion and Recovery Act permit applications have been received for treatment,
storage, or disposal facilities located in areas where abandoned underground
mines are known to exist. The potential exists for subsidence under a hazard-
ous waste facility to result in uncontrolled release of hazardous constituents
to the environment.
The investigation was approached in two phases. Phase I involved a
literature review and data compilation to gather information on the subsidence
phenomenon, available predictive models, and on the adverse effects that can
result from mine-related subsidence; and Phase II consisted of an evaluation of
available equations and models used to predict subsidence and an assessment of
the applicability of these models to predict subsidence problems at hazardous
waste sites.
Predictive models of subsidence fall into two broad categories: empirical
and analytical. In order to use the empirical approach, two major requirements
must be met. First, the model must be used in an area for which a large data
set has already been gathered. Second, the investigator must have an accurate
account of the dimensions of the mine, including height of the overburden,
width of the coal seam, 'location and condition of all remaining pillars, and
percentage of coal removed. A large data set has not yet been collected for
the American coal fields but, regardless, meeting the second requirement would
be difficult. It is not possible to obtain necessary measurements to verify
pillar dimensions and locations. Therefore, the empirical approach is not
applicable.
The analytical approach is derived from deformation mechanisms and
strength parameters of rock. The elastic theory and finite element models are
applicable only to longwall mining.
Despite the difficulties inherent in modeling subsidence from room-and-
pillar operations, every effort should be made to gather as much information as
possible. Using all available data, subsidence prediction should be attempted
using a model recommended for use in this report or elsewhere. It is important
to evaluate the adequacy of the results with a clear understanding that failure
of the model to accurately represent the system could, if a hazardous waste
facility were disrupted, have a detrimental effect on the environment. There-
fore, if it cannot be said with certainty that subsidence will not occur, then
one should be hesitant to place a hazardous waste facility in the location
under consideration.
-------
CONTENTS
Page
1. Introduction 1-1
2. Description of Subsidence Phenomenon and Its Impact on
Hazardous Waste Facilities 2-1
2.1 Mechanism of subsidence 2-1
2.2 Geological aspects of subsidence 2-4
2.3 Definitions of common terms 2-8
2.4 Potential impact of subsidence phenomenon on hazardous
waste facilities 2-9
3. Factors That Affect Subsidence 3-1
3.1 Mining method 3-1
3.2 Mine dimensions 3-5
3.3 Multiple seam mining 3-5
3.4 Major surface changes 3-6
3.5 Roof bolting and cribbing 3-6
3.6 Rate factors 3-6
3.7 Time 3-7
3.8 Summary 3-7
4. Subsidence Models 4-1
4.1 Background information 4-1
4.2 Descriptions of individual models 4-4
4.3 Summary 4-11
5. Costs of Using Subsidence Prediction Models 5-1
6. Conclusions and Recommendations 6-1
6.1 Conclusions 6-1
6.2 Recommendations 6-5
References A-l
List of Contacts A-4
Glossary A-5
-------
FIGURES
Number Page
2-1 Geological/Mining Conditions Related to Underground
Mine Roof Failures and Resulting Surface Subsidence 2-3
2-2 Summary Diagram of Geological Factors Related to
Underground Mine Collapse and Resulting Surface
Subsidence 2-5
2-3 Angle of Draw and Critical Width 2-9
TABLES
Number Page
3-1 Major Factors Associated with Subsidence 3-8
4-1 Investigated Models 4-2
4-2 Matrix of Subsidence Models 4-12
VI
-------
SECTION 1
INTRODUCTION
EPA has discovered a number of uncontrolled hazardous waste sites in
close proximity to abandoned underground mines. Further, several Resource
Conservation and Recovery Act permit applications have been received for
treatment, storage, or disposal facilities located in areas where abandoned
underground mines are known to exist. The potential exists for subsidence
under a hazardous waste facility to result in uncontrolled release of hazard-
ous constituents to the environment.
Subsidence is defined as a lowering of ground surface which occurs as a
result of the deformation caused by removal of subsurface mineral deposits.
Coal extraction is responsible for more than 90 percent of all subsidence-
related problems. Subsidence has the potential to create hazardous condi-
tions for structures situated both above and beneath the ground. Subsidence
has already affected more than 2 million acres of land across the United
States.2
The purposes of this study are to evaluate the currently available
models that predict subsidence and to determine the applicability of these
models to predicting potential subsidence problems at hazardous waste sites
throughout the Appalachian coal fields. These models may serve as tools for
the selection, cost analysis, and design of remedial actions at hazardous
waste sites.
1-1
-------
The investigation was approached in two phases. Phase I involved a
literature review and data compilation to gather information on the sub-
sidence phenomenon, available predictive models, and on the adverse effects
that can result from mine-related subsidence; and Phase II consisted of an
evaluation of available equations and models used to predict subsidence and
an assessment of the applicability of these models to predict subsidence
problems at hazardous waste sites.
The intent of this report is 1) to present an overview of available
predictive models and 2) to provide the user community (e.g., EPA inspectors,
permit application reviewers, permit writers, and EPA contractors) with a
basic understanding of the subsidence phenomenon and its potential impact on
hazardous waste facilities.
This report contains five main sections (Sections 2 through 6), a list
of references, a list of contacts, and a glossary. Section 2 contains a
description of the subsidence phenomenon, including the actual mechanism of
subsidence and the geological aspects. Section 2 also contains a discussion
of the potential impact of subsidence on hazardous waste facilities. Section
3 describes the factors affecting subsidence that arise as a result of human
activity. Sections 2 and 3 have been included to provide basic definitions
and background data that will enable.the user to better understand how the
models work and how they can be used to assess potential problems at
hazardous waste sites.
Section 4 is divided into two parts. The first part provides background
information on categories of predictive subsidence models, the state of the
art, and some commonly-held assumptions. The second part describes the
1-2
-------
characteristics and limitations of the individual models that were investi-
gated for this study. Section 5 gives general cost estimates for utilizing
selected models. Section 6 contains conclusions and recommendations regarding
the applicability of the investigated subsidence prediction models to hazard-
ous waste sites.
The information in this report is based on a review of subsidence liter-
ature provided by representatives of the Bureau of Mines, West Virginia
Geological Survey, United States Geological Survey, Office of Surface Mining,
the National Research Council Committee on Ground Failure Hazard, West Vir-
ginia University, and West Virginia Institute of Technology, and on conversa-
tions with the persons who provided the literature.
For those who are interested in obtaining more details on subsidence and
its potential impact on hazardous waste sites, this report includes a list of
references and a list of contacts who are involved in ongoing research.
1-3
-------
SECTION 2
DESCRIPTION OF SUBSIDENCE PHENOMENON AND ITS IMPACT ON
HAZARDOUS WASTE FACILITIES
The excavation of a subsurface mineral deposit causes a progression of
underground changes, the end result of which may be a lowering of the ground
surface, or subsidence. This section describes the progression of events
that results in subsidence and presents a discussion of the associated geo-
logical aspects of subsidence. This section also contains definitions for
several common subsidence terms. They are useful for understanding the
subsidence phenomenon and models.
2.1 MECHANISM OF SUBSIDENCE
The creation, by mining, of an underground void disturbs the equilibrium
of the layers of rock, or strata, that comprise the overburden above the
mine. This disturbance results in a redistribution of rock stresses. Stress
is defined here as the resistance of the rock to compressional, tensional or
tortional force, and is measured as force applied per unit area.
When stresses in a selected area build up to a point where they exceed
the strength of the surrounding rock, the rock strata above the cavity frac-
345
ture and fall into the mine void. ' ' To release pressure, strata can move
inward toward the excavation from all directions. Therefore, the surface
area affected is often larger than the area of extraction. The pattern of
fracture and collapse may extend upward to the ground surface. Thus, subsi-
2
dence results from a time-dependent redistribution of forces. However,
2-1
-------
"the length of time over which significant movements occur depends upon the
mechanism by which subsidence is [sic] taken place." The redistribution of
forces is affected primarily by physical and chemical properties of the rocks
that comprise the overburden and by the presence or absence of water. The
geological and mining conditions related to various types of subsidence are
illustrated in Figure 2-1.
Mine cave-ins cause the underground formation of a dome-shaped section
of fractured rock. Above such a dome-shaped section, self-supporting strata
sag like a long beam. These sagging strata may result in the formation of a
subsidence trough at the surface. The trough's magnitude is a function of
the thickness of the coal seam, depth of the seam from the surface, and the
4
total extraction area.
The trough, a shallow, broad depression, forms as a result of a sagging
overburden whereas a sinkhole, identified by an abrupt boundary between its
edge and the ground surface, forms more often as a result of a fractured,
collapsed overburden. Sinkholes are more often associated with shallow
mines.
Strain is the deformation that results from applied force. Within
elastic limits, strain is proportional to stress. It is measured as change
in length per unit length in a given direction. Strains transmitted through
strata have horizontal as well as vertical components. Although vertical
shift is the striking feature of a subsidence trough or sinkhole, horizontal
strain also plays a prominent role in trough formation. It is considered the
o
primary cause of structural damage to buildings affected by subsidence. In
evidence of the prominent role played by horizontal strain, horizontal stress
may exceed vertical stress by a factor of 1.5 to 4.
