Prediction/Mitigation of Subsidence
Damage to Hazardous Waste Landfill Covers
(U.S.) Army Engineer Waterways Experiment
Station, Vicksburg, MS
Prepared for
Environmental Protection Agency, Cincinnati, OH
Mar 87
EPA 600-2-87-025
PB87-175386
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«. — 3 / ~ ' 7 J J
;:i'.\."v;n, 4- •,,
P:ttUlCrH)N/MlTloATlUN OF SUBS I!*.NCt UAMAtih
HAZARDOUS UASTK L,\NUKILL CUVtKS
by
Paul A. Gilbert
and
William L. Murphy
Geotechnlcal Laboratory
U. S. Army Engineer Waterways Experiment St^tton
PO Box 631, Vicksburg, MS 39180-0631
Agreement Mo. DW21930680-01-0
Project Officer
Robert P. Hartley
Land Pollution Control Dlvisioa
Hazardous Waste Engineering Research Laboratory
Cincinnati, Gd
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI. OH 45268
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,B, TECHNICAL REPORT DATA
If lease rnu ,'niirjctiont on me went be/ore cample ti/it.1
3 REIlPiyjT S ACCESSION NO.
rioT 1 ( ~ 3 £
•^••^•^^•••••^^^^^^^^^^••••^fcfc^^^B^m^^S^^—^^__
4 TITLE ANO SU8TITI.C
Prediction/Mitigation of Subsidence Damage to
Hazardous Waste Landfill Covers
S. REPORT DATE
March 1987
6. PERFORMING ORGANIZATION COOe
AuTHOR(S)
Paul A. Gilbert and William L< Murphy
• . PERFORMING ORGANISATION REPORT SO
PERFORMING ORGANIZATION NAME ANO AOOOESS
l.S. Army Waterways Experinent Station
P.O. Box &:i
Vicksburg. Mississippi 39180
10. PROGRAM ELEMENT NO.
1 1. CONTRACT /GRAN TNO.
DW 21930680-01-0
2. SPONSORING AGENCY NAME 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 COOE
EPA/600/12
IS. SUPPLEMENTARY NOTES
^natatteristics of Resource Conservation and Recovery Act hazardous waste landfills
and of landfilled hazardous wastes have been described to pen.it development of
models and other *;ialytical techniques for predicting, reducing, and preventing
landfill settlement and related cover damage by subsidence. Differential settle-Ben
across short distances is more threatening than relatively uniform settlement
across longer distances. The potential for differentiallll settlement is considers
to be greater in heterogeneous landfills than in monofills. Settlement of bulk
waste landfills is relatively predictable and is expected to be essentially
complete before final closure. Settlement of landfills with containerized
wastes is more difficult to predict because the containerized wastes may remain
relatively undeformed until the containers degrade and collapse. Bulk waste
(monofill) landfills can be analyzed by consolidation theory. The potential for
differencial settl»--ent can be analyzed by treating the final cover as a beam
and determining the tensile stresses. Differential settlement can also be
analyzed by determining the deformation of two or more central columns. Damage
to the final cover by differencial settlement can be minimized by compacting
wastes during placement, by eliminating void space within the landfill, by
stabilizing liquids before placement, by not disposing of waste in containers,
and by adjusting cover component specifications to minimize the effects of
KfV WORDS ANO DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTIPIERS/OPEN ENDED TERMS c. COSATI Field/CfOup
21. NO. Of PAGES
91
«. DISTRIBUTION STATEMENT
Release to Public
i9. SECURITY CLASS
iim" laia^i f^o H
2O. SECURITY CLASS
Unclassified
23. PRICE
f*rm 2239-1 (»•». 4-77) mcviou* COITION n o«o«.«TC
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This report has been revioved in accordance with the U.S. Environmental
Protection Agency's peer and administr-itive review policies and approved for
presentation and publication. Mention of trade names or commercial products
does act constitute endorsement ,>r reeoimnendation for use.
II
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FUREW'JKD
Today's rapidly dev*l->ping aru* c' >nSing technologies and industrial
products and practices frequently carry with them the increased generation of
solid and hazardous wastes. These materials, if impr >p-irly dealt with, can
threaten both public health and the environment . Abandoned wast>» s-ltes and
accidental releases of toxic and hazardous substances to the environment also
have important environmental and public health Implications. Th* 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 i>e rmi ". c ' nj? and other responsibilities of the State and
local governments, and the needs of both large and snail businesses in handling
their wastes responsibly and economically-
This repcrt describes the causes and effects, prediction methods, and
technologies that may be applied for the prevention of subsidence In hazardous
waste landfills. The information should be of assistance to those Involved
in evaluating landfill permit applications. The goal is to help prevent
damage to, and resulting leaks through, landfill covers caused by subsidence-
induced stresses.
Thomas R. Haus=r, Director
Hazardous Waste Engineering Research Laboratory
iii
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ABSTRACT
Characteristics of Resource Conservation and Recovery Act hazardous waste
landfills and of landfilled hazardous wastes have been described to permit
development of models and other analytical techniques for predicting, reduc-
ing, and preventing landfill settlement and related cover damage by subsi-
dence. Landfill settlement results from the consolidation and secondary
coar«»«io« of the waste nass and from the collapse of voids In the fill and
of containers and other debris by corrosion, oxidation, combustion or biochem-
ical decay. Landfills may be described a? containing a single type of waste,
a mnoflll. or as containing different types of wastes heterogeneously such as
bulk, in containers, and 35 c'elris. referential settlement across short dis-
tances is more threatening thar relatively uniform settlement across longer
distances. The potential for differential settlement is considered to be
greater in heterogeneous landfills than in ironofills. Settlement of bulk
waste landfills is relatively predictable and is expected to be essential y
complete before final closure- if adequate provisions are made for internal
drainage of fluids. Settlement of landfills with contairerized wastes is more
difficult to predict because the containerized wastes may remain relatively
urdeformed until the containers degrade and collapse. The void space arnjnd
nnd in containers can be a major contributor to total postclosure settlement.
Accordingly, steps should be taken to minimize the void component of settle-
ment by backfilling voids during waste placement or by eliminating the dis-
posal c-f drums and other vaste containers. Settlement of some landfills can
be predicted by analyzing the deformation of a central col-imn consisting of
layers of wastes and intermediate cover material. Bulk waste (monoflll) land-
fills can be analyzed by crnsclidation theory. The potential for differential
settlement can be analyzed by troafing the final cover as a bean and determin-
ing the tensile stresses that develop in the cover layers. Differential set-
tlement can alsc be Analyzed by determining the •'eformation of two or more
central columns. Damage to the final cover by differential settlement can be
nlniaized by compacting wastes during placement, by eliminating void space
within the landfill, by stabilizing liquids before placement, and by adjusting
cover component specifications to minimize the effects of tensile strain.
IV
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CUNftNTS
Page
Abstract ................................ xv
Figures ................................ vl
Tables ................................. vil
Acknowledgments ............................ vtii
1. Infodm-t ion .........................
Fip.ck ground .......................
Purpose .........................
2 . Conclusion* arJ Recommend at ions ............... '
1. Landfill Ch ir.icteristics ................... 5
Current landfill designs ................ 5
Characteristics of landfill wastes ........... 9
4. An.ilvsis of Potentinl Settlement and Subsidence ....... 15
Sett 1 eirent-causinp mechanisms .............. '5
Trtdtctine landfill settlement ............. 20
Analvsis of differential cover subsidence ........ 32
5. Vittpacien of Settlement ?nd Efferts of Settlement ...... i4
Landfill treatment to reduce settlement potential .... ^4
DesipTi and ror?truction of covers to
.Tcconw'ij ite subsidence ................ ^°
Correcfive action for subsidence ............ 53
Referrncer. .............................. 57
Appendixes
A. Characteristics of Selected Haznrdous Waste Landfills .... 59
B. Cl assi f ir.vtirn of f.eomenbranes ................ ^^
C. Field experimental F.xample of Settlement Analysis
bv Standard Consolidation Theorv .............. 71
D. Consolidation Equation .................... 80
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F1CVRF.S
bo£ "age
: Final covers of actual RCRA landfills ?
