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United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/035 May 1985
s>EPA Project Summary
Settlement and Cover
Subsidence of Hazardous
Waste Landfills
W. L Murphy and P. A. Gilbert
Numerical models using equations
for linearly elastic deformation were
developed to predict the maximum
expected amount of settlement and
cover subsidence, and potential crack-
ing of the cover by differential settle-
ment in uniformly, horizontally layered
hazardous waste landfills. The hazard-
ous waste landfill models represented
landfills in which unsaturated wastes
were contained in standard steel drums
that were assumed to deteriorate ulti-
mately in the landfill. The models were
analyzed using methods of linear elas-
ticity to estimate the maximum amount
of subsidence of the final cover to be
expected before and after landfill clo-
sure and after deterioration of the waste
containers. The model landfill consisted
of alternating layers of intermediate
inert cover soils and steel drums filled
with simulated waste materials. Waste
drums, waste materials, and intermed-
iate cover soils were assigned values of
density. Young's modulus, and Pois-
son's ratio for the analysis. Landfill
geometry, layer thicknesses, waste
drum placement, steel drum stiffnesses,
and laboratory consolidation tests on
the soils and simulated wastes were
also considered. To simulate postclo-
sure waste layer deterioration, compres-
sion of the fill was calculated for de-
creasing values of the Young's modulus
of the waste layers. The analyses indi-
cate that as much as 92 percent of the
expected subsidence of the cover is
caused by closure of cavities (void
space) inherent in landfilling. The maxi-
mum expected subsidence was calcu-
lated to be approximately 12 percent of
the total landfill thickness. If all of the
waste drums are assumed to contain 10
percent void space (a "worst case"
condition) the maximum subsidence
could be as high as approximately 20
percent of the fill thickness.
Finite element method (FEM) analysis
of differential settlement across the
landfill indicates that tensile stresses do
not develop, and therefore that cracking
by settlement does not occur in the final
cover of the landfill as modeled. Numer-
ical and FEM models were based on
data gathered for several active com-
mercially operated hazardous waste
landfill facilities in the United States.
Other settlement and subsidence mech-
anisms, including karst and subsurface
mining experiences, sanitary and low-
level nuclear waste landfill experiences,
and classic soil consolidation theory
were addressed in addition to linear
elastic deformation.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Background
Settlement of sanitary and low-level
nuclear waste landfills with subsequent
damage to or compromise of the integrity
of the covers is a recognized and docu-
mented occurrence. The U.S. Environ-
mental Protection Agency (EPA) is con-
cerned that settlement within hazardous
waste landfills may produce similar sub-
sidence problems with cover systems.
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The failure or distortion of the cover
system may cause consequences that are
totally unacceptable to managers of haz-
ardous waste. Little documentation exists
of the extent, potential for, or mechanics
of settlement/subsidence in hazardous
waste landfills.
Purpose
This study was conducted to determine
the potential for settlement of the fill and
subsequent subsidence-related damage
to cover systems of hazardous waste
landfills and to provide information nec-
essary for developing and improving
interim and future regulatory guidance. A
goal was to develop predictive numerical
models to estimate the amount of sub-
sidence and strain that would be expected
to occur in the landfill and cover as a
result of settlement. It was first necessary
to characterize the hazardous waste land-
fill with respect to design, operation, and
physical properties. This study examined
the characteristics of several hazardous
waste landfills with special attention to
those features expected to influence the
potential for settlement of the fill and
subsidence of the final cover. Numerical
models were developed from assessment
of landfill characteristics and suspected
settlement/subsidence mechanisms. The
potential for degradation or compromise
of the final cover is examined through
models developed in the full report.
Scope
The study addresses hazardous waste
landfills operating under the interim
standards imposed by EPA under author-
ity of the Resource Conservation and
Recovery Act (RCRA) of 1 976. The pre-
dictive models developed for the investi-
gation are based on facilities operated by
the commercial hazardous waste disposal
industry. Other landfill types and subsid-
ence mechanisms operating in geotech-
nical circumstances other than hazardous
waste landfill situations were reviewed in
developing hazardous waste landfill mod-
els, but the mechanisms and character-
istics used to construct the models are
those derived from an assessment of
several hazardous waste landfills oper-
ating with interim status permits under
RCRA.