2-2
-------
SAC AND ROOF COLLAPSE
SAG~AND PILUHTSQUEEZE
PILLAR COLLAPSE OR PILLAR REMOVAL
DOMING- TYPE ROOF FALL
MINING TOO FAR UPDIP
MINING INTO FA UL T
MINING TOO CLOSE TO ALLUVIAL
OR GLACIAL OVERBURDEN (H j
MINING TOO CLOSE TO AN OVER- OR
UNDER-LYING MINED-OUTSEAM (§)
MINING INTO CHANNEL SAND OR OTHER
HETEROGENEOUS ROCK STRA TA
Figure 2-1. Geological/Mining conditions related to underground mine
roof failures and resulting surface subsidence.9
2-3
-------
The curvature of the ground surface due to differential downward verti-
cal movements results in horizontal displacement toward the center of the
subsidence trough. If vertical displacements were uniform, no horizontal
strain would be produced; however, vertical displacements are non-uniform,
and they result in horizontal displacement per unit length. These differen-
tial displacements cause the strain that actually produces structural damage.
Subsidence models are not yet able to accurately predict horizontal displace-
ment.
A subsidence trough consists of areas of compressive strain, which
causes an elastic body to shorten in the direction of applied force, and
tensile strain, which causes a body to lengthen in the direction of applied
force. At the ground surface, tensile strain occurs outside the extraction
area, while compressive strain occurs over the extraction area.
2.2 GEOLOGICAL ASPECTS OF SUBSIDENCE
The geological factors that affect subsidence can be divided into two
major areas, stratigraphic and water-related. A number of these factors are
illustrated in Figure 2-2.
Stratigraphic factors include the material properties, types, and struc-
tural features of the rocks that comprise the strata above and beneath the
mined space. More specifically, rocks are characterized by their elastic
properties, strength, degree of fracturing, moisture content, homogeneity,
degree of compaction, and permeability.
The overburden, which consists of the rock material above the coal seam,
is characterized by the unique site-specific interaction of the rock layers,
including joints, faults, surface and sub-surface fracturing, tension cracks,
bedding and foliation planes.
2-4
-------
01
c
22
-a
c
IB
a.
10
o
u
ai
o
3
O
0>
0)
a
a
CD
U
CO
'aj
I/)
O
+J
U
fO
M-
ro
U
CD
O
o
at
4_ a,
o u
E 0)
i/>
s_ u
IQ (O
3 3
CM
I
CM
OJ
s_
3
01
2-5
-------
Structural features affect the timing and the extent of subsidence.
When strata above the mined space are strong and hard, such as massive sand-
135
stone or limestone, subsidence is minimized. ' When strata are soft and
weak, such as thin bedded shales, mudstone, siltstone, or unconsolidated
deposits, incidences of subsidence increase. Thick, or massive, sandstone
3 11
above the roof of a mine acts as a rigid beam. ' Because strong rocks,
such as limestone and massive sandstone, tend to break at a steeper angle,
the area affected by subsidence in an overburden consisting of harder rock is
normally smaller than it would have been had the overburden consisted of
softer rocks, but it is still larger than the caved area in the mine. '
Subsidence associated with strong rocks is usually characterized as
violent, sudden, and delayed. Subsidence associated with weak rocks is
usually characterized by a gradual lowering of the surface.
The amount of clay in the overburden is also an important factor. Clay
has the ability to reseal after fracture, thereby keeping man-made structures,
12
such as buildings, perched above it. The presence of clay on a mine floor,
however, has its own unique set of problems. These will be examined shortly.
The length of time required for rock deformation to spread from the mine
to the ground surface is proportional to the depth of the mine. This is
because rocks deform as jointed, layered media, and not as uniform, intact
bodies. Therefore, subsidence does not occur all at once. Rather, deforma-
tion progresses upward in stages until it reaches the ground surface, at
which point it is termed subsidence. Also, the duration of surface movement
is proportional to the depth of the mine. The deeper a mine is, the greater
volume of overburden rock will be deformed. Deformation in the various
involved sections of overburden rock will progress at different rates and
2-6
-------
will therefore reach the surface over an extended period rather than all at
once.
Most experts agree that preexisting surface topography has no effect on
the maximum amount of subsidence, but a small number believe that it does
3 7
play a minor role. This latter group is of the opinion that irregular
preexisting topography is indicative of irregular stress distributions within
the overburden. They claim that more unstable stress configurations in the
overburden enhance the likelihood of subsidence.
Water-related geological factors can affect subsidence in a variety of
ways. Water enhances chemical and physical deterioration of mine pillars.
Laboratory tests have shown that the amount of water present in an enclosed
vessel is inversely proportional to the amount of time required to crush the
rock in that vessel under constant stress.
Water also causes the claystone floor of coal mines to soften over time.
Once the floor rock becomes significantly softer than the coal pillars, the
pillars actually sink, or "punch," into the floor. This causes an overall
lowering of the mine roof and the resultant deformation is transmitted through
the rock strata to the ground surface. '
Water may enter a mine directly, as a result of the proximity of the
coal seam to transmissive rock strata, or indirectly, as a secondary effect
of altered surface drainage patterns. For instance, in a small town in
Pennsylvania, a new ordinance calling for installation of gutters and leaders
resulted in flooding of the mines beneath the town. Where rainfall had
previously filtered through lawns and topsoil to groundwater, it was now
collected and drained directly into the underground sewerage system. The
local sewerage system was not large enough to carry the increased drainage,
and so it overflowed into the mines. Whereas the direct entrance of water
2-7
-------
into a mine is likely to occur soon after extraction of the coal seam, indirect
entrance might not become operative for years after abandonment of a mine.
This factor contributes to the difficulty of predicting subsidence for certain
types of mining methods, such as 'room-and-pillar' mining. ('Room-and-pillar1
mining is described in Section 3.1.2.)
2.3 DEFINITIONS OF COMMON TERMS
This subsection contains definitions for four common subsidence terms.
They are useful for understanding the subsidence phenomenon and models.
2.3.1 Angle of Draw
The total ground surface area affected by subsidence is determined in
part by the angle of draw, defined as the angle of inclination from the
vertical of a line connecting the edge of a workings and the edge of a sub-
sidence area (see Figure 2-3). The average angle of draw is approximately
25°s3 with a range of 10° to 35°.1
2.3.2 Maximum Subsidence, S
_._ __.. ., . ...... ,.,._ "(Ha A
The maximum amount of subsidence possible is directly proportional to
the thickness of the mined coal seam. Maximum subsidence, or S , . is equal
max
to 0.9 multiplied by the seam thickness. S av will not be reached until the
max
width of the area of complete extraction (longwall or room length) exceeds a
value equal to 1.4 times the depth to the mine. Thinner seams and shorter
void spans result in a decrease in the depth of the subsidence basin.
2.3.3 Critical Width
The concept of critical width is fundamental to an understanding of
subsidence. Critical width is defined as the smallest extracted width that
will result in maximum subsidence at one point on the ground surface.
2-8
-------
GROUND SURFACE
SUBSIDENCE
PROFILE:
CRITICAL WIDTH
ry
***T~ ANGLE OF DRAW
SUBSIDENCE
PROFILE:
SUPERCRITICAL WIDTH
GROUND SURFACE
OF DRAW
*-*.L~.* SEAM
SUBSIDENCE
PROFILE:
SUBCRITICAL WIDTH
GROUND SURFACE
ANGLE OF DRAW
fe^r^. SEAM
Figure 2-3. Angle of draw and critical width.
2-9
-------
Subsidence occurs as a U-shaped trough. It will reach a maximum possi-
ble vertical displacement, $_..,, when the critical width of extraction is
max
reached. Where extraction occurs over an area smaller than the critical
width, subcritical width extraction results in formation of a trough whose
lowest point is less than S . Where extraction occurs over an area exceed-
ing the critical width, supercritical width extraction results in formation
of a flat-bottomed trough where every point in the bottom section is equal to
SmaV' The stylized drawings in Figure 2-3 illustrate surface subsidence
iNdA
resulting from critical, subcritical, and supercritical width extractions.
2.3.4 Subsidence Factor
The subsidence factor is an area-specific measure for predicting Smav.
(TtflX
Used primarily in empirical models, it is defined as the ratio of the depth
of the subsidence trough to the height of the coal seam. In Appalachia this
4 3
value ranges from 0.22 to 0.72, with an average value of approximately 0.6.
A subsidence factor remains fairly constant within regions, thereby providing
a rough estimate of the depth of subsidence troughs, relative to the height
of the coal seam, that can be expected in that region.
2.4 POTENTIAL IMPACT OF SUBSIDENCE PHENOMENON ON HAZARDOUS WASTE FACILITIES
Subsidence from abandoned mine workings could affect the structural
integrity of hazardous waste facilities located at ground surface above the
mine workings through a variety of mechanisms. Cracks that develop in a
subsiding overburden may continue to propagate upward into the barriers of a
surface impoundment or storage facility. Collapse of stratified layers
beneath a hazardous waste site may cause part of a landfill, impoundment, or
storage facility to drop into the void created by the collapsing rock.
Because subsidence has the potential to cause a breach in the integrity of a
2-10
-------
liner, containment system, building, or holding tank, hazardous waste facili-
ties should not be located in areas of potential subsidence. A breach in the
integrity of a containment system could result in an uncontrolled release of
hazardous materials to the environment.
The way in which room-and-pillar mining affects hazardous waste facili-
ties differs from the effects of longwall mining on these facilities. Gener-
ally, room-and-pillar mine collapse occurs many years after cessation of
mining operations. Moisture, number and size of pillars, depth of overburden,
and many other factors (see Figure 2-2) interact to create a unique, site-
specific situation. This uniqueness makes the prediction of the extent,
location, and time of room-and-pillar subsidence difficult, if not impossible.
There is no guarantee that the ground above a room-and-pillar mine will not
subside. In fact, if the pillars are too small to support the roof of the
mine, and moisture is present, and the overburden is heavy, factors are
optimized for subsidence to occur and, given time, it probably will. There-
fore, based on the above, it is recommended that hazardous waste facilities
not be constructed over room-and-pillar mines, and neither should room-and-
pillar mining be conducted under a hazardous waste facility.