.' EPA-recorwerJed RCKA landfill cover R
3 Thickness .ind volume relationship for
A soil tnnss of thickness H and volume V^ IS
i Constrain*-: elastic roduius of waste
siirulater. h\ "Virtv litter"
5 Stress-str.iin characteristics of kitty litter . .
6 Stress-strain data fit by oscillating polynomial
7 Beam repre«entadci of a cover svstem
P ^tress-strain behavior of clav sfectmens at
different compaction condition 35
1 Mo.inls of beams on the elastic foundations 37
10 ." / • versus avernjre tend«»x ^>
12 ? 'nd densfi'icat ion usin? vibratorv rollers 4fi
13 >nil compaction curve 49
C-! Hxperimental Inncftll, plar view 7ft
C-2 Tvpical crr-ss section of experimental landfill 77
C-3 Load-depth diagram 79
C-i fc»ad increment added to n sludge laver *8
C-5 Consol4dation characteristics of sample sludge 79
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TABLE?
NuirK-r
I Reported Wa-te fVnsities for ..and;'. lied
Hn.'artious Wastes
Fraineering Char.ii'ferl sties .ind Physical Properties
of Selected Un<=te« and Simulated Wastes
Settlement r>ue to Linear nnd Nonlinear Stress
Strain Properties ir. Kittv Litter ............. - '
Orrpnrt ion Kquipn-.ent nnd Methous ...............
Rnnkir.R of I'Sf^ Soil Tvpes bv Performance
of Tover Functions .....................
Soil Conservation Services Recommended
Bentonire Application Rate for F.-.nn Ponds ......... 55
Characteristics of Selected Hazardous Waste Landrllls .... 59
Phv^ual Properties of P.iper-Xi1! Sludee ........... 75
vii
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ACKNOVLFIM'.MFNTS
This study was authorized by the U.S. tnviroiunental Protection Agency
(hl'A), Hazardous Waste um-inmTing Research Laboratory, Cincinnati, Uhio, by
Interageney Agreement No. UW.119]u6St)-01-
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SECTION 1
INTRODUCTION
BACKCROUND
Section' 3004 of the Resource Conservation and Recovery Act (RCRA) of 1976
requires Che Administrator of Environmental Protection Agency (EPA) to estab-
lish standards applicable to owners and operators of hazardous waste treat-
ment, storage, and disposal (TSD) facilities. Among the standards are
requirements for "treatment, storage, or disposal of all such waste received
by the facility pursuant to such operating methods, techniques, and practices
as may be satisfactory to the Administrator." The Implementing regulations
for landfill cover? are found in 40 CFR 264.310, "Closure and postclosure
care," which states that the final cover must be designed and constructed to
(H provide long-term minimization of migration of fluids through the closed
landfill; (2) function with minimum maintenance; (3) promote drainage and
minimize erosion or abrasion of the cover; (4) accommodate settling and sub-
sidence so that the cover's Integrity is maintained; snd (5) have a permeahli-
lt v less than or equal to the permeability of any buttom liner system or
natural soils present.
Monitoring and maintenance, including necessary cover repairs, are also
required throughout the postclosure period. The postclosure period is des-
ignated in 40 CFR 264.117 as 30 years after completion of closure.
EPA recognizes the need to provide guidance In implementing the cover
requirements. This document addresses the fourth requirement listed above
regarding settlement and cover subsidence.
PfRPOSE
This report presents technical guidance directed at predicting, reducing,
and preventing landfill settlement and related cover damaged by subsidence.
The report is Intended to be used by regulatory personnel and by operators of
hazardous waste landfill?.
SCOPE
The Information in this report pertains to hazardous waste landfills
designed, constructed, and operated within the t'nited States under the RC7A
regulations. Landfills constructed and capped before the passage of RCRA in
1976 may not meet RCRA's relatively stringent waste placement, liquid wast,e
limitations, liner specifications, and leachate collection and control
requirements, and thus may not be amenahle to the analytical, construction,
and remedial guidance presented in this report.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
Hazardous waste landfills meeting RCRA requirements have physical charac-
teristics that influence their potential for settlement and subsidence.
Attention to those characteristics can minimize postclosure subsidence damage.
Data on physical properties of real and simulated hazardous waste are
available to assist the landfill operator or permitting agency in assessing
long- and short-term settlement potential.
Landfill subsidence results from primary consolidation and secondarv com-
pression of the waste mass, and from collapse of voids or cavities in the fill
and around c~ir.-.lners by corrosion, oxidation, combustion, or biochemical
decay of landfllled materials.
Rarely, a landfill may be a monoflll, that is it may contain uniform
layers of drummed wastes or uniformly placed bulk wastes. More often, the
landfill will consist of different types of wastes placed nonuniformly across
the landfill in layers separated by Intermediate covers of soil. The poten-
tial for differential settlement must be considered to be greater in landfills
with nonuniform wastes and waste placement procedures.
Bulk wastes behave differently from containerized (e.g., drummed) wastes
in settlement characteristics. Bulk wastes behave relatlvelv predictably,
much like soils, becoming increesingly consolidated with time, but at a
decreasing rate. Containerized wastes may remain relatively undeformed until
the containers degrade and collapse, at which time voids will be created, and
consolidation will begin.
Settlement by consolidation and secondary compression of bulk waste land-
fills in which drainage layers are provided will probably be essentially com-
plete before final closure. Compaction of waste materials and Installation of
drainage layers are recommended to lessen the potential for postclosure set-
tlement and cover subsidenca.
The approximate time required for primary consolidation to occur can be
estimated for a waste or soil layer if the liquid limit is known for the mate-
rial and if the shortest distance to a drainage path (e.g., a drain layer) is
known. Time, for any degree of consolidation, can be computed more precisely
if the compressibility or coefficient of consolidation has beer, determined for
the mat«r:'al.
Of the controlling factors, the distance to a drainage path has the most
pronounced effect on consolidation time for a waste layer. This fact
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Indicates the desirability of including frenuent drainage layers and of remov-
Ir? liquid from the landfill mas* so that most of the consolidation will occur
before closure.
The time required tor ultimate settlement of containerized (drummed)
waste to occur cannot he computed without knowledge of the drum deterioration
time. The time carrot be determined, although it is expected to be several
vears, perhaps several decades, if water infiltration is prevented by an
impervious can and liner system.
The void space around drurrs or other containers in a landfill can be a
major contributor to total postclosure settlement and should be filled with
solidifying agents or a free-flowing backfill to minimize the void component
of total settlement.
The surest way of avoiding problems associated with postclosure deterior-
ation of drums and the delayed settlement and cover subsidence associated with
it may be to ban drums from landfills. Instead, drums can be emptied and
crushed or reclaimed. Drum contents can be treated and disposed as bulk
waste.
Equations for calculating settlement time should be used more to identify
operational landfilling and waste treatment procedures that will minimize set-
tlement tine than to predict precise values from theory.
Differential settlement across relatively short distances that may occur
within subcells comprising a larger landfill cell is. more threatening than
relatively uniform settlement across longer distances that may occur across
large monofills. For the former, tenslonal stresses may be sufficient to
cause cracks in the cover resulting in leakage of water into the landfill.
Those tensional stresses may not develop over longer distances, but ponding of
water may occur on the cover barrier, weakening its ability to repel water.
Similarly, tensional stresses are anticipated to cause few or no problems
with flexible membrane barriers over large subsidence areas. Locally severe
differential subsidence can cause strain sufficient to rupture a flexible mem-
brane or otherwise cause its premature failure.
Two or more central column models for analyzing landfill deformation
(settlement) can be used to predict differential settlement between columns
and thereby to determine the effect of differential subsidence on the final
cover.
Expressions for analyzing the deflection of a beam can be used to iden-
tify parameters controlling the deformation of a landfill cover subjected to
differential settlement. Once identified, the parameters can be adjusted by
cover design and construction procedures to minimize distress to cover
components.
Differential settlement can be minimized by compacting wastes during .
placement, eliminating void space within the landfill, stabilizing liquids
before placement, and othsr considerations. The length of the cover (repre-
sented as a beam) subjected to subsidence can be raduced by placing wastes as
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uniformly as possible to provide uniform support to the cover. The cover poll
components can le rc.irle more resistant to distress by compacting the cover har-
rier soils wet ot optimum water content.
Final cover components will stretch under differential settlement and
must be constructed Co withstand tensile strain. The average tensil- strain
in the cover can be computed, and the maximum value of the differential set-
tlement that can he tolerated bv the cover soils can be estimated from that
computation.