Approach
The approach was to determine from
the literature the known mechanisms
and analytical techniques of settlement
and subsidence in all geotechnical areas
including mining, landfill, engineering
fills, karst (geologic), tunneling and foun-
dation investigations; to determine the
mechanisms most likely to be active in a
hazardous waste landfill; and to select
the proper methods of analysis. A review
was made of literature pertinent to set-
tlement and subsidence in landfills. The
initial computer data search used the
information retrieval systems DIALOG,
National Technical Information Service
(NTIS), GEOREF, and COMPENDEX to
compile listings and abstracts of pertinent
reports and documents. Review and
updating of literature continued through-
out the study. Site characterization, nec-
essary for model development and prob-
lem assessment, was accomplished by
contacting the hazardous waste disposal
industry and through site visits to selected
facilities to obtain real data on site
geometry, liner and cover design and
properties, waste and fill placement pro-
cedures, waste and fill physical proper-
ties, compaction efforts, leachate collec-
tion and control, subsidence experiences,
and other relevant information. Sites
were selected for their location, repre-
senting several areas of the nation and
several climate and soil conditions; for
their size, representing for the most part
large facilities; and for their current
activity, i.e., their status as viable com-
mercial landfill facilities operating under
RCRA guidance.
In addition, selected state agencies
familiar with hazardous waste disposal in
their states were contacted for further
information including information about
potential or existing subsidence prob-
lems, and to obtain an indication of the
amount and kinds of variation in opera-
tional procedures to be expected nation-
wide. The authority for implementing
RCRA interim standards on the state level
is vested in the state Departments of
Health, Water Resources Boards, special
environmental regulatory commissions
or departments. Natural Resources De-
partments, Pollution Control or Solid
Waste Management Boards, or state
EPAs. Some states have multiple juris-
diction for implementing the standards.
The National Directory of State Agencies
was used to establish an initial list of
contacts. Other regional or site-specific
data were obtained from consulting engi-
neering firms and scientists and engi-
neers of state agencies.
Numerical FEM models were developed
for selected hazardous waste disposal
situations using the data compiled in site
characterization and by modifying exist-
ing FEM analytical models. The approach
in the modeling analysis was to simulate
worst-case conditions that would be
expected in RCRA landfills, i.e., those
situations producing the greatest amount
of settlement. Accordingly, the larger
facilities where wastes are buried in
drums were used to develop the model
landfill. The subsidence problem was
analyzed by modeling pre- and postclo-
sure maximum settlement in the middle
of the landfill and differential settlement
across the landfill.
Landfill Modeling
Several commercially operated landfills
observed during this study were large pits
excavated in natural earth to depths of 50
to 100 ft. The natural earth was typically a
soil or rock of low permeability. Landfill
structures observed were generally lined
with clay and/or synthetic polymeric
membranes and equipped with a leachate
collection and withdrawal system. The
structures were typically filled with alter-
nating waste layers 2 to 3 ft thick and
intermediate cover layers about 1 -1 /2 ft
thick. After filling, the landfills are capped
with a permanent clay cover layer which
is continuous with the sides and bottom
liner system. A polymeric membrane
may or may not be used in the cover. A
representative model landfill section
chosen for the purpose of mathematical
analysis and demonstration for this in-
vestigation is 50 ft deep and 200 ft wide
across the cell bottom. Sides slope up to
the original ground level at 3 horizontal
on 1 vertical and the cover layer slopes up
to the crown at 5 percent to give a total
depth of 62.0 ft at the center of the
landfill. The model landfill is assumed to
be constructed and filled exactly as
several observed representative landfills.
Solidified material buried in steel drums
is expected to make up the most signif-
icant portion of the waste in the cells
under consideration. A mathematical
equation based on the theory of elasticity
was developed to allow calculation of
maximum subsidence. Use of the equa-
tion requires input of waste properties
such as density, depth of burial, and
stiffness (Young's modulus). Subsidence
occurs as a result of postclosure cavity
collapse and waste-drum deterioration
and softening. The mathematical equa-
tion predicts subsidence as a result of
waste layer softening which is simulated
by lowering the Young's modulus of the
layers.
The equations developed in this report
and their related subsidence prediction
curves apply as well to horizontally lay-
ered landfills of varying depths, physical
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properties,and geometries and are not
restricted to landfills with the specific
dimensions presented in this report. The
analyses are for homogeneous and iso-
tropic materials in layered, unsaturated
waste cells. Subgrade materials are pre-
sumed to be rigid (noncompressible). The
site-specific amount of maximum sub-
sidence depends on the fill depth and the
physical properties of the waste, fill,
cover and liner materials. Waste proper-
ties are presumed to show the most site-
specific variability of all the contributing
materials and are critical to the accurate
prediction of subsidence. Strengths and
stress-strain data of actual or simulated
waste materials would permit more ef-
fective predictions of expected subsid-
ence with the models developed for the
full report.