The situation is somewhat different for longwall mining. Subsidence
resulting from longwall mining occurs virtually concurrently with the mining
operation. Ninety-five percent of the total expected subsidence occurs
immediately, and the remaining 5 percent occurs in the following one to five
years. Therefore, hazardous waste facilities may be constructed over areas
that were mined by the longwall method five or more years previously. How-
ever, it is recommended that longwall mining not be conducted under existing
hazardous waste facilities. This practice would likely result in subsidence,
associated disruption of the containment system of the facility, and release
hazardous materials to the surrounds.
2-11
-------
SECTION 3
FACTORS THAT AFFECT SUBSIDENCE
Certain factors that affect the occurrence of subsidence arise as a re-
sult of human activity. They include mining method, mine dimensions, multi-
ple seam mining, major surface changes, roof bolting and cribbing, rate of
mining, and time of mining.
3.1 MINING METHOD
As this report is concerned with subsidence that occurs as a result of
coal mining, the following explanations will be restricted to coal mining
only.
Coal is mined underground by two different methods, the room-and-pillar
method and the longwall method. The more traditional room-and-pillar mining
is more common, less efficient, and less expensive than longwall mining,
which requires a substantial investment in costly machinery.
While subsidence due to longwall mining occurs almost concurrently with
the mining process, and usually produces a smooth, predictable surface set-
tlement, subsidence due to room-and-pillar mining may be delayed as much as
50 to 100 years, and has been characterized as erratic, intermittent, and
delayed. The different timing and character of subsidence from longwall and
room-and-pillar mining have different ramifications for hazardous waste
facilities built on the surface above longwall and room-and-pillar mines.
3-1
-------
This report addresses the mechanisms and predictive models for both
room-and-pillar and longwall mining. Because the highly consistent nature of
subsidence from longwall mining makes it easier to model and predict than
subsidence from room-and-pillar mining, more work has been conducted on
developing models for longwall solutions.
3.1.1 Longwall Mining
Deformation of strata due to longwall mining is transmitted through to
the surface almost immediately, and results in a smooth subsided area at the
11 14
ground surface. ' When mining ceases, subsidence ceases; when mining
resumes, subsidence resumes. By the time mining is completed, 95 percent
of the total eventual subsidence has occurred. Subsidence will be complete
within 1 to 5 years maximum, * the former number being more widely accepted.
Subsidence due to longwall mining proceeds as follows. After the coal
is extracted, the roof collapses immediately. When the ratio of the coal
seam height to overburden height exceeds some critical value, generally
between about 0.1 to 0.5, displacements will then be transferred to the
surface. The critical value of that ratio is affected by the strength and
structure of the overburden rock. Subsidence will not begin until a critical
minimum mine void size is exceeded, resulting in the collapse of a compression
arch in the solid rock above the mined area. Subcritical extraction widths
produce a relatively small and shallow trough, and supercritical extraction
widths produce a deeper trough with a fairly flat bottom.
For longwall subsidence in Appalachia, the angle of draw typically
ranges from 12° to 34°.15
3.1.2 Room-and-Pi11ar Mining
In room-and-pillar mining, a series of parallel entries are driven into
a coal seam. Interconnecting breakthroughs are driven, at right angles,
3-2
-------
through pillars into adjoining entries, thus creating "rooms and pillars."
Th^Rumber and size of pillars is determined based on the amount of support
deemed necessary to maintain the integrity of the roof during the entire
mining operation.
Anywhere from 40 percent to almost 100 percent of the coal may be ex-
tracted by this method. The higher number reflects the results of "retreat"
mining, where pillars are "robbed" in the final stage before abandonment of
the section of the coal seam being mined.
A major difficulty in predicting subsidence from room-and-pillar mining
is the inability to know whether reports of the amounts of coal removed are
accurate. In some cases, the available maps of mine workings do not accurate-
ly portray the size and number of remaining support pillars. This problem is
especially pertinent to older, abandoned mines, particularly those from the
1940's and 1950's and earlier. In those years, before the enactment of
niner safety laws and surface protection rights, miners took more coal than
is taken now. It was not uncommon for coal mine operators to "rob" pillars
in their final retreat from the mine. When more coal is taken than is re-
quired to support the roof, the remaining pillars are too small to support
the weight of the overburden. Over time, the pillars deteriorate, lose
strength, and are finally crushed. ' fll»16 Although additional data could
)e gathered by a drilling survey, they would not provide sufficient informa-
:ion to determine the stability of the area with regard to subsidence.
Jnderground character could vary greatly over short distances, and ascertain-
lent of the presence and quantity of mined space would not provide informa-
:ion on the strength of the pillars and mine roof or on the imminence of
ubsidence.
3-3
-------
Unlike the immediate subsidence which occurs due to longwall mining,
subsidence resulting from room-and-pillar mining is strongly time-dependent.
The creep effect of coal in the pillars, the constant load over time, the
oxidation and strain all contribute to the eventual buckling of pillars.
Water hastens failure of the supports by progressively softening claystone
mine floors until pillars gradually sink into the weak underclay. This
particular kind of pillar failure often results in the development of wide
areas of subsidence, as opposed to the limited areas characteristic of the
kind of pillar failure caused by deterioration of the pillars themselves.
Due to the aforementioned factors, subsidence from room-and-pillar
operations is characterized as erratic, intermittent, delayed, and difficult
to predict. In addition, if the percent recovery of a room and pillar
operation is unknown, it becomes extremely difficult to predict the occur-
3
rence or extent of subsidence. The potential always exists for a room-and-
pillar mine roof to collapse and cause subsidence. Irregular room develop-
ment, non-uniform barrier pillars and poor panel definition further contribute
to the difficulty of predicting room-and-pillar subsidence. "Under such
circumstances, frequently associated with old mining areas, it is usually
impossible to predict the time, magnitude or occurrence of subsidence."
Opinions differ as to the length of time required for complete subsidence.
One study maintains that 100 years are required. After a mine floods,
failure is likely to occur within 10 years. With respect to the Pittsburgh
coal seam, if it has not occurred within 30 to 40 years, it is unlikely to
begin. Once it does begin, however, it is likely to continue for several
years.
Certain patterns specific to subsidence from room-and-pillar mining have
emerged near Pittsburgh, Pennsylvania. Where shale overlies the coal seam,
3-4
-------
the roof usually falls along northeast mine passageways, and parallel to the
joint direction in the coal. Where thick sandstone overlies the coal, severe
and frequent falls occur in the rock overlying the coal, with no consistent
orientation relative to joints or passageways.
3,2 MINE DIMENSIONS
Dimensional factors include depth from the ground surface to the coal
seam (equal to overburden height), the thickness of the coal seam, the length
and width of the coal panel extracted, and the amount of support provided by
the gob, ' ' ' where gob is defined as the coal refuse left in the mine.
Although some experts believe that subsidence will not occur where mines
are located more than 500 feet below the surfce, others point, in disagree-
ment, to those instances where the ground has subsided above mines located
some 1000 feet below the surface.
What is more clear is that a direct relationship exists between the
occurrence of subsidence and the ratio of mined coal seam thickness to the
overburden thickness. The thicker the coal seam relative to the height of
the overburden, the more likely it is that subsidence will occur.
The entire quantity of coal removed is equal to the thickness of the
seam times the area (length times width) of the extracted coal panel. The
greater the total amount of coal removed per unit area, the more likely it is
that subsidence will occur.
3.3 MULTIPLE SEAM MINING
Where multiple seams are being mined in an area, stress concentrations
within the rock comprising the overburden increase beyond those expected as a
result of mining a single seam. This increase in stresses, in turn, causes
3-5
-------
1 18
an increased likelihood of subsidence. The greatest difficulty in pre-
dicting subsidence is in assessing potential subsidence problems associated
with horizontally or vertically adjacent mined out areas.
3.4 MAJOR SURFACE CHANGES
Large man-made structures on the ground surface above a mine increase
the surface loading. The increased load is transmitted to the overburden
strata. Where stress concentrations increase within the overburden, the
likelihood of subsidence increases. In other words, the loading acts as
additional overburden.
3.5 ROOF BOLTING AND CRIBBING
The practice of bolting and cribbing lends additional support to the
mine roof and, therefore, delays or prevents the collapse of the roof into
the mine. Where practiced, roof bolting and cribbing decrease the likelihood
of subsidence, until the bolts and cribbing deteriorate. Steel bolts corrode
under highly-corrosive mine conditions, and the wooden cribbing rots.
3.6 RATE FACTORS
In longwall mining, the time for subsidence to occur is a function of
the time required to work the coal face through the critical width (defined
in Section II, "Description of Subsidence Phenomenon"), which, in turn, is
controlled by factors such as mine depth, angle of draw, and rate of advance.
A smooth, consistent advance rate is associated with a consistent, predict-
able surface settlement. No such clear time relationship exists for room-
and-pillar mining.
3-6
-------
3.7 TIME
Where a coal seam has been mined by the room-and-pillar method for a
period of 50 years or more, a sharp increase is noted in the likelihood of
18
the occurrence of subsidence. Fifty years ago, before the enactment of
miner safety laws and surface protection rights, miners removed more coal
than they remove now. In mines where coal was removed under those conditions,
the remaining pillars are too small to support the weight of the overburden
on a long-term basis. Losses in strength that occur as a result of deteri-
oration cause the pillars finally to collapse.
Subsidence in abandoned room-and-pillar mines has occurred as soon as 10
years and as long as 100 years after retreat from the mine. More than half
of the cases of subsidence have occurred more than 50 years after the mine
was closed. Since it is virtually impossible to know in which mines sub-
sidence will occur, with respect to the situation of hazardous waste facili-
ties above them, it must be assumed that subsidence will eventually occur at
all mining sites.