Plastic soils (soils with high plasticity indexes) should be selected for
use as cover components to produce a cover resistant to tensile strain.
Laboratory investigations by others Indicate the flexible membrane lin-
ers (FML's) (components of the barrier layer in covers) may fail at lower
strains than would be expected from manufacturer's data. Every effort should
he made to reduce differential settlement potential of the landfill and to
design the cover to resist tensile strain.
I.andfilled wastes should be compacted or treated where possible to reduce
potential settlement. Compaction methods include standard compaction tech-
niques, vibratory rollers, and precompression (preloading and surcharging).
Waste treatment methods include addition of fixative agents to render the
wastes permanently less compressible.
The stabilization of liquid wastes with pozzolanic materials has been
shown to increase compressive strength "and lessen settlement potential. Such
stabilization could be especially beneficial for containerized wastes.
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SECTION 3
LANDFILL CHARACTERISTICS
Landfill settlement and subsidence can always be related to the physical
design characteristics of the landfill, the character of the emplaced wastes,
and hov the filling process was conducted. Careful attention to these factors
can minimize subsidence damage. Typical aspects of currert characteristics
and practices are outlined below.
CURRENT LANDFILL DESIGNS
Hazardous waste landfills that meet RCRA requirements have the following
characteristics:
- Pits (cells) are excavated in native soil or rock of low permeability
(aboveground facilities enclosed by soil embankments are less common).
- Single- or multicell construction is practiced, the cells isolated by
berns and the multicell groups Isolated by benns, liners, and cov.rs.
- Depths are commonly 15 to 50 feet but are as great as 100 feet.
- The base of the cell is usually above the water table or aquifer.
- Cells are lined with single or multiple natural or synthetic barriers
—7 -*8
with low permeability to water (10 to 10 cm/sec).
Cells are equipped with leachate collection and monitoring systems.
A final cover (cap) of more than one layer is installed; the cover
includes a synthetic and/or natural barrier layer.
Wastes are placed with some care in layers generally 3 feet thick or
less and covered with less than 2 feet of crushed rock or soil fill
(intermediate cover). Waste and intermediate covers are alternated as
the cell is filled.
Compaction of liners and caps is usually controlled and monitored;
compaction of waste and fill Is limited and is that obtained by pas-
sage of tracked and wheeled waste placement vehicles.
Final cover caps on closed cells are grassed and may be equipped wlfh
settlement plates for subsidence monitoring.
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Operators are required to solidify all liquids enclosed within the
cell (no free liquids are permitted).
Several types of landfills are commonly found in the United States.
Characteristics of landfills for which descriptive information has been'
obtained are tabulated in Appendix A. One of the most common is excavated
with the vaste fill almost entirely below the original ground surface and only
the cover above ground. Another type is built essentially above ground with
the waste enclosed within embankments or dikes. These two types may be com-
bined to maximize the waste volume in a limited area. In hilly terrain and
more commonly in the western United States, cut-and-fill landfills may be con-
structed by partial excavation of natural valleys and gullies with the con-
struction of an embankment or retaining wall at the lower end. Tn the past,
it was common to take advantage of abandoned quarries or gravel pits, where a
lar^e excavation had been created for other reasons. Unfortunately, many
quarry or pit types became uncontrolled dump sites.
Hazardous waste landfills vary greatly in areal size. Those observed by
the authors range from 1 to 37 acres for a single landfill under one cover. A
single facility may contain several landfills under separate covers, collec-
tively enclosing hundreds of acres. Landfill depths are commonly less than
50 feet (fill and liner thickness) but are as great as 100 feet. Associated
landfill volumes of the largest fills are as much as 1,250 acre-feet or more
than 2 million cubic yards of waste and soil fill. Landfill size is an impor-
tant parameter in developing models for analysis of settlement and subsidence.
All RCRA-permitted landfills have been required to be lined with natural
or synthetic materials capable of preventing contact of vaste and leachate
with the ground water. The "minimum technology requirements" of the Hazardous
and Solid Waste Amendments of 1984 require that new landfills be double-lined
with a leachate collection layer between the liners. Draft guidance from
EPA's Office of Solid Waste has suggested a membrane liner as the top part and
a membrane on a clay layer as the bottom part of the double liner.
Further minimum technology requirements dictate that the landfill cover
(cap) be no more permeable than the bottom liner. EPA has interpreted this to
mean that the cover must include at least both a membrane and a clay component
as the barrier layer.
Existing Itners and covers vary substantially from the new requirements
and from site to site. Liners may vary from none (relying on the impermeabil-
ity of the cut soil) to elaborate and thick clay layer and membrane combina-
tions. Covers on recent landfills also vary but are commonly a combination of
layers including both a membrane and clay. A classification of geomembranes
is presented in Appendix B. Figure 1 illustrates the variety of existing
cover configurations, and Figure 2 illustrates a cover that will meet the cur-
rent RC&A regulations.
As-bull final cover surface slopes vary from 1 to 30 percent but are •
commonly 2, 5, or 8 percent. Draft guidance from EPA's Office of Solid Wastes
recommends from 3 to 5 percent.
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TOTAL
THICKNESS
TOTAL
THICKNESS
TOTAL
THICKNESS
8FT
FT
4FT
FML
.„
/2 FT
"! GEOTEXTILE
4FT
3-1/2 FT
4FT
FML
6FT
31/2 FT
4 FT
12-U FT
SOIL/BENTONITE
N0101V£N
I Ml
•.j»j FML
GEOTEXTILE FML ^_^_^_^_^^^__
1FT '/S////77/7*
I"*" - • '•• 1
* ' 1
LEGEND
TOP SOI LOR
SOIL BUFFER
DRAINAGE
BLANKET
GRAVEL
W////////.
FML
4-1/2 FT
COMPACTED
CLAY
FML
Figure 1. Final covers of actual RCRA landfills.
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VEGETA TED TOPSOIL. 2 FT MIN
— FIL T£a CLOTH OR FIL TER
DRAINAGE LAYER. J FT MIN
I— SYNTHETIC MEMBRANE IFMLi. 20 MIL
CLA Y SOIL. 2 FT
SAND BUFFER. 6 IN. MIN
SECTION THROUGH LANDFILL
Figure 2. EPA-reconnaended RCRA landfill cover.
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Settlement and cover subsidence analysis must consider the effects of
settlement on the clav and membrane barriers of the cover. Th« clay portion
Is potentially subject to tensile cracking, thinning, and ponding. The mem-
brane portion is subject to stretching and ponding. The final cover surface
slope Is subject to being decreased by cover subsidence. Anv of these changes
increases the possibility of cover leakage and infiltration of water Co the
waste below.
All landfills meeting RCRA requirements incorporate a leachate collection
system Into the base of the landfill. Systems vary considerably but commonly
consist of plastic perforated pipe imbedded in a granular drainage blanket or
in drainage trenches which slope toward a sump for monitoring and removal of
leachate via a riser through the fill and cover. The riser most often used
appears to be 4-foot sections of concrete sewer pipe added to each lift. Geo-
textiles are emplaced over the collection pipe of some systems to filter out
fines and prevent clogging of the collection lines. In a few landfills accom-
modations are made to drain the upper portion of the fill by installation of
additional drainage arrays within the fill as the landfilling progresses.
Commonlv. however, the leachate collection array is emplaced only along the
base of the landfill.
Good drainage of leachate is desirable for several reasons, including
lessening the potential for long-range settlement by allowing more rapid pr- -
closure consolidation and by decreasing pore water pressure.
CHARACTERISTICS OF LANDFILLED WASTES
Commonly, hazardous wrste landfills accept a variety of wastes from sev-
eral types of industries. Soire of the more frequently occurring and abundant
wastes include paint waste; eleclroplatlng waste; wastewater treatment
sludges: baghouse (collector) dust; fly ash; intact, damaged, and crushed
steel drums; waste oil and oil-contaminated soil; electric arc furnace dust;
filter cake from various dewatering operations; and steel mill pickling
liquor.
Hazardous waste landfills may also contain, generally in lesser quanti-
ties, more noxious organic chemical wastes such as polychlorlnated
biphenyls (PCB's) and pesticide waste.
Waste Texture
The liquid content of wastes is extremely important in evaluating settle-
ment potential,, for it is often deliqueficatlon or the squeezing out of liquid
that accounts for a great part of consolidation.