Response of Cover to
Maximum Subsidence
Analyses of the numerical settlement
models developed for this study indicate
that the maximum landfill cover subsid-
ence that would be expected under
worst-case conditions (deep fill, deteri-
orated drums, and low-stiffness waste
layers) would be approximately 111/2
percent which for the representative
landfill would result in a final cover slope
of about 2 percent from the crown to the
landfill boundary. Most of the subsidence
(around 10 percent) occurs by closing of
cavities incorporated during filling. The
111/2 percent subsidence would not create
ponding (negative slope) for the repre-
sentative landfill assuming a 5 percent
cover slope (12.5-ft crown) was estab-
lished at closure. The resulting 2 percent
slope is, however, less than the minimum
suggested by EPA (3 percent) to promote
drainage. Drainage off the cover is desired
to prevent infiltration of standing or slow-
draining precipitation with the danger of
filling and overflowing the landfill (the
bathtub effect) and of creating excessive
leachate. An additional subsidence of 8 to
9 percent must be considered if drums
are assumed to contain the maximum
allowable 10 percent void space when
placed in the fill. However, an assumption
that all the drums would contain 10
percent void is considered realistic by the
authors.
This study has shown that consolida-
tion of the unsaturated intermediate cover
(fill) layers occurs relatively soon and
prior to closure. Landfills whose waste
layers consist predominantly of waste-
filled steel 55-gallon drums will exper-
ience additional settlement after probably
several years during which time the
drums and waste deteriorate to progres-
sively lower elastic moduli and strengths.
Estimations of waste layer and fill moduli
for intact and deteriorated waste con-
tainer conditions were made and applied
to the prediction curves to derive the 111/2
percent maximum expected subsidence.
Response of Cover to
Subsidence Cracking
(Differential Settlement)
The analysis of the assumed hazardous
waste disposal cell described shows that
tensile cracki ng does not occur within the
body of the landfill configuration modeled
for this analysis. For the soil and filling
techniques assumed, the stresses which
would cause tensile cracking do not
develop. The stress and displacement
fields observed in the landfill body are
well behaved and smooth with no un-
expected singularities. There are several
reasons for this, including the fact that
the landfill material behaves plastically,
yielding rather than rupturing. The found-
ation completely supports the waste
material and is very stiff relative to the
waste cell contents. An analysis of the
vertical surface displacement shows that
vertical movement is maximum in the
center of the cell and subsidence is almost
uniform over the flat-bottomed portion of
the cell. This confirms that a central
column analysis of subsidence isjustified,
and allows the subsidence at any point
across a typical cell to be estimated if the
center subsidence is known. Tensile
stresses from differential settlement oc-
curring in other landfills (nonuniform
waste layers, for example) were not
analyzed.
Other Considerations
The settlement models for this study
were developed under the assumption
that drained, unsaturated conditions pre-
vailed within the landfill and that con-
tainers of free liquids were not included
in the fill. It is prudent to consider the
effects on settlement of including sub-
stantial volumes of free liquids, or stabil-
ized liquids that become unstable with
time, in the landfill. A liquids-filled waste
drum that had deteriorated sufficiently
would release its contents into the sur-
rounding soil or soil-like intermediate
cover or into bulk wastes surrounding the
drum. Assuming the surrounding fill and
bulk wastes were less than 100 percent
saturated, the freed liquids would be
absorbed into the pore spaces and would
increase the saturation of the fill mater-
ials. Stabilized or "solidified" liquids that
might be released after deterioration of
drum contents would be expected to act
similarly, but with a smaller volume of
liquid. The intermediate cover layers
placed will be compacted with construc-
tion equipment as discussed earlier.
Regardless of the compactive effort ap-
plied, two conditions of compaction are
possible; compaction wet of optimum
water content and compaction dry of
optimum water content. If the layers are
compacted wet of optimum, allowed to
consolidate to 100 percent consolidation
under the applied load, and then exposed
to free liquid (water), volume change of
the clay layer will usually be insignificant.
However, if the layers are compacted dry
of the optimum water content, two effects
might conceivably be expected as a result
of post-closure release of liquid; the cover
layer could tend to absorb liquid and swell
(increase in volume) or collapse (decrease
in volume) upon exposure to the liquid.
Laboratory investigation has shown that
collapse of soil usually occurs at low
water contents and at high stress levels,
which would mean that the layers at
greater depths in the landfill would tend
to collapse. Volume change due to col-
lapse is irreversible, that is, the settle-
ment or subsidence due to this phenom-
enon is not recoverable. Swell usually
occurs in soils at low water contents and
low stress levels, which would mean for
this study that layers at shallow depths in
the landfill would tend to swell when
exposed to water. Swell, however, may
be reversible in that, as surplus water
which was absorbed into the soil diffuses
with time into dryer regions of the layers,
shrinkage may occur and the soil may
tend to return to its original volume.
Laboratory tests suggest that to mini-
mize the problem of either swell or
collapse of intermediate cover layers,
compaction wet of the optimum water
content is desirable. In practice, waste
and intermediate cover layers of hazard-
ous waste landfills are placed at the
existing water content of the soils and no
special compactive efforts are made.