Subsidence from longwall mining occurs virtually concurrently with the
coal extraction process. There is a slight additional (5 percent) settlement
of the gob with time; however, 95 percent of the subsidence occurs during the
extraction process.
3.8 SUMMARY
Table 3-1 summarizes the effect of major factors on increasing or decreas-
ing the likelihood of subsidence.
3-7
-------
TABLE 3-1. MAJOR FACTORS ASSOCIATED WITH SUBSIDENCE
Factor
Effect
Overburden height
Overburden material
Faults
Coal seam thickness
Mine age
Mining method
Amount of support
remaining
Multiple seam mining
Major surface changes
Moisture
The greater the overburden height (the deeper the
mine), generally, the less likely subsidence becomes.
Subsidence is minimized in overburdens comprised of
hard rock such as massive sandstone and limestone.
It is more likely in areas of softer rock, such as
mudstone, siltstone, and bedded shales.
Inherent weaknesses in the overburden increase the
likelihood of subsidence.
The thicker the coal seam, relative to the overburden,
the more likely subsidence is to occur.
Older room-and-pillar mines (over 50 years old) are
more likely than recent operations to have been
robbed of coal meant to support the roof. They are
more prone to subsidence than mines with adequate
numbers and sizes of pillars. Some younger mines
too, however, have also been robbed of sufficient
coal to support the roof.
1) Longwall method - Smooth, predictable surface
settlement; occurs almost concurrently with mining.
2} Room-and-pillar method - Erratic and intermittent
surface settlement; may be delayed as much as 50 to
100 years after mining.
1) The larger the amount and/or numbers of gob or
pillars remaining, the lower the likelihood of
subsidence.
2) Roof bolting and cribbing provide additional
support and delay or prevent roof collapse. There-
fore, they decrease the likelihood of subsidence.
Causes increased stress concentrations in overburden
and, therefore, increases the likelihood of subsidence.
Placing man-made structures on the ground surface
increases surface loading, increases stress concen-
trations, and therefore increases the likelihood of
subsidence.
Increasing amounts of moisture in the mine contribute
to heightened rates of deterioration and increase
the likelihood of subsidence.
3-8
-------
SECTION 4
SUBSIDENCE MODELS
4.1 BACKGROUND INFORMATION
Predictive models of subsidence fall into two major categories, empirical
and analytical. Empirical models are based primarily on observations of
ground movement, and do not take into account principles of deformable body
19
mechanics or the mechanics of continuum. The empirical approach is a des-
criptive technique. While it is simple and practical, it is area-specific,
requires a large data set, and is best applied to areas in which 100 percent
of the coal has been removed.
The analytical approach employs deformation mechanisms based on the
theory of elasticity, and the strength parameters of rock. The value of the
tensile strength parameter assigned to rock varies widely among the analytical
models. While analytical models are more widely applicable than empirical
models, they are mathematically complex and their idealization of the subsi-
dence phenomenon precludes an exact description. Table 4-1 provides an over-
view of the investigated models. It categorizes each model by theory, method
of analysis, and means of analysis. It is divided into two sections, empirical
and analytical.
One major shortcoming of many of the subsidence models that have been
developed, both empirical and analytical, is that most models have been tested
at only one site. Few have been tested at more than one site. Because of
the unique geological environment of each coal mine, models that are developed
4-1
-------
LaJ
C3
00
UJ
u
I
QQ
91
U
C
01
i.
01
4.
Ol
M
IA
C
<«-
o
IA
C
I/I
VI
C
<*-
o
JC
4J
>»
o
Ol
t
CM
Ib
CM CM
CM
* f^
i I CM
CM
* CTI
O i
CM
" CO
O
in in in
ft tt ft
t-> CO f! *
VI
o
r"
O
<4-
O
C
o
4J
§
r** r*° fm ^"*
4-> 4->
IB IB IB IB
4J 4J +J 4J
IB
o
i
- ?
3 B -8
C 0 C
3 *i
U C ^ VI
-C C T- -C
Q. O <*- U
IB a. o o
t. X S. *J
o uu a. 01
1
a.
58
Ol u
ec.
T3
O
1
o
!_
en
i
c
o
4-J
2
en
Ol
c
o
CM
O O
s.
Ol
J2
J3
3 U
i. - Ol
VI -C
>, *J
U -C IB
i- o. X
VI
LU
U
>I
|
§
3
C
4J
e
o
u
o
*
co
*
V0
tf>
«
o
C
4-1
i
>-> >>
01 S
s.
01 O
B
VI C
VI IB
<
C
o
"o
VI
*J
c
Ol
Ol
4-t
"c
Ll»
(^
CM
O O
« o
s_
IB
01 1
C C
* o
i US
C -MM-.
Z, *^
Q. VI *J
O >> E O.
1-1- r- -D
u ai 01
^
CM
\Q
CM
in
CM
in
t«t
B
O
Ol
u
c
0)
3
C
M_
O
>,
01
.C
4-> I/I
C O
O "*~
4-*
a u
VI 3
B
01
i-
Ol
c
o
1-sl
^4
CM
1
L. VI
Ol -^
4J U
C O
4->
*^B
in en
u t-
c
I4_ 2
<»- 4->
i- Ol
O £,
01 VI
4J Ol
c ia
r- t-
u.
4-1
c
E
J2
4J
U
C
VI
Q
CO
f-4
in
i
i.
a o
c u
VI
8
u
I/I
»
rtf
-M
(A
5
||0
1
4-2
-------
for use in one specific region are usually not applicable for use in other
s. The National Coal Board (NCB) model, developed for use in Great
Britain, is one model that has been tested at more than one site. Predictions
by the British NCB model do not coincide with field observations in the Appa-
lachian region. In the Appalachian coal field, 30 to 50 percent more sub-
sidence occurs than is predicted by the NCB model.
Many analytical models require input parameters that are either unavail-
28
able, irrelevant, or too cost-prohibitive to gather. Analytical methods are
further criticized for the 'arbitrary' way in which the values of certain
parameters (e.g., Young's modulus, rock strength) are adjusted to improve
correlation of the model with field observations at a particular site. '
"... The predominant obstacle of transforming ... models into predictive
methods is the indefinite and subjective process by which the magnitude of the
pi
input parameters are [sic] chosen".
In Europe, comparatively uniform conditions and a long history of field
measurement have facilitated development of accurate and reliable empirical
subsidence prediction models. In the United States, on the other hand,
diverse conditions and a comparative dearth of field data have resulted in a
subsidence prediction methodology that is less well developed. Attempts are
still being made to accurately define such basic parameters as subsurface
rock, soil and ground water conditions, and the distribution, duration and
29
intensity of stress changes. Currently available models can predict the
degree of expected subsidence, but not the length of time required for it to
develop.
No subsidence model yet exists that accounts for the rates of coal oxida-
tion, degradation and creep. Rate is one key to determining the involvement
of time in prediction.
4-3
-------
A predictive model that accounts for time is in the early stages of
development at West Virginia University. In this model, time is linked to the
viscoelasticity of rock. Viscoelasticity is a property of rock in which
application of a stress gives rise to a strain that approaches its equilibrium
value (limit) slowly. As the viscoelasticity of a material approaches its
limit, the material is considered failed. Because a certain, fixed period of
time is associated with the failure of different types of rock, viscoelasticity
of rock may be considered a function of time. Therefore, with prior knowledge
about the viscoelastic properties of rock, a model can theoretically be used
to calculate the length of time after which subsidence will occur. It is
difficult, however, to obtain viscoelastic property values of rocks for use in
the time-dependent model.
4.2 DESCRIPTIONS OF INDIVIDUAL MODELS . ' .
4.2.1 Empirical Theories
Graphical Method
The graphical, or National Coal Board (NCB), method best predicts subsid-
ence due to longwall mining of thin seams at moderate depths and in areas
where the overburden is composed of soft rock (i.e., marl, siltstone or shale)
and contains little or no hard rock (i.e., sandstone or limestone).
The NCB method is based on observed phenomena. It was developed in
England, and its existing data base is extensive. Predictions by the NCB
method do not correlate well with observations in the Appalachian coal fields.
In order to use this method, values are required for these parameters:
width, length, and thickness of the proposed panel, and height of the over-
burden.1'5'10'20'21'22
4-4
-------
Exponential Function Method--
Three steps are involved in this method. First, the model determines
S at the center of the proposed mine void space where excavated coal has
max
exceeded the critical width. Next, it determines the extent of subsidence
where excavated coal has not exceeded the critical width. Finally, it deter-
mines subsidence at various distances from the center of the proposed opening.
The thickness of the excavated seam and the overburden must be known in
advance.
Profile Function Method
The profile function is a technique for fitting a curve to measured data.
Once the curve fittings are determined, subsidence can be determined along any
cross section, in arbitrary positions and orientations, by a mathematical
expression of the distribution of displacements over a two-dimensional area.
The mathematical equation shows one-half the subsidence profile.
The profile function predicts vertical displacement and surface curvature
caused by longwall mining. Its applicability is restricted to complete subsi-
dence over rectangular areas of extraction. Because it cannot explain cause-
effect relationships, it is of questionable value for new situations lacking a
data base. It can be used only in a region that can provide constants for its
development.
The profile function is more accurate and repeatable than the graphical
(NCB) method. With slight modifications, it has shown good agreement with
field data in the Appalachian coalfield.3'5'6'8'19'21'23'24
The hyperbolic function, a profile function developed by Brauner, accu-
rately predicts the depth at the center of a subsidence trough (Sm ), but not
max
24
ihe overall shape of the subsidence trough.