Hazardous wastes since about 1980 have been treated with solidification
(absorption) agents or other materials before being landfilled. Older land-
fiUs may contain wastes with much higher liquid volumes, some or most of
which may have been in drums. When released, the drainage of the liquids may
initiate significant subsidence. Even recent landfills contain liquids from
precipitation and nin-on that occur during filling.
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The recent general ban on free liquids in landfills has been interpreted
and enforced in different ways by the regulating agencies. Some stat«s use
the EPA-recommended paint filter test to determine whether a waste is a liq-
uid. Several other less sophisticated methods are in use, such as rappinz a
drum and interpreting the sound, or measuring the "free liquid" over the
solids in a drum. From a leachate standpoint, the determination by these
methods of whether a waste is a liquid may have merit, but from the srandpoint
of landfill settlement analysis, it may be more beneficial to evaluate consis-
tency on the basis of compressibility.
Landfilled bulk wastes usually resemble soils in that they can in most
cases support the heavy vehicles used to place them within the landfill. Some
treated wastes have pozzolanic qualities and "set up" to relatively strong
materials of low compressibility. Some solid landfilled materials such as
wood and metal products, including steel drum containers, have initially high
strengths and compressibilities but are presumed to degenerate and corrode to
conditions of lew strength and high compressibility with time vithin the land-
fill. Prediction of settlement in landfills must consider the delayed com-
pressibility potential of the landfilled materials as well as the short-term
potential. The delayed potential may, in fact, be much more significant, as
will be seen later.
Waste Densities
Density Is an important property of wastes in evaluating the settlement
and subsidence potential. In general, greater density means less void space
and *nus less settlement potential. Densifying the waste Is one way of reduc-
ing that potential.
Table 1 lists densities for some landfilled bulk wastes and wastes in
drums, presumed to be as delivered and measured at the landfill before place-
ment and compaction.
Engineering Properties of Selected Wastes
Reported properties important to settlement analyses include natural
(as-delivered) or optimum (laboratory-determined) water content, unit weight,
unccnfined compressive strength, elastic modulus, shear strength (trlaxial
compression) data, ami compressibility and consolidation data. Table 2 pre-
sents selected data for several wastes and simulated wastes. A full report
presenting the results of these tests and others is in preparation by the
authors. Materials 1 through 12 of Table 2 were laboratory-tested before and
after being enclosed in large lysimeters for a number of years and are desig-
nated "prelysimeter" and "postlyslmeter," accordingly. The three industrial
wastes used in the lysimeter tests were an electroplating waste sludge, a
chlorine production brine sludge, and a glass etching sludge. Both raw
(untreated) and stabilized (treated by mixing with portland cement and fly
ash) samples of the wastes were tested.
Materials 13 through 17 are mixtures simulating wastes and consist of '
mixtures of fly ash and water, fly ash and oil, "kitty litter" and oil, and
"kitty litter" and water. Material 18 is an electroplating waste sludge
treated with a pozzolanic (portland cement-like) fixation agent. Data for
10
-------
T*BLE 1. REPORT^ u*cyTf nirvSITIES FOR LANDFI1.LEP HAZARDOUS WASTES
Waste
Density
Wastewater treatment plant sludge press cake
(bull:, 762 nonvolatile ash, 24Z volatiles)
Vastewater treatment sludge (hard, dry cake)
Lime sludge (55Z total solids)
Metal hydroxide sludge
Electric arc furnace dust (dry)
Electric arc furnace dust (dry powder)
(pellets)
Enamel powder (ury)
Fly ash
Cement manufacture kiln dust
Cement manufacture baghouse dust
Wastes In Drums
Lab pack (inorganic oxidizers in jars, cans)
Lab pack (oxides, salts in containers)
Lab pack (salts, alkalines, solids, and pastes
in jars, cans)
Lab pack (organic solids, solids, and pastes
in jars, cans)
Lab pack (organic acids, solids, and pastes
in Jars, cans)
Sludge
Lab Pack (organlcs in glass bottles)
Sludge
Lab pack (mixed wastes)
Hydroxide waste
Paint sludge (dry cake)
Cured polyester resin still bottoms (moist gel)
Waste placing sludge (mud filter case)
85 Ib/cu ft
37
86 (wet unit weight)
44 (dry unit weight)
69
62
70
100
74
63
80*
40*
250 Ib/drum
300 Ib/drum
SCO Ib/drum
500 Ib/drum
350 Ib/drum
450 Ib/drum
400 lb/drvm
300 Ib/drum
300 Ib/drum
425 Ib/drum
500 Ib/drum
600 Ib/drum
545 Ib/drum
* Boynton, Robert S., 1980. Chemistry and Technology of Lime and Limestone,
John Wiley and Sons, Inc., NY, p 305.
11
-------
' '"• MH» .*
«Ht* tttlffr * %OI *ll
f!*••«:«•t«r
. J «(c#r 1 <
I It cttcr *.'
«tt*r ,t *«••
. i. ra >I^M •* •^•ff-
:<•»«•»' - > O.M O.M
12
-------
material 18 were reported by Webster . Materials 19 through 24 were two
limestone scrubber flue gas desulfur^zaC1011 (FGD> sludges treated with differ-
ent proportions of sludge solids, fly Jsh and/or port land cenent, and water
and tested for unconfined comprespive strengths (UCS) at optimum water con-
tents. Data for materials 19 through 21 were for one FGD sludge and 22
through 24 for another. Compression tests were performed on remolded (com-
pacted) samples. Table 2 also shows values for UCS after varying setup times
after mixing.
The values presented in Table 2 are not a comprehensive collection of
war.te property data but do represent some common and abundant landfilled waste
materials. As such, the values can supply approximate unit weight, strength,
and compressibility data for estimating initial and long-tern settlement of
landfills. Unconfined compressive strength values for treated sludges also
show the tendency for wastes mixed with pozzolanic agents to gain strength
rapidly with time.
Waste Placement Characteristics
In most hazardous waste landfills, the wastes are placed in rather stan-
dard configurations. Unless the landfill is a monofill (containing only one
general type of waste), it will be divided into cells for wastes of different
chemical, character. This segregation is usually to prevent the possibility of
•Detrimental chemical reactions among the materials. The cells will ordinarily
be separated by clay berms that are maintained as the landfill progresses
upward.
The area and depth of the monofill or the cells are important in their
influence on potential cover damage from subsidence. Shorter horizontal
dimensions and deeper depths tend to accentuate tensile stresses in the cover.
Cellular subdivision of the landfill acts similarly in shortening horizontal
dimensions. But this subdivision will also usually segregate the waste into
masses of different physical characteristics, separated by berms with still
another set of physical characteristics. All of these differences will tend
to accentuate differential settlement and cover subsidence.
Wastes are generally placed in the landfill in "lifts" that are simply
layers of wastes. It is probably rare that a lift is totally uniform in its
physical characteristics across a landfill or a cell. It is probably unreal-
istic to require uniformity, although that would be ideal for the evaluation
and prediction of settlement.
A lift of bulk waste will generally be comprised of many loads of mate-
rial dumped and spread across the cell. Spreading is most likely to be done
by relatively heavy equipment which simultaneously compacts the waste. The
lift of bulk material may be a foot or nore thick. In some landfills, bulk
waste and containerized (most often in steel drums) waste will be found in the
same cell, and sometimes in the same lift. Usually drummed wastes will be
grouped together, but the horizontal locations of the drum groups may change
from one lift to the next. However, in some landfills, containerized waste
may comprise an entire cell or even the entire landfill. On the other hand,
some operators disallow drummed waste altogether and, if it is received, the
drums will be emptied and crushed and landfilled separately.
13
-------
Intercell and intracell configuration of bulk and containerized waste Is
one of the most important considerations in evaluating the potential for cover
subsidence. In general, bulk waste Is the easiest to manage to eliminate a
danger of Its contributing to postclosure subsidence. This is not to say that
It Is impossible to contro' he potential danger from postclosurr settlement
of containerized waste, but it will be much more difficult.