At this time, no specific statement may
be made regarding the effects of liquid
released in a landfill. The ultimate effects
of such releases would depend on site-
specific factors such as the amount of
liquid released, the water content of the
layers at the time of compaction, the
mineralogy of the intermediate cover
layers, the compaction characteristics of
the layers, and the stress levels within
the layers exposed to liquids.
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Conclusions
Conclusions reached as a result of the
study are:
1. The dominant mechanisms of set-
tlement of the fill and subsidence of
the covers of horizontally layered
hazardous waste landfills are ex-
pected to be closing of the inherent
drum-placement void spaces and
compression of cell contents includ-
ing intermediate cover soils, waste,
and waste containers. Cavity relat-
ed piping and sinkhole phenomena
are not expected to play a major role
in predicted subsidence. This con-
clusion is based on review of repre-
sentative active waste disposal
practices in industry and govern-
ment and analysis of documented
and theoretical subsidence mech-
anisms reported in other, related
activities.
2. The maximum postclosure subsid-
ence of the cover (cap) of a simu-
lated hazardous waste landfill op-
erating under interim RCRA guide-
lines, from compressibility alone, is
predicted to be about 11 !/2 percent
of the total height of the fill and
cover at the center of the landfill.
For a 62.5-ft-thick fill and cover the
maximum expected subsidence,
after deterioration of the waste
containers, was 87 in. The final
cover slope with that subsidence
would be about 2 percent, which is
undesirable under current RCRA
guidelines. An additional 8 to 9
percent subsidence must be consid-
ered if waste drums are assumed to
contain the maximum allowable 10
percent void space when placed in
the fill.
3. Actual landfill settlement and cover
subsidence may be less than the
worst-case maximum predicted
11 Ya percent because more of the
voids between containers will prob-
ably be filled as a result of less than
optimum (tight) stacking of contain-
ers during placement. The 11 Vi
percent figure is considered an
effective value for design purposes.
practiced at representative RCRA-guided
secure landfills as determined from field
observations of this study. Recommended
operational methods include:
1. Landfill operators should make an
effort to reduce voids when placing
wastes and fill within the cell by
insuring that intermediate cover
soils are allowed to sift between
waste containers and debris. Drums
or other containers of wastes
should be filled to minimize the
volume of void within the contain-
ers. Much of the potential settle-
ment from compressibility can be
eliminated by preventing the inclu-
sion of cavities in the waste place-
ment process in the typical haz-
ardous waste landfill.
2. Layering of waste and intermediate
cover in thin lifts so that some
compactive effort is achieved during
filling.
3. Control of liquids by installation of
efficient leachate collection sys-
tems and stabilization of liquid
wastes to prevent saturation of the
fill and to allow consolidation to
occur as rapidly as possible.
4. Installation and monitoring of cover
settlement plates so that the sub-
siding surface can be maintained at
the proper slope. Hazardous waste
landfills should be documented.
instrumented, and monitored after ^
closure. Subsidence of the cover as I
well as the settlement of internal
waste layers should be monitored
with time in an effort to gain
understanding of postclosure in-
ternal changes, how they occur,
and how they affect the overall
behavior of the landfill. Many of the
mechanisms at work within these
landfill cells can be understood only
by study and experience with repre-
sentative landfill cells. The data
obtained by field monitoring will
permit evaluation and improvement
of settlement/subsidence predic-
tion models developed in this study.
Landfill operators should remember
that, while the cover surface can be
maintained at a proper runoff slope
by the addition of soil or other
material, the internal cover liner,
whether a clay layer or a flexible
membrane liner or both, may have
been deformed and stressed by
subsidence. Internal cover liner
damage or deformation cannot be
remedied by simple cover surface
cosmetic actions.
5. Placement of a buffer thickness of
intermediate cover soils above the
uppermost waste layer and beneath A
the final cover to lessen the poten- "
tial for collapse of the cover directly
above locally compressible zones
such as deteriorating drums.
W. L. Murphy and P. A. Gilbert are with the U.S. Army Engineer Waterways
Experiment Station, Vicksburg, MS 39 J 80.
Robert P. Hartley is the EPA Project Officer (see below).
The complete report, entitled "Settlement and Cover Subsidence of Hazardous
Waste Landfills." (Order No. PB 85-188 829/AS; Cost: $13.0O, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
Recommendations
Cover subsidence from settlement of
compressible fill is expected to occur.
Therefore, to minimize the severity of
subsidence, managers of hazardous
waste landfills should continue at least
those operational methods that are being
. S. GOVERNMENT PRINTING OFFICE: 1985/559 111/10836
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
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