4-5
-------
Many researchers have developed profile functions. Knothe's description
is applicable for single seam, longwall mining of rectangular panels. It is
limited to mines where the area exceeds critical width and is no longer in-
fluenced by time. Other profiles, developed for use throughout Europe by
Martos, King and Whetton, Marr, and Warden, differ considerably from Appala-
chian coalfield data.5'19
A computer program for the profile function, developed by Hall and Dowd-
ing, is based on an analytically obtained formula of approximation for the
subsidence profile curve. It includes empirically obtained factors. The
program requires input data for each extraction area and calculation point at
ground surface, a characterization of the trough area, coordinates and depths
21
of corner points, the thickness of the seam worked, and a stowage factor.
The stowage factor represents the percentage of mined void space that is
refilled with other materials (e.g., crushed rock). Normally, only about
50 to 70 percent of the space can be filled in, representing stowage factors
of 0.5 to 0.7. Because of operational difficulties, the stowage factor never
exceeds 0.8. The practice of refilling is common in Europe only.
Stochastic (Random) Media Theory Method--
The stochastic media theory predicts a subsidence profile using the
characteristic bell-shaped, normal distribution curve. It does not incorpo-
rate knowledge of material properties, but transfers field measurements from
known to new areas by an empirical approach. The only physical principle it
employs is conservation of mass.
Horizontal strain and displacement can be computed by the stochastic
media theory. '
4-6
-------
Integration-Grid Method
The integration-grid method is applicable to nonrectangular, irregularly
shaped extraction areas. It uses a grid derived directly from observed sub-
sidence movements. The critical area is drawn on tracing paper and divided
25
into sectors. Then the relative influence of each sector is determined.
4.2.2 Analytical Theories
Elastic Theory Method
An early physical model, developed by Berry and Sales, models a mine as a
displacement that causes discontinuity in an otherwise continuous elastic rock
mass. It uses two- and three-dimensional solutions to represent a longwall
panel as a single, rectangular dislocation. The model is applicable only to
longwall mining.
Salamon developed a mathematical model based on analog and digital tech-
niques. The model calculates stresses and displacements of complicated exca-
vation geometries in the plane of a single seam. It too is applicable only to
longwall mining.
Finite Element Method
In the multi-layered finite element model, bedding planes are the primary
factor affecting subsidence. This model simulates material behaviors and
boundary conditions. It incorporates the discontinuities inherent in hetero-
geneous materials and assumes anisotropy. It creates a stress distribution,
and determines the extent of the caved zone based on the mechanical properties
of the overburden.5'6'8'10
The finite element model is applicable only to longwall mining in deep
mines, and the gob area must be settled. Another limitation is that certain
parameters for rock strength must be obtained in the laboratory. Since only
4-7
-------
the more competent samples remain intact for testing, estimates of rock strength
are usually overestimated. ' '
The NASTRAN computer program of the finite element model determines the
combined stress on the overburden from mining and body weight. It uses the
Mohr-Coulomb Failure Criteria and Young's Modulus to model the progressive
collapse of strata, from the mine roof to the ground surface. Those elements
that fail are reassigned revised Mohr-Coulomb Failure Criteria and Young's
Modulus values, and the stress field is recalculated. The system reaches
equilibrium, or complete subsidence, in five to six iterations. NASTRAN is a
software package developed at NASA in 1977 by C. W. McCormick. It was origi-
nally designed to calculate stresses on homogeneous materials, particularly
metals, but has more recently been adapted for use in modeling heterogeneous
rock materials.
Isotropic Model Method--
According to Gray et al. (1974), Crouch developed a homogeneous isotropic
model for the two-dimensional case of a single seam parallel to the surface.
The rock is assumed to be isotropic and linearly elastic. However, the sub-
sidence profiles do not match field observations. Gray et al. (1974) provide
no reasons for this.
A three-dimensional transversely isotropic model shows reasonable agree-
ment with field data. No further information was available on this model.
Elastoplastic Theory Method-
Fracturing of strata above a mine roof occurs for some distance. At
greater distances, strata deform without fracturing. These strata are ac-
counted for in the Dahl elastoplastic model, a three-dimensional model that
represents the development of zones of failure as plastic phenomena. ' '
4-8
-------
The elastoplastic model has been used for modeling longwall and room-and-
llar mine subsidence in the bituminous coal regions of Pennsylvania and West
Virginia, and it shows agreement with field measurements. However, if chosen
elastoplastic constants yield results that do not correlate with field mea-
surements, constants are 'adjusted' until correlation is achieved. In other
words, the model is made to fit the available field data. ' '
The mathematical model developed at Conoco explains subsidence as a
result of rock mass deformation at the mine opening. The deformation is
governed by elastic-frictional plastic stress-strain relations where the yield
20
condition is dependent on confining pressure.
Zone Area Method
The zone area method is based on the theory of influence functions. The
theory of influence functions states that the total subsidence is equal to the
sum of the individual subsidences resulting from each of the infinitesimal
extractions comprising the excavation. It is based on mathematical expres-
sions for summations of the effects of minute extraction elements. ' ' '
The zone area model requires input of an area-specific zone factor and
influence constant. The zone factor is a weighted constant that relates the
amount of mined-out area underground to the amount of expected resultant
surface subsidence. The influence constant is an exponential value that has
been developed to correct for ribside subsidence, described below. The zone
area model can be useful in the case of both homogeneous and heterogeneous
overburdens. It can handle uniform and nonuniform extraction patterns, and
individual sections of an extraction area can be evaluated separately.
This model has a limitation that is called ribside subsidence. Where a
surface point is located at the edge of a critical excavation, one-half the
4-9
-------
zone has been excavated. Therefore, subsidence at this point should equal
one-half the maximum value. However, field observations show that subsidence
is equal to only about one-fifth the thickness of the seam. Ribside subsi-
dence is corrected for by deducting a 'compensation zone1 from the 'extraction
zone.' The compensation zone is a zone of incomplete closure at the ribside
of the excavation; for purposes of calculation it is treated as if unmined.
This lowers ribside subsidence values to less than the original values of
one-half the total subsidence, and more closely approximates measured field
27
observations. If a zone factor and an influence constant are available for
the region, the zone area model may be useful for modeling subsidence at a
hazardous waste site. However, availability of the zone factor and influence
constant for a specific area is, in all probability, limited.
The computer program model of the zone area method, developed at Virginia
Polytechnic Institute, establishes a series of six annular rings around a
surface point, with each zone representing 0.2 of the working depth, given a
27
critical width-to-depth ratio of 1.2. Implementation of the model requires
these input parameters:
coordinates of the area of interest
rectangular coordinates of panels and pillars
zone intervals
draw angle, zone factors, influence constant (representative of area
being investigated)
seam and surface dips and gradients
extraction height
graphics input (optional)
Distinct Element Method--
The distinct element model is based on the assumption that the joint
system in the overburden is the single most important mechanism in failure of
the overburden. Strata over the extracted area are presented as a series of
21
rigid blocks. Deformation occurs at interfaces between the blocks.
4-10
-------
Viscoelastic Solution Method--
A viscoelastic model is being developed at West Virginia University. As
explained in the first part of this section, viscoelasticity of rock is a
function of time. As the viscoelasticity of a material reaches the limit, the
material can be considered failed. Therefore, once the viscoelasticity of a
material is determined, a relation can be found to time.
The viscoelastic model takes into account local geology and mine geom-
etry, and is applicable to multiple-seam mining. These factors increase its
applicability to different areas. Close agreement has been shown between
theoretical predictions and observations in the British coalfields. One known
5 13
disadvantage is that it is of limited use with increasing mine depth. '
4.3 SUMMARY
Presently, subsidence models exist that can be used to predict the degree
of subsidence under various circumstances. Computer programs are available
for several of the models.
None of the models that are presently available is able to predict when
subsidence will occur. However, one such model is in the developmental stages
at West Virginia University.
Table 4-2 provides a summary of the models, their characteristics, and
limitations. It is divided into an empirical and an analytical section.
Section 6 provides a discussion of the applicability of these models to
subsidence prediction at hazardous waste sites.
4-11
-------
OO
_J
UJ
o
O
I/)
ca
^
00
u_
o
CM
I
UJ
_j
CO
KB
I
(A
*-
IA
U.
01
u
2
u
X
0
OI
M IA
£ g
OI O
o ta
o 4-> o
goo-
C i/)
*-> c a.
-O -F- .C
I/I £ VI
£ v» U C
o ' -^
.C T3 1-
*-» 4> C
4- O. LI
0 g(g
en o <*-
e c *- oi
0> -M 1
^ at .a &!
|S°§2
O U Ot » **^*
O£ J5"§.-£
VI X (A
3 O VI
<*- *- Ot S£ 01
VI e oi ot
"O "O VI
CTt W T5
e o o>
(A (A (Q
f 01 W
X 3 +»
01 1 X
01 >"
> *»»
S £ *
01 *- ^
** 3 O»
X O**«~
UJ OI OI
o
s = g
o +» ot
c »- c
a.
*- -c
01 3 vi
> u o
t-o a.
a w
Of
a **
at at ot
all
m .-
CD CM
(A "O CM
S* 4-*O
£ r-O
* a.
u *.
0
0
-M .C
fd »->
c a.
I-
SI
«A°X
1
at
jS 1.
o
5
i
e
o
u
c
3
U.
£
!«
.
01 'Q
O C »-»
C TJ U C
c *- o
rt» Ot i-
iO V) U
J3 O) TJ
ex t.