Waste lifts are often separated by soil layers, especially where lifts
are containerized waste. Tn this case, the primary purpose of the soil layer
Is to provide a working surface for the next lift. A conscious effort may or
may not be f;iven to filling the void space between containers. Bulk wastes
may not be separated by soil layers. It does not appear that a great deal of
attention is given to the properties of soils that may be used, even though
such attention could have a dramatic effect on the amount and rate of ensuing
settlement. For example, free-draining soil layers in bulk wastes can accel-
erate settlement during the preclosure period before the cover is placed.
Pozzolanic solidification of drummed waste and the placement of pozzolanic
material between drums might eliminate the danger of postclosure settlement
and damaging cover subsidence by permanently increasing the compressive
strength of those waste layers.
Most hazardous waste landfills are equipped with vertical riser pipes, as
noted earlier, extending completely through the waste mass and cover. These
riser pipes help to drain run-on and leachate from the waste mass, thus accel-
erating settlement to some extent during and after the filling process. The
riser pipes also help ro vent gases that may be generated in the waste,
although several vent«: specifically for gas venting are features of some
covers.
Preloading of the waste mass with a temporary soil cover for a period of
time before the installation of the final cover has been suggested and occa-
sionally used as a means of promoting settlement prior to final closure.
-------
SECTION 4
ANALYSIS OF POTENTIAL SETTLEMENT AND SUBSIDENCE
SETTLKMFNT-CAt'SING MECHANISMS
Several mechanisms have been "recognized to cause subsidence at sanitary
and low-level nuclear waste landfills. These include primary consolidation
and secondary compression, raveling or piping of fill soils or debris into
voids or cavities, and enlarging and subsequent collapse of voids or cavities
in waste fill bv corrosion, oxidation, combustion, or biochemical decay. All
of these mechanisms are not perMnent to hazardous waste landfills constructed
according to current RCRA requirements. Those considered important in hazard-
ous waste landfills, and discussed herein, Include primary consolidation,
secondary compression, collapse of voids created by waste container deteriora-
tion, and decav of waste debris. Primarv consolidation and secondary compres-
sion are the dominant mechanisms of settlement in soil-like bulk wastes.
The settlement mechanisms cause changes in the waste volume which in turn
cause stresses and strains In the overlying cover that may result in surface
subsidence.
Framework of the Analysis and Evaluation of Assumptions
Real world situations involving in situ stress, strain, deformation,
material properties, and time dependent factors which Influence these quanti-
ties can never .be completely known or modeled precisely. Additionally, there
is an element of uncertainty In the Reometrv of structures such as hazardous
waste landfills. Therefore, in the development of a model to predict behavior
in such structures, certain simplifying assumptions must be made. The assump-
tion will typically be those made in the development of rhe theory of consoli-
dation, and it may be important to state these assumptions because the
conditions within a hazardous waste landfill may be worse than those within a
compacted earthen embankment. Additionally, it should be stated that predic-
tions made on the behavior of well controlled compacted earthen embankments
using consolidation theory can be in considerable error simply because real
world sltuatipns rarely conform to Idealized theory.
The assumptions made for this analysis are discussed below.
- The material under analvsls Is homogeneous. Homogeneity is never
fully realized In the very heterogeneous mass of a hazardous waste
landfill. The mass is spatially heterogeneous and violates the
assumption of homogeneity.
15
-------
- The material under consideration is saturated with liquid. Saturation
tn hazardous waste landfills Is seldom complete, but complete satura-
tion influences the rate of settlement and subsidence. Settlement
occurs more rapidlv in an unsaturated fill, so time predictions made
with the assumption of complete saturation are conservative.
- One-dimensional compression does occur within a large portion of a
landfill which Is large in areal extent compared with the depth. How-
ever, one dimensional compression does not occur in rones in which
there are appreciable shear stresses, such as areas in close proximity
to physical boundaries.
- The mass is isotropic. The materials involved are generally soil-like
materials which are not isotropic; that is the properties of the mate-
rials may vary with direction. Applied compaction may increase the
anisotropy of the materials. However, laboratory tests performed on
representative materials should be performed on material treated in
such a way as to duplicate, as closely as possible, the placement and
hence the anisotropy of the material in the landfill.
- Darcy's law is valid, and one-dimensional flow occurs in the landfill.
Both f these conditions are in general violated because of inhoreoge-
neity and anisotropy of the materials in question.
- The material is linearly elastic. The materials involved are soils
which are not linearly elastic. However, effort Is made to develop a
treatment which accounts for the nonlinear behavior of the materials
in question.
- The action of an infinitesimal mass is no different than that of the
larger representative mass. This assumption relates to the fact that
a representative small specimen of material may be tested to determine
properties which may be used to predict the behavior of rhe mass.
Realistically, the accurate representation of the mass by a small
specimen is unlikely because of the heterogeneous nature of a hazard-
ous waste landfill.
The serious violation of many of the stated fundamental assumptions is
fully recognized. Similar violations of fundamental assumptions are recog-
nized for beam models (presented later in this section) because beam theory is
based on the assumption of small strains, and there Is no insur.-.nce that
strains will remain small in hazardous waste landfills. Evidence will be pre-
sented to show that strains may become large. However, it must be realized
that in general these models will not be used to quantify the various factors
associated with distress in the structures under analysis. Instead the models
will be used to identify parameters associated with distress, how these param-
eters relate to each other, and how they may be manipulated to minimize the
effects of distress. The models will be used for qualitative rather than
quantitative analysis. In this light, the assumptions necessary for the •
development of the models become less disturbing.
16
-------
Primary Consoliddtion
CorifolidatIon of a soil (or w;iste) is Che decrease in void ratio (the
ratio of the- volume of voids to the volume of solids) bv expulsion of fluids
from the voids under excess hydrostatic pore pressure (primary consolidation)
and by deformation of the skeleton of the mass and compression of gases in the
voids (secondary compress ion). The decrease in void ratio by consolidation
represents a decrease in volume of the mass and can cause the surface of the
mass to subside.
The classic Terzaghi theorv for one-dimensional consolidation of a soil
assumes that the soil is saturated and that deformation of the soil mass is by
change in volume caused bv expulsion of water from the consolidation.
If a mass of soil of thickness H , diagrammed in Figure 3, is com-
pressed, the change in its thickness, AH , can be expressed as a change in
the void ratio, Ae . An estimate of settlement expected to occur in a soil
bv consolidation can be obtained by combining field data with laboratory data
on soil compressibility in the equation
where
AH » amount of settlement
C - laboratory-determined coefficient of compressibility
e£ * initial void ratio
p » initial overburden or self-weight stress in the field
ip * increase in strers by the added load
Equation 1 might be used to compute the subsidence in a hazardous waste
landfill. However, Equation 1 is developed from the t' -;ory of consolidation
md therefore suffers the limitations resulting from the assumptions made in
rh2 development of the theorv. These assumptions and ' .-.e associated limita-
tions are listed and discussed separately in Section 3. A procedure to com-
pute settlement based on the integration of measured stress-strain properties
circumvents some of the assumption of the consolidation theory.
Consolidation of soils by lowering of the water table has been identified
as a possible cause of ground subsidence in some locations. The effect of
lowering the water table in a soil is to surcharge the soil by increasing the
effective stress (the vertical stress minus the pore water pressure) through a
decrease in pore pressure. Similar effects can be expected in soil and waste
materials in a hazardous waste landfill where the extraction of landfilled
fluids through the leachate collection system would result in compression of
the mass.
Secondary Compression ,
Settlement from secondary compression (deformation of the soil mass)
occurs later in the loading history of a fill as the applied stress is
17
-------
V
1
K
aT
«2
Ae
T
VOIDS
SOLIDS
A/y
T
H
Figure 3. Thickness and volume relationship for a.
soil mass of thickness H and volume V .
18
-------
transferred from the pore fluids to the soil skeleton. Secondary compression
raa>' be calcill;1t*d from tht? following equation:
"sec \
W
where C » coefficient of secondary compression from lab
'sec * tlm« for which settlement is significant
t . » time to completion of 100 percent primary consolidation
The total settlement in bulk waste is the sum of the primary consoHd. -
tion and the secondarv compression settlements. It is likely that most bulk
wastes initially contain a significant Amount of liquid. If that is the case,
primary consolidation will .be a greater contributor to total settlement than
will secondary compression in bulk vnste landfills.
Appendix C provides an example of the calculation of total settlement for
a landfill.
Container and Fill Deterioration and Cavity Collapse
The dominant settlement mechanism for heterogeneous landfills containing
mixtures of debris, bulk, and containerized wastes is not expected to be con-
solidation. Instead, long-term settlement of heterogeneous hazardous waste
landfills should be analyzed on the basis of deformation of the waste layers
and .Deteriorating waste containers.
Most of this type of settlement is likely to take place after, perhaps
long after, closure of the landfill. THUS, settlement caused by the collapse
of containerized waste may have more potential for subsidence damage to the
cover than consolidation settlement, much of which can occur, or can be made
to occur, prior to closure. However, it must be emphasized that there is no
documentation of subsidence-related problems in controlled (RCRA-regulated)
landfills, probably because none are old enough for deterioration to have
occurred.
Settlement should result from later filling of larger structural voids
witnin the la: dfill that remain through the filling process or are created by
waste degradation. These voids are expected to survive the primary consolida-
tion and secondary compression because they are supported by initially very
stiff materials. Drummed wastes are the most significant case in point.
Initial structural voids consist of unfilled landfill space. Incomplete
filling of containers and the space between them is probably the most preva-
lent example of how such voids are created. Random space in large debris and
space created by decay of organic materials are other exairp es.
The maximum amount of potential settlement should approximate the volume
of the larger voids. A small additional amount should result from the consol-
idation of wastes after they are released from rigid containers.
19
-------
It should not uc construed that the potential settlement resulting from
the filling of larger voids will necessarily be significant. Careful place-
ment of containers and debris-type materials with attention to filling voids
with lift (intermediate) cover material will keep cavity size small. Sinkhole
development by piping should not occur because liner systems preclude the
development of escape paths or pipes, and leachate removal systems prevent
excessive heads and gradients that might trigger cavity collapse or growth.
PREDICTING LANDFILL SETTLEMENT
A layer or zone of waste or fill soil within a hazardous waste landfill
possesses engineering properties that control its deformation (strain) under
the load (stresses) imposed on it by materials above and around it In a con-
tinuum mechanics model of the landfill. Variable properties including stiff-
ness (Young's modulus), unit weight of materials, and Poisson s ratio (ratio
of transverse normal strain to the longitudinal strain in a sample compressed
longitudinally) reduce waste layers or zones to units that can be mathemati-
callv analyzed (if the landfill satisfies the requirements of the mathematical
model). Thus the .mount of settlement to expect in initial and degraded waste
fill conditions may be estimated. Values of the variables can be changed to
reflect changing conditions of stress and material properties in the landfill
with corresponding charges in the deformation or settlement. Material proper-
ties such as unit weight, modulus, and Poisson's ratio can b« determined in
the laboratory for actual waste materials and container* or can be estimated
from tests on simulated waste materials- and standard containers.
Mathematical models constructed to aid analysis of deformation of land-
fills should recreate the stress conditions and loading history of the fill.
For example, because wastes and fill are placed in th« landfill gradually over
a period of months or years, and the fill depth increases gradually, deeper
fill materials are compressed at different rates and under increasing loads as
the filling progresses. A model should be used that sinulates the process,
building up the total structure by stacking one layer at a time on top of the
preceding layer ->nd allowing vertical stress and lateral confinement to
increase in a systematic manner as the layers are placed. Deformation after
closure is controlled by changing strengths and stiffnesses of the waste mate-
rials as they degrade and deteriorate, with relatively constant vertical
stresses. This later or postclosur* settlement can be analyzed based on sud-
den loading or "gravity release" loading whereby the load to the entire land-
fill is applied all at once. Such a loading condition would apply after
closure (cessation of filling and application of final cap to the fill), after
the landfill has undergone initial settlement. Deformation of the post-
closure landfill then depends on decreasing elastic moduli of the deteriorat-
ing fill contents. Earlier Investigations of settlement in hazardous waste
landfills used these approaches to predict settlement.
Settlement in Bulk Waste Landfills
In bulk waste disposal, liquid and solid wastes are deposited in the
landfm and stabilized if necessary, then compacted into the landfill using.
some pr&ctlcal, effective, economic compactiv* effort. Liquid content and
compaction effort applied to the waste will determine the amount of settlement
which will occur, and there may be a certain economic pressure on the landfill
20
-------
operator to maximize liquid waste content and minimize compaction effort.
Such an approach m?v lead to postclosure settlement problems if taken to
extreme. Central column analysis mav be used to estimate postclosure set-
tlement based on assumed in situ stress and strain conditions and the abilifv
to select waste samples from which stress-strain properties representative of
the mass may be measured. In using the central column model for estimating
settlement, stress-strain data from one-dimensional compression tests are
required. In using this approach for the analysis of drum disposal, it was
convenient and conservative to assume stress-strain linearity. Such linearity
may also be assumed for bulk waste disposal analysis, bur a more precise
method based on actual stress-strain data will be presented..
Assume that stress-strain data from a one-dimensional compression test
can be presented in the functional form
e - ((a) O)
where
e « vertical strain
a * vertical stress
The typical shape of such stress-strain data is seen in Figure 4. The "soil"
in question is again "kitty litter," a material often used to stabilized haz-
ardous waste.
Assume further that the stress-strain curve may be leas'-squares fitted
to he represented by a polynomial of degree four. (Note: Many computer codes
exist which will curve fit polynomials.) From the least squares polynomial
fit, the stress-strain data may be written as
E - aQ + ajO + a2a2 + a-jj + a^a (4)
where an, a., a,, a., a, are the coefficients of the curve fit. The instan-
taneous change in stiffness may be obtained by differentiating Equation 4 and
is
Jj£ - a. + 2a,a + 3a.CT2 + 4a,o3 (5)
do 1 I -1 *
Substituting a - YY into Equation 5
where
Y - material density, assumed initially constant
y - vertical distance below the surface
results in
ry f\
•
Substituting Equation 6 into
21
-------
60 r
^1 MOIST "866PCF
8 12-16
P6RCENT 3TRAIN
24
Figure A. Constrained elastic modulus of waste simulated by
"kitty litter."
22
-------
C de
J Yyd^
AL - / YV -j- dv (7)
and Integrating, the result
(8)
is obtained where
AL •» subsidence in a central column with r.onlinear stress-strain
properties
L - depth of landfill
To compare the results obtained from the linear modulus (Equation 3) versus
the nonlinear modulus (Equation 8) Table 3 was prepared showing predicted sub-
sidence in a bulk landfill having the stress-strain characteristics of kitty
litter.
Stress-strain data for kitty litter are shown in Figure 5. Table 3 shows
how the two models predict different values of settlement for different land-
fill depths and material densities. The table shows that settlement predicted
by the nonlinear model is always less than that predicted by the linear model.
The nonlinear model predicts less settlement because the stress-strain stiff-
ness modulus of soil increases as soil is deformed in confined compression.
Because of the shape of this curve, the secant stiffness modulus value is
always less than the average tangent stiffness modulus of the nonlinear curve,
and therefore the subsidence predicted by the nonlinear model will be less
than that predicted by the linear model. However for shallow depths of land-
fills (represented by the initially flat part of the curve) the linear and
nonlinear models will predict essentially the same value of subsidence. As
the landfill becomes deeper and the stress-strain modulus increases, subsi-
dence predicted by the more precise mode] will diverge, as shown in Table 3.
Figure (5 also shows actual data and the data which would be predicted by the
polynomial and demonstrate that there can be good agreement between actual and
fitted stress-strain data.