Wl 19 +»
Q.4-> X
f- to 41
IA XI-
§"O 01 u
?~S,°£
01 3 41
«a -w at *-
u * o ~o +*
"fl C*U 3 01
0 01 V» U
a>*^ a. T5
X O> VI 01 "O
4) 01 -w iq
i_ (A 01
2^SI8
J u a.
SS^-g
O>-*-» -M t-
w'a.^ oJ
O vi ^ w
l«i|
III*
oi oi e **
Ul
Is.
Z CM
e
o <^
2 CM
e co
3
U_ *
OI
Ic
1
2^*
O) fO
«-» 3
U
ot fl
J- ^J
»i
'Q VI *
01 41 VI
I- O 01
oi e o
(J O -M
T*= U U
oe v> a.
u
41
4J
i
o>
3
2
W
«
01
1
(A
§
IA
1
X
IA
+*
U
I
C
o
u
3 ^
*4- CM
(J e
^ CM
O
!«;
i
o
^ e
3 Ol
01 fc-
c «
u -c
^'s
e *
c u
f *u
If
O T3
41
. *
iO 01 Ot
01 s
(A -^
.£«>,
5?
W U
W fO C
O > 01
u*-
~ «*=
U (A i.
Q, C C
< a^
e
o
u
c
41
<4-
o
v>
U
4J
IA
*W
01
**
S
i
0*
|
a.
IA
4t
IA
~41
5
a an
c *-
i
T3
C
U
ITJ
O.
a.
|
»»-
Mderably
§
u
1
"O
01
2 IS
Ok, tD
e
o
u
3
01
14-
2
S-
o
tA
U
7
(A
*C
haracte
01
01
1
a.oT
^ .i
u
(A ^
01 C
-o *
IA fQ
2 ?
i- 3
1
C
0)
i
IA
01
£ -3
o
o
ot ^=
f at
M *-
(4-
o -c
u
z^
e ta
o
"4- IS)
or
"w
haracte
8
CA
§2
X O
ll
U
Ol U
C IA
i- 01
1
"O
o c c
19 <*-
C 4J *» O
o ,0 c
*- o * c
-M 1 O O VI
IZcSl
X <« O isi O
O U Q.
i. v* ** V.
aw u ot s.
~- to -M at
O IA O
C 01 *» O *-»
»- > -o -a c v>
-Q l 01 3 Q O V)
U (TO,
(0 C 01 »'*-=
C 01 Of. C <« O
*O "O O 01
§(A -4-) »
CJ A
i
I
cy *->
(fl Ul
O O)
£*"D
o -o
U t)
c ^
*~ o
o
c v>
vi O
o
(A
,. oi
X **
O 01
o s
01 s
0 0
*4o OJ U
^- f O
u X *»
Ot O (J
^ ****
C 1
IA a oi
«-» w
-2S ^
o .a -t~ .£
IA f ' >4-
§ra k
1 ss
«*- O **- 3
o e vi o.
c g
C * "3 5
o ^ w y
at w
MO, C
222<3
k. (A
Sri "*
o 'oJ o 1
JI 1-
II
T3
OJ
O
u
4-12
-------
-a
a>
*-
> oi
155
Ol -C C
S U fl
Ol f
5 *
"vi s -a
v» <*- -i-
O vt i_
(u L. at
> o «-»
I- 4-> 01
01 U T3
*Jf
O C 0
f- U
1^
*- o
^5°
Oi i- oi
(. v> 01
Ol 01 3
a x> F-
T3 > *S
Si5 1
>
O
U
*
o>
c
s
r- O
i 4V
IX
01'*-
§«
S
s-
v»
>> V)
13
Ol 4
TJ O
U C
ss
e
TJ
C 1 fl
ss-a £
4-1 C
3 Ot
C I S
i C\l fl
»-» £X
C vt
4J O 01
« t- vt C
f y £5
F- Q.*J
** ^ Ol TI
C (/> i- U
S oi *-» v»
O -F-
-o - vi -a
"5.3 0 i-
-0 C 3 3
*- F Ol
VI 4-1 O C
SU r U
»T3
S £S
=
«
a.
at
Ot
Ol
u
fl
Ol
c
c
t-
^- o
0 W
4^
*c u
"O
"S a
U C
O- at
<°
Ol
Ol 4-1
v< c
w S
vt t-
Ol *J
"S §
u at
0 0
VI >
4> TJ
3 U
O" X
*- 41
e
-c-o
ot w
TJ /
-i
onS
0 S
(*- a>
1°"
I'll
«^a *
o £»°O
T3 T3 Ol
* C
in S 'a.
41
I
o
u
4^ C
t> t/1
1
at
OJ C
TJ 0) TJ
F- L. 3
VI 4J VI
C i/) 3
O
U ^ Ot
u s-
4-» O fI3
C ^
-F- -0 0
C Ol «-»
.*s ? 2
i. 4-» 0
o i at A
4J F- VI TJ
(J 03 r
rg 2 Ol
(f- 01^2 C
c -
Ol O **
F= 3 ai
-S»5
vi , Ol *-*
O) F t. j3 °
t- C fl OT3
O O 41
-0 ^1 -4J
4J ^ at -F-
O TJ 4-» +»
C U 4> vi
F- .C S 41
vt , w rg i-
ai CL a. t- ii
o a. oi TJ >
a «t -a a. o
u
o
.w .-a
TJ a >
"-o 23
^ S S "o1*-
§'F* VI C
VI 4-*
-F- I. i- C 4-1
1- O 3 -F- Vt 4-1
Ol >4-> C
T '£ TJ ai S
F- O ^ i- ^
W Ol C 4> 41
F-**- 3 J= 4J
z *£ ^ .5 a
5
o^
C
UJ lO
Sin
c -
*- CO
U-v
c
at
3
f over
o
4) *
4-1 U
Ol 4-»
o
*- £
4^
3 §
e 4-
J= VI
C F
4t
"S °*
-a "g
II
o c
-s
i
c
Ol
Ol
+J
c
^
VI
§
*-
4J
'e
ai
at
vt
^mS
3 4-t f
323- .
O 4-i Ol 41
o - F- VI
F- 4-» ai
T3 O C -M
C (J Ol 41
ai vi
nj c -M u
U. 0 C
E
at
e
a.
ai
1
o
a:
CO
1
vt
o
£
a.
o
vt
.C
4-1
1 .5 1_
s- S 0
vi >. u
TJ 4J «*-
C C
3 at at
O 4J S
S- VfF=
ai- *->
VI
O C vt
*-» O vi
U Ol
r L.
O> 4J T3
F O "O
TJ
1- 0 -w
a. c
VI
fOI vt
F- 01
"* 0°
41 t,
F- Ci e
O> VI
c qj c
*- u O
vi C
41 *->
0 TJ TJ
4-" -F- >
V) 1-
Ot 3 Vl
4-> LO ,O
U 0
vt U OJ
Ol TJ -F-
Q£ **- »«
0 ^
U
4t O
VI O£
S
- Ol
S3
0'*-
- k
I«
S § o'
§2 w
en L«
ifl
'oJF^ TJ
(j P~
F- e -o
0.
ss
U1-0
c ^~
« u
L. -r-
H- U-
o
g
1
C
o
1
-C U"J
O)
TJ >
ai o
> i-
^ 0,
ai E
.a s-
o o o
"CTJ T»
TJ ai "4-
O vi 01
4V 3-S
O "O 4V
T3 vi
Ol 41 vt
S- i- U
a, fl t.
C Vt T5
at 4-> TJ
Us
01 vt C
J3 e
O vi
c u at
SJO
T( O
4i Q. C
L. Vt -F-
O -M
O Tl
VI ^~
ai ai at
S. 3 I-
air- t.
J= Ti 0
3 > 0
vl I
41 T» «
C -*-» C
0 TI4J «
IS) S. 3 1
t-t O S
*- wt jn a
4-t Vt X
CO T3
1 J= S C .
F- o at F-
OJH--O X
c S Z ^
01 U t) 4^
1/1 < tu a
41 !- J3
a. ot
a> TI F- c
£ C 0
4-t ai i- S
f U
F- Q. U .
at oi o oi
*O U -C **- U
S -M 4)
vt S 3 -0
c a,<«- vi ^3
O 33
i" vi 01 vt
C C Ol
at oi O
vt g
TI
a. !c
4-1 a
V)
UJ
CD
o
u
4-13
-------
-a
t~-
O J3 4»
L, f-
O C
i > C
3 0 O
"5 £
vt *- I. 4»
C 0) 4V l_
01 O vi 3
-O 1 vi
-* Vt VI
ui vt 01
3 c £ o.
fill
(J
4V
o
O CVJ
1
o
c
41
o
M
X
01
0)
01
*'
01
u
i
vs
3
v) O
.a 4v
*- u
0 g
4J *»-
U -4V
£S
3 fc»
83
O 4
vt
*» 4V .
3 C VI Vt
O 41 4V
U Qi 4 **
*^ o &. o
x U v e vt
01 c 4v o e
*-» 4) 41 O
41 l~ O 4V
4) t- 'O 4 (J
M e c u oi
»-
«*- 4 § 3 C
0> 4C C
U 4) O 4»
c c.e e^
4) O 4V 4V
3 N O T3
-Q C >,
4- U 4 .
<*- (J 4> O t-
&?"3§S3
o a u i.
0) « X
03 3 OCJ 41
^"»
CM
in
fc
i£-
o
4V
4
1
i
O
,
|
V
£
s.
i.