Assumptions made in developing this model are that the density at all
points along the column element was initially homogeneous, the stress-strain
properties used are representative of the entire column, and the column was
suddenly "released to gravity" from a weightless state. The last assumption
will never be physically approached except in the case of a column in a satu-
rated landfill with very low permeability which was filled rapidly. As was
mentioned above, subsidence begins to occur as soon as the first layer of
material is deposite ' in a landfill. This nonlinear central column model will
predict the tota1 amount of subsidence which will occur in columns of the
waste, in short, an upper bound of subsidence. If this upper bound of subsi-
dence can be tole ated, then the amount of subsidence which is likelv to occur
23
-------
TABLE 3. SETTLEMENT DUE TO LINEAR AND NON-LINEAR STRESS STRAIN
PROPERTIES IN KITTY LITTER
0.0168557
a* - -0.0005803
_ ^ _ i nnft i i P_
a« " i »uvwi, i i-—^
a? . .6.47878 E-8
4
r
pcf
84
86
88
90
92
84
86
88
90
92
84
86
88
90
92
L
ft
30
3C
30
30
30
50
50
50
50
50
70
70
70
70
70
0
max
psi
17.5
17.9
18.3
18.8
19.2
29.2
29.9
30.6
31.3
31.9
40.8
41.8
42.8
43.8
44.7
Ed - v)
DSl
109
109
109
109
109
150
150
150
150
150
191
191
191
191
191
Linear
£L
ft
2.40
2.46
2.52
2.58
2.63
4.86
4.97
5.09
5.21
5.32
7.48
7.66
7.83
8.02
8.19
Nonlinear
AL
ft
1.93
1.94
1.95
1.95
J.96
3.27
3.26
3.26
3.25
3.25
4.60
4.61
4.63
4.65
£.67
24
-------
50 -
40
LEGEND
• OBSERVED DATA
O DATA PREDICTED BY CURVE FIT
0.25
Figure 5. Stress-strain characteristics of kitty litter.
25
-------
will be less severe since some of the subsidence invariably occurs during
filling/construction.
It must be mentioned that the curve fit of stress-strain properties imjst
be carried out with caution since higher order polynomial curves will oscil-
late between data points. A high coefficient of correlation may be indicated,
and the polynomial may predict points on the curve with a high degree of
accuracy. However between points the polynomial nay oscillate in an undesir-
able manner as is shown in Figure 6. If *nch a polynomial wera used to pre-
dict subsidence, incorrect and meaningless results would be obtained.
Oscillation occurs on this stress-strain plot because a few widely spaced
points are being fitted with a high degreo polynomial. This problem will be
avoided if enough closely spaced points on the stress-strain curve are used
such that there is no room between points for oscillation. Finally, it may be
a good idea to plot the polynomial fit against the actual data to ensure that
no undesirable oscillation is occurring and the desired stress-strain data are
accurately fit.
Time is not addressed in this nonlinear model. The amount of settlement
predicted is the maximum amount which may occur in an unspecified time inter-
val. If the steps outlined below are taken to minimize the time for consoli-
dation and the wastes are properly treated and compacted so as to minimize
settlement, then the element of tire may be eliminated as a point of consider-
ation. Operating such that time for primary consolidation is minimized may be
the only effective means of dealing with time since time effects are poorly
understood and therefore very difficult to model.
Settlement in Containerized Waste Landfills
It is the settlement occurring after closure that causes surface subsi-
dence and possible cover (cap) damage. Although, as indicated previously, the
landfill can and should be constructed so that most of the settlement will
occur before closure, it is inevitable that some will occur later.
Pcstclosure settlement is likely to be .dominated by compression resulting
from the closure of structural voids. Only a minor amount will result from
the continuation of primary consolidation and secondary compression of bulk
wastes. A relatively small amount of postclosure settlement may also occur
due to the primary consolidation of wastes released from deteriorated, but
formerly rigid, containers.
Structural voids, as noted earlier, are likely to result from the close
placement of containers, usually drums, and the inability to completely fill
both the drums and the space between them. Some, probably lesser, void space
may result from degradation of organic materials and from the unfilled space
characteristic of coarse debris waste. The amount of settlement to be
expected from closing of structural voids will approximate the total of the
structural void space.
It was shown previously (Equation 1) that the void space around drums may
be as much as 10.73 percent by volume for drums disposed by burial on their
sides and 9.31 percent by volume for drums disposed by on-*nd (upright)
burial. Void space inside drums is difficult to quantify, but current
25
-------
50 i-
_ 30
v)
a.
10
LEGEND
A OBSERVED DATA
• POLYNOMIAL FIT PREDICTION
0.4
STRAIN
0.5
0.6
0.7 0.8
Figure 6. Stress-strain data fit by oscillating polynomial.
27
-------
regulatory practice limits it to !0 percent. Assuring that this void space is
filled completely with a solidifying agent is an obvious way to reduce event-
ual settlement. Free-flowing backfill such as dry sand or gravel will be the
most effective material to fill the void spaces under and between drums to
minimize the void component of settlement.
Subsidence caused by the change in stiffness of the waste material inside
the drum, after drum collapse, is difficult to quantify accurately. The
expression developed from Equation 1 was
,L , ^ l_^v I_l2v (9)
where
AL - the subsidence due to the change in stiffness between the
barrel and waste
Y • density of the waste material
L » thickness of the combined waste layers
(l+v/l-v)(l-2v/E) ~ reciprocal of the slope of the constrained modulus from
one-dimensional compression of the material in question
The subsidence predicted by Equation 9 will be conservative (more than
actually occurs) because it assumes linearity of the constrained modulus.
Actually the stress-strain curve is nonlinear, with the rate of strain
increase diminishing as stress increases (see Figure 5).
nrums are usually placed in layers, one to three drums thick, with an
intermediate cover of soil separating the layers. The intermediate cover lay-
ers are generally well-compacted during construction and do not pose a long-
term consolidation problem. However, with tine, the mild steel of which most
waste drums are made will corrode and may weaken to the point of total col-
lapse, subjecting the contents of the drum to compression and volume change
which will cause subsidence in the landfill. In this light, the use of drums
may create the problem of prolonging the time over which subsidence occurs.
It is not possible to predict the time of drum collapse. Maintaining the
integrity of the landfill cap, liner, and leachate collection system will tend
to keep the drums dry and extend their lifetime. However, the contained mate-
rial may be more or less corrosive in themselves. In addition, there is no
reason to expect that containers will all degrade uniformly. It would seem
more likely that they would degrade, each on its own schedule, over an
extended period. The beginning of deterioration might begin with the first
drum perhaps a decade after closure, while the last might occur a century or
more later. The surest way of avoiding problems with drums is to ban drums
from landfills, or to ban intact drums. Drums of waste can and have been
emptied of their content, crushed, and then placed in the landfill. The drum
contents are fixed or treated and then applied to the landfill where they are
less of a problem. Drums can also be emptied and recycled (reclaimed). Drum
recycling center or services are available in some states.
Intentionally increasing the compressive strength of the contained mate-
rials and the fill materials between the drums scy prevent compression and
28
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subsidence from this cause even if the drums fall. Mixing the waste and fill-
ing void spaces with pozzolanic materials such as lime and fly ash could pro-
vide Che needed strength.
Analysis of Settlement Time
An analysis of time is necessary to estimate the portion of total settle-
ment that occurs prior to closure. Any preclosure settlement reduces the
amount that can occur after closure and is therefore beneficial in preventing
cover subsidence. In addition, preclosure settlement benefits the operator by
allowing more space for disposal of additional wastes. As indicated earlier,
preclosure settlement is likely to be limited to consolidation of the bulk
waste and soil portions of the fill material. Preclosure settlement goes
largely unobserved and unmeasured, and thus it is difficult to quantify.
Consolidation time can be estimated. If it is less than the time
required for waste placement, consolidation of the bulk wastes and intermedi-
ate soil layers can be assumed to have occurred prior Co capping and will not
contribute to subsidence. An expression to estimate the time required for
90 percent consolidation was derived from the theory of consolidation and is
as follows:
t^-H^x 10(0.0168LL-2.2) (IQ)
where
C • Cime in days for 90 percent primary consolidation
H - shortest path to drainage in a saturated medium, cm
c«
LL • the liquid limit of the material, percent
Certain simplifying assumptions were necessary in Equation 1C, the details of
which are given by Murphy and Gilbert. However, the Cime computed using
Equation 10 will.be conservative because the theory assumes complete water
saturation which will be slower than for the case of partial saturation, and
the curve fit incorporating liquid limit into the equation was chosen as an
upper (conserva. ive) bound.
If, as : n example, a waste or soil layer is 18 inches thick and has
access to drainage (e.g., a drainage layer) on either side, than H in the
equation is 9 inches or 23 centimetres. If the liquid limit of the soil is
60 percent, then the time computed for 90 percent primary consolidation from
Equation 3 is 34 days, meaning that 90 percent of the settlement which will
occur in that layer will take place in 34 days. Therefore, most of the com-
pression which will occur in layers of a landfill to which drainage is pro-
vided will probably occur during construction.
Varying the thickness of the waste or soil layer or the distance between
drainage layers illustrates the great afreet that layer thickness has on the
time of consolidation. Halving the thickness cuts the consolidation time by a
factor of it.