§
b.
a>
3
1
t
|
U
1
VI
VI
£
^g
§
VI
S
O
VI
0)
U
VI 4
01 4V 14-
.a c t-
41 01
4v at e
Its
I .2
L, 4J 3
0
§CW
M-S 1
tSZ i
^ 3 £
e
i
4V
41
Ut
U
C
VI
o
.1
'i
01
c
VI
£
(J
C
_£
4V
"5
1
'i
t
VI
£
|
VI
a
u
+j
4> a,
S-8
VI O O
4) £ U
3-"1*
^ 4 e vi
01 U « T-
a o 4) -4V
Ns"
U 3 ^ C
^ o *-
4V U 4V
vt u e 4
4 flS 4J *V
"*) O 4t "O
O 4V 4)
u c u-a
0) 4) O)
^^G"°
z- .1
>t X ^ vi
^ 01 4J 41
O 4) O *»5
4V 4) 4| >
U VI OI
4- 4> 4»
4» C 4V
ll-5
** o? £
VI ** rtj i
41
vt ^ >t e
vt U OI 41
2! 2,3 * 2
O *O -*v ^
S«4- 4» 41 t-
0 CT^-0 4-
_
n
*
-^
|
3
'o
VI
u
4V
V)
S1"
U
VI
»
c
5
VI
u
i!
4V
O
4V
S
3
O
41
-C
4V
C
1
I
o
*4-
c
41
JS
u
i
|
VI
g
£
£
V
1.
«
4V
4V
e
represe
8
4)
1
U
4t
01
4V
4
VI
01
Vt
S
£
a
c
VI
s_
i
4-14
-------
SECTION 5
COSTS OF USING SUBSIDENCE PREDICTION MODELS
Certain factors must be considered in the cost evaluation of running a
subsidence prediction model at a hazardous waste site.
First, a site survey must be performed. The site survey includes collec-
tion of geological, geotechnical, engineering and environmental data. These
data would be collected by various means, including a drilling program,
topographical survey, and gathering of preexisting geological records and
mining records. Such data would include coal seam thickness, height and
character of overburden, amount of coal extracted (i.e, sizes of rooms), and
amount of support remaining (i.e., sizes of pillars), as well as other factors
summarized in Table 3-1.
Secondly, the cost evaluation must account for a computer-aided analysis
of collected data. Included here are actual computer time, and operator
time. The operator oversees preliminary data analysis, input of the data
into the computer program, and analysis of the computer-generated output.
Estimates for a complete site survey and in-house analysis range from
$150,000 - $200,000. Of this, the costs for running a computerized subsi-
dence prediction model range from $250 for a single finite element model or
NASTRAN, to $2,000. This range reflects the costs of computer time only.
5-1
-------
-------
SECTION 6
CONCLUSIONS AND RECOMMENDATIONS
This section contains conclusions about the major factors that affect
the likelihood of subsidence, the different effects of room-and-pillar and
longwall subsidence on hazardous waste facilities, and the models that will
probably be most valuable for predicting the occurrence and extent of subsi-
dence beneath a hazardous waste facility. Recommendations are made regarding
the various aspects of subsidence prediction, from collection of data through
evaluation of results, as well as the state of the art of subsidence predic-
tion itself.
6.1 CONCLUSIONS
The primary goal of this subsection is to provide a conclusion as to
which models are most valuable for predicting the occurrence and extent of
subsidence beneath a hazardous waste facility. This discussion will be
preceded by a review of those factors that increase the likelihood of subsi-
dence and of the type of subsidence most likely to be problematic for hazard-
ous waste facilities.
Several major factors contribute to increasing or decreasing the likeli-
hood of mine collapse and resultant subsidence. These factors are: height
of the overburden, strength of the rock comprising the overburden, extent of
geological faults in the overburden, presence of moisture, thickness of the
coal seam (relative to the height of the overburden), and percentage of coal
6-1
-------
removed from the mine. Greater overburden height, stronger rocks comprising
the overburden, absence or minimum number of faults in the overburden, in-
creased amounts of moisture, thinner coal seams (relative to the height of
the overburden), and smaller percentages of coal removed all decrease the
likelihood of subsidence, while the opposite in each case increases the
likelihood of subsidence.
Removal of smaller amounts of coal decreases the likelihood of subsi-
dence because the remaining coal acts as a support to the mine roof. The
chances of mine roof collapse are diminished with increasing support. Where
large amounts of coal are removed from a mine, little or none is left to
support the roof and to prevent or delay subsidence. Such is the case with
longwall mining, where the percentage of coal removed is fixed at 100 percent
and subsidence is intended to occur concomitantly with coal removal. Because
approximately 95 percent of the total expected subsidence always occurs at
exactly the same time as longwall mining, longwall mining should not be
conducted under or near an existing hazardous waste facility. Also, because
the remaining five percent of the total expected subsidence requires an
additional one to five years for completion, any construction of a hazardous
waste facility above an area that has been longwall mined should be postponed
until five years after completion of mining. By then, subsidence is reason-
ably expected to be complete. In review, longwall mining should not be
conducted under an existing hazardous waste facility, but a facility may be
constructed over an area that has been completely free of longwall mining for
five years or more.
Unlike longwall mining, the percentage of coal removed by the room-and-
pillar method is highly variable. This is the major factor associated with
the difficulty of predicting the occurrence and extent of subsidence over a
6-2
-------
room-and-pillar mine. As discussed in Section 3, maps of abandoned mine
workings often do not accurately represent the size and number of remaining
support pillars. Drilling surveys are not a solution. The problems with
conducting a drilling survey are several-fold. First, underground character
may vary greatly over short distances, so that mine roof collapse may have
occurred in one section but not an adjacent section. Second, stresses from
mine roof collapse may be in the process of being transmitted to the surface,
but not yet have resulted in subsidence at the surface. Third, knowledge of
the location of rooms and pillars does not provide data on rates of pillar
deterioration that may or may not be occurring within the mine as a result of
coal oxidation, pillar creep, presence of moisture, and so on. It is not
possible to accurately and dependably characterize the number, size, and
condition of support pillars within the room-and-pillar mine; therefore, a
hazardous waste facility should not be constructed above any room-and-pillar
mine workings, and neither should room-and-pillar mining be conducted under
an existing hazardous waste facility. The potential for subsidence to even-
tually occur over any room-and-pillar mine, coupled with the high risks
involved in disrupting the integrity of the containment structure at a hazard-
ous waste facility, lead to the conclusion that no hazardous waste facility
should be constructed above a room-and-pillar mine, and that no mining of any
kind, longwall or room-and-pillar, should be conducted underneath an existing
facility. Although one might use the aforementioned factors as guidelines,
the high risks involved preclude any considerations of situating a new facil-
ity in an area where subsidence is "probably unlikely."
Certain models may provide rough approximations of the likelihood of
subsidence. These models may be used in combination with area-specific
6-3
-------
information about the aforementioned factors to arrive at a conclusion regard-
ing the likelihood of subsidence in a particular area.
Predictive models of subsidence fall into two broad categories: empirical
and analytical. In order to use the empirical approach, two major requirements
must be met. First, the model must be used in an area for which a large data
set has already been gathered. Second, the investigator must have an accurate
account of the dimensions of the mine, including height of the overburden,
width of the coal seam, location and condition of all remaining pillars, and
percentage of coal removed. A large data set has not yet been collected for
the American coal fields but, regardless, meeting the second requirement
would be difficult. For reasons discussed previously, it is not possible to
obtain necessary measurements to verify pillar dimensions and locations.
Therefore, the empirical approach is not applicable.
The analytical approach is derived from deformation mechanisms and
strength parameters of rock. The elastic theory and finite element models
are applicable only to longwall mining.
Several other models have limitations that may or may not preclude their
usefulness. What little information is available about the two-dimensional
isotropic model states that its use is restricted to single seams that are
parallel to the ground surface. The elastoplastic theory has shown poor
correlation between predicted and observed values. Although the zone area
model requires development of an area-specific 'zone factor,' it is useful in
the case of nonuniform extraction patterns, and can evaluate separately the
individual sections of an extraction area.
The distinct element and three-dimensional isotropic models do not have
any specific limitations other than those applicable to all analytical models
6-4
-------
(see Section 4.1). These models may be most useful for predicting subsidence
over longwall mines, but the zone area model, taking into account its limita-
tions, is probably more useful in predicting subsidence over room-and-pillar
mines.
The viscoelastic solution model, still in the developmental stages, is
the only model that addresses the time factor. This is less important than
the ability to predict the occurrence or extent of subsidence, but it could
be useful in the following way. If the time period of subsidence could
someday be estimated for room-and-pillar mining as it now can for longwall
mining, it might be possible to plan to construct a hazardous waste facility
over a room-and-pil'lar mine once it had experienced complete subsidence. The
viscoelastic model could be used to predict the period of time necessary for
subsidence to occur and be completed.
6.2 RECOMMENDATIONS
Despite the difficulties inherent in modeling subsidence from room-and-
pillar operations, every effort should be made to gather as much information
as possible. Using all available data, subsidence prediction should be
attempted using a model recommended for use in this report or elsewhere. It
is important to evaluate the adequacy of the results with a clear understanding
that failure of the model to accurately represent the system could, if a
hazardous waste facility were disrupted, have a detrimental effect on the
environment. Therefore, if it cannot be said with certainty that subsidence
will not occur, then one should be hesitant to place a hazardous waste facility
in the location under consideration.
6-5
-------
Subsidence prediction is still in its early stages, and results of
subsidence prediction models should not be viewed as completely reliable.
Or. Syd Peng of West Virginia University expressed the belief that the state
of the art of subsidence prediction is such that a model providing 30 percent
reliability is considered serviceable. In view of this, it should be kept in
mind that the results of subsidence prediction models may not reflect future
observed conditions.
6-6
-------
REFERENCES
1. Lee, F. T. and J. F. Abel, Jr., "Subsidence from Underground Mining:
Environmental Analysis and Planning Considerations," USGS Circular 876.