More precise (but still approximate) estimates of primary consolidation
time may be made by performing laboratory consolidation tests on the actual
29
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materials to determine the coefficient of conso)idacion. Then the time t
to achieve an average percent consolidation, U . can be predicted wj-h'rhe
equation
where
T - a dimensionless time factor which is a direct result of the mathe-
matical solution of the partial differential equations describing
the consolidation process.
H^ - length of drainage path for expulsion of water from the soil voids
(for single drainage, as with soil overlying an impervious harrier,
H^ " H ; for double drainage, as with soil bounded above and below
by pervious zones, HC - H/2 ; for multiple drainage paths, as with
soil interspersed with alternate layers of pervious zones, Hr *
fraction of H). c
Cv - coefficient of consolidation, a laboratory-determined value
dependent on the soil's compressibility, permeability, and density
(void ratio).
The exact values of T must be determined by evaluating a rather complex
series expression by trial and error. However, this is not necessary since it
has been found that T may be evaluated with high precision using the empir-
ical expression
T" i (rib) (u *60Z) (12>
\ /
T - -0.9332 Iog1() (l - yM - 0.0851 (U > 60T) (13)
where U - percent consolidation desired.
Consequently, if the coefficient of consolidation. C , is determined
for a material in a hazardous waste landfill, then the time for any desired
percent of consolidation may be computed from Equation 11. In a more general
form. Equation 11 may be written (see derivation in Appendix D).
where
Y. * dry density of the waste material
slope of the dry density versus pressure relationship determined
from a one-dimensional compression tast on the waste material
30
-------
vw - density of water
It - coefficient of permeability of the waste material
The various factors of Equation 14 may be evaluated to determine how they will
affect the time to achieve desired percentages of primary consolidation.
Obvously the factors in the numerator of Equation 14 must be minimized and
factors in the denominator must be maximized to minimize the time for
consolidation.
The time factor, T , and the density of water, y , cannot be changed
in the equation, but the other factors may be manipulated to achieve consoli-
dation In the shortest possible time. For example, H may be manipulated to
advantage by installing drainage layers within the landfill. It should be
mentioned again that H has the most pronounced direct effect on consolida-
tion time. c
The compressibility of the waste material is given in this treatment as
dy./dp , and this quantity will be minimized as the strength and density y,
of the material are maximized. This can be accomplished by applying compac-
tion effort to the landfill wastes and cover layers; selecting a material of
low compressibility (low plasticity index) to serve as intermediate cover
where possible; stabilizing the wastes and intermediate cover with pozzolanic
agents to increase their compressive strength; and compacting the intermediate
cover layers dry of optimum if they are clay-like, also to Increase their
compressive strength (a caution here is that subsequent wetting can cause
collapse* of low plasticity material).
Finally, the time for consolidation may be minimized if the coefficient
of permeability, k , of the landfill materials is maximized. Since perme-
ability generally decreases as density increases, efforts to maximize both may
be counterproductive. A good compromise may be to compact the soil or waste
to optimum density for the effort applied.
In order to calculate time from Equation 11, C must be determined.
This parameter Is usually evaluated using a curve-fitting orocedure applied to
the time—consolidation curves from one-dimensional compression tests. The. ,
procedure (logarlthm-of-time method) Is given in many standard references. '
The values of C^ are different for each load Increment and therefore
must be evaluated for all load increments used in the compression test. To
compute the time for various degrees of consolidation for layers of material
in the field, an appropriate value of C , corresponding to the average
* Collapse is a phenomenon which can occur in- low plasticity soils at low
density (compacted dry of the optimum water content) when exposed to water.
Wetting such a low plasticity soil may soften clay binder between larger
silt and sand size particles causing a loss in strength which is accompanied
by a large volume decrease. Collapse usually occurs rapidly when compared
to th« time for comparable volume change due to the process of
consolidation.
31
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pressure in the field situation Is selected and the time for field consolida-
tion is computed from Equations Jl, 12, and 13.
Because of the simplifying assumptions made in the development of the
theory and uncertainty in the evaluation cf C^ , time predicted by the theorv
is at best approximate and at worst only an order oj magnitude estimate. The
problem of selecting an appropriate value of GV is additionally complicated
by the inhomogeneous nature of the contents of a hazardous wants landfill and
the difficulty of representatively sampling these materials for testing. The
problem is additionally complicated because of the inherent differences in
behavior between laboratory samples and in situ soil.
Rather than trying to predict precise values from the theory, it may be
much more practical to use the geometric and material properties dictated by
the theory to identify general operational procedures that will minimize set-
tlement time. Drainage layers to control the effective thickness of waste
layers (H ) appear to be a practical measure to monitor and control the
internal movement of fluid within the facility and to eliminate extended peri-
ods of settlement. Previous soil drains and, in more recent time, geotextile
fabric drains4'6*7 have been used to relieve pressure and control flow within
earthen embankments. The same techniques may b« used to great advantage in
hazardous waste landfills.
Differential Settlement
Problems with differential (uneven) settlement may occur if drummed waste
must be disposed of in a landfill with bulk waste. The time for deterioration
of the steel drum may be quite long and drummed waste layers may remain very
stiff in the Interim. In such instances, if there are not many drums for dis-
posal In a landfill, they may be dispersed about the landfill, or emptied, the
contents stabilized, and the drums crushed or reclaimed. Obviously drums of
unstabilized liquid are to be avoided because once the drum is corroded, the
entire volume of the drum becomes a large void.
As discussed below, differential settlement is aggravated if stiff and
undeformlng columns o* material are placed in close proximity to flexible
deformable columns. Since the central column model is actually based on
column elements, If spatial properties within the landfill are known with
sufficient confidence, the subsidence of two columns may be computed with the
central column model. The difference between th« subsidence of the two col-
umns la the quantity described below. Knowing the distance I between the
columns, the index of the differential settlement, 4/A , may be computed and
the methods used to analyze the effect of this amount of differential settle-
ment on the cover system.
ANALYSIS OF DIFFERENTIAL COVER SUBSIDENCE
Identification of Causative Factors
Settlement of the waste mass In a hazardous waste landfill will result in
subsidence (sinking) of the cover (cap). Differential settlement can lead to
cover damage and leakage caused by the tensile stresses created. In such a
32
-------
case, the cover system would be required to brtdge the zone of lost support.
For this reason, It is reasonable to formulate a model to determine rhe impor-
tant factors involved in differential settlement using elementary beam theory.
The model assumes that the cover system will lose support over a length, {. ,
and as a result will undergo a differential settlement. The model representa-
tion is therefore a beam with fixed supports at either end and is distorted
when one support settles an amount a (see Figure 7).
Expressions for vertical shear, moment, slope, and deflection of the ide-
o
alized cover may be determined by integration using elementary beam theory.
The mathematical expressions for maximum stress due to shear and moment in the
beam model in Figure 7 are
and
ashear "'-' IT/ ^"Tl (15)
°moment
where
o . , o " maximum stress due to shear, moment
shear moment ____,
i - length of beam
E » Young's modulus of the cover material
h - cover thickness
Although these expressions were developed using small-deflection beam theory
and may not be appropriate for the large deflections observed in soil struc-
tures, the expressions identify parameters which quantify distress caused by
differential settlement. For example, Equations 15 and 16 suggest that stress
is minimized if A/J , E , and h/1 are minimized. Obviously A/I is mini-
mized if the differential settlement, A > is minimized. This may be accom-
plished by minimizing total settlement and involves compacting wastes during
placement, eliminating void space within the landfill, stabilizing liquids
before drum disposal and other considerations (Equation 7).
Additionally, A/Z mav be minimized by maximizing I . This will reduce
cover stress by spreading the distortion over a greater length and therefore
reducing the effect of the distortion. The I may be maximized by placing
the landfill wastes as homogeneously as possible to give uniform support to
the cover.
Minimizing Young's Modulus, E , of the cover material can be accom-
plished by compacting the cover soil wet of the optimum water content. This
will result in a cover with lower strength but with greater pliability and
capacitv to distort without rupture. This is shown in Figure 8 which is taken
4
from Lambe and Whitman . The figure shows that samples 1 and 2, which are
compacted dry of optimum water content, offer high strength and stiffness .
(Young's Modulus) but exhibit brittle behavior in that they develop maximum
strength and fail at