2. Karmis, M. and C. Haycocks, "Computer Simulation of Mining Subsidence
Using the Zone Area Method," Office of Mineral Institutes, Bureau of
Mines, March, 1983.
3. Jones, T. S. and K. K. Kohli, "Mine Subsidence in West Virginia," Pre-
sented at the Annual Fall Meeting of the West Virginia Section of the
American Institute of Professional Geologists, October, 1983.
4. Martin, J. S., "The Big Problem Underground," Kentucky Coal Journal,
April, 1985, p.45.
5. Kohli, K. K., "Prediction of Surface Subsidence Profile Due to Under-
ground Mining in the Appalachian Coalfield," Ph.D. Dissertation, West
Virginia University, 1984.
6. Peng, S. S., "Panel Discussion on Subsidence Prediction," Proceedings of
Workshop on Surface Subsidence Due to Underground Mining, sponsored by
West Virginia University, Department of Energy, Bureau of Mines, Novem-
ber 30-December 2, 1981, p. 288.
7. Gray, R. E. and R. W. Bruhn, "Coal Mine Subsidence - Eastern United
States," Man-Induced Land Subsidence, ed. T. L. Holzer, Geological
Society of America, Boulder, Colorado, 1984.
8. Peng, S. S. and C. T. Chyan, "Surface Subsidence, Surface Structural
Damages and Subsidence Predictions and Modeling in the Northern Appala-
chian Coal field/' Proceedings of Workshop on Surface Subsidence Due to
Underground Mining, sponsored by West Virginia University, Department of
Energy, Bureau of Mines, November 30-December 2, 1981, p. 73.
9. "Use of Photo Interpretation and Geological Data in the Identification
of Surface Damage and Subsidence." Appalachian Regional Commission.
ARC-73-111-2554. April, 1975.
10. Gray, R. E. et al., "State of the Art of Subsidence Control," General
Analytics, Inc., Report ARC-73-111-2550, December, 1974, pp. 11-37-
11-45.
11. Personal communication with Drs. Thomas Jones and Kewal Kohli, West
Virginia Tech, April 23, 1985.
A-l
-------
12. Personal communication with William Hobba, U.S. Geological Survey (Mor-
gantown), April 24, 1985.
13. Personal communication with Dr. Syd Peng and Wen Su, West Virginia
University, April 24, 1985.
14. Gentry, D. W. and J. F. Abel, Jr., "Surface Response to Longwall Coal
Mining in Mountainous Terrain," Bulletin of the Association of Engineer-
ing Geologists, Vol. XV, No. 2,
15. Karmis, M. et al. , "The Potential of the Zone Area Method for Mining
Subsidence Prediction in the Appalachian Coalfield," Proceedings of
Workshop on Surface Subsidence Due to Underground Mining, sponsored by
West Virginia University, Department of Energy, Bureau of Mines, Novem-
ber 30 - December 2, 1981, p. 48.
16. "Study and Analysis of Surface Subsidence Over the Mined Pittsburgh
Coalbed," GAI Consultants, Inc., Contract J0366047, Bureau of Mines,
July, 1977.
17. Personal communication with Bradley Johnson, Bureau of Mines (Washing-
ton, D.C.), April 26, 1985.
18. Personal communication with Dr. Lessing, West Virginia Geological Sur-
vey, April 22, 1985.
19. Gill, D. £., "Subsidence Associated With the Mining of Bedded Deposits,"
Subsidence Subcommittee of the Canadian Advisory Committee on Rock
Mechanics, Montreal, February, 1971.
20. Choi, D. S. and H. D. Dahl, "Measurement and Prediction of Mine Subsid-
ence Over Room-and-Pillar Workings in Three Dimensions," Proceedings of
Workshop on Surface Subsidence Due to Underground Mining, sponsored by
West Virginia University, Department of Energy, Bureau of Mines, Novem-
ber 30-December 2, 1981, p. 34.
21. Hall, B. M. , and C. H. Dowding, "Prediction of Subsidence From Full
Extraction Coal Mining," Int. J. Rock Mech. Min. Sci. and Geomech. ,
Abstr. , V. 19, No. 3., June 1982, pp 305-312.
22. "Subsidence Engineer's Handbook," National Coal Board, London, 1965.
23. Tandanand, S. and L. R. Powell, "Consideration of Overburden Lithology
for Subsidence Prediction," Proceedings of Workshop on Surface Subsid-
ence Due to Underground Mining, sponsored by West Virginia University.
Department of Energy, Bureau of Mines, November 30-December 2, 1981, p.
17.
24. Hood, M. et al., "Empirical Methods of Subsidence Prediction - A Case
Study," Proceedings of Workshop on Surface Subsidence Due to Underground
Mining, sponsored by West Virginia University, Department of Energy,
Bureau of Mines, November 30-December 2, 1981, p. 100.
A-2
-------
25. Kratzsch, H., Mining Subsidence Engineering, Springer-Verlag, Mew York,
1983.
26. Adamek, N. and P. W. Jeran, "Evaluation of Existing Predictive Methods
for Mine Subsidence in the U.S.," Proceedings of Workshop ^on ^Surface
Subsidence Due to Underground Mining, sponsored by West Virginia Univer-
sity, Department of Energy, Bureau of Mines, November 30-December 2,
1981, p. 88.
27. Karmis, M., and C. Haycocks, "Computer Simulation of Mining Subsidence
Using the Zone Area Method," Dept. of Mining & Mineral Engineering.
Virginia Polytechnic Institute, March 1983.
28. Martin, J. S., "A Problem We Can't Ignore," Kentucky Coal Journal,
January, 1985, p.35.
29. "Survey of Ground Surface Conditions Affecting Structural Response to
Subsidence," 6AI Consultants, Inc., Contract J0295014, Bureau of Mines,
June, 1983.
A-3
-------
LIST OF CONTACTS
Brad Johnson
Bureau of Mines (Washington)
(202) 334-3312
Dr. Thomas Jones
West Virginia Institute of Technology
(304) 442-3339
Dr. Michael Karmis
Virginia Polytechnic Institute
(703) 961-7057/6671/7918
Dr. Kewal Kohli
West Virginia Institute of Technology
(304) 442-3337
Dr. Lessing
West Virginia Geological Survey
(304) 594-2331
Dr. Syd Peng
West Virginia University
(304) 293-5696
Mike Trebitz
Bureau of Mines (Pittsburgh)
(412) 675-6602
Dr. Barry Voight
Pennsylvania State University
(814) 865-3437
A-4
-------
GLOSSARY
Analytical Model - A mathematically complex subsidence prediction method
based on the theory of elasticity, mechanics of continuum, and an ideali-
zation of the subsidence phenomenon.
Angle of Draw - The angle of inclination from the vertical of a line connect-
ing the edge of a workings and the edge of a subsidence area.
Critical Width - A site-specific value, the exceedance of which results in
development of the maximum possible depth of subsidence in the center of a
subsidence trough.
Empirical Model - A mathematically simple subsidence prediction method based
primarily on field observations and data, a descriptive technique.
Gob - The refuse coal and other minerals that are left in the mine because
they are not marketable.
Longwall - A system of mining in which the entire seam is removed in one
operation, with heavy machinery, by means of a long working wall.
Maximum Subsidence - A value that is equal to 0.9 multiplied by the thickness
of the coal seam.
Overburden - Rock material of any nature, consolidated or unconsolidated,
that overlies a coal seam.
Pane_1_ - A large rectangular block or pillar of coal; an area or district in
which coal is being extracted.
Room-and-Pillar - A system of mining in which the coal is mined in rooms
separated by pillars, and about 50 percent of the coal is removed on the
first working.
Seam - A stratum or bed of coal. This term is usually applied to a large
deposi t.
Strain - Deformation resulting from applied force, measured as change in
length per unit length in a given direction. Within elastic limits, strain
is proportional to stress.
Stress - Resistance of a body to congressional, tensional or tortional force,
measured as force per unit area.
A-5
-------
Subsidence - A lowering of ground surface which occurs as a result of the
deformation caused by removal of subsurface mineral deposits.
Subsidence Factor - The ratio of the depth of the subsidence trough to the
height of the coal seam.
A-6
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing]
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
5. REPORT OATE
Evaluation of the Applicability of Subsidence
Models to Hazardous Waste Sites
6. PERFORMING ORGANIZATION CODE
7. AUTHO«(SI
8. PERFORMING ORGANIZATION REPORT NO.
PEI Associates, Inc.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEI Associates, Inc.
11499 Chester Road
Cincinnati, Ohio 45246-0100
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY IMAMS AND ADDRESS
Hazardous Waste Engineering Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY COOS
EPA 600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACT
EPA has discovered a number of uncontrolled hazardous waste sites in close proximity to
abandoned underground mines. Further, several Resource Conservation and Recovery Act
permit applications have been received for treatment, storage, or disposal facilities
located in areas where abandoned underground mines are known to exist. The potential
exists for subsidence under a hazardous waste facility to result in uncontrolled re-
lease of hazardous constituents to the environment.
The investigation was approached in two phases. Phase I involved a literature review
and data compilation to gather information on the subsidence phenomenon, available pre-
dictive models, and on the adverse effects that can result from mine-related subsidence
and Phase II consisted of an evaluation of available equations and models used to pre-
dict subsidence and an assessment of the applicability of these models to predict sub-
sidence problems at hazardous waste sites.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lOSNTIFlgRS/OPSN ENDED TERMS
c. COS ATI Fieldy Group
13. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS {Tnts Report)
Unclassified
21. NO. OF PAGES
20. SECURITY CLASS (This page I
Unclassified
22. PRICE
SPA fatm 2220-1 (R«y. 4-77) Previous EDITION is oeaousre
------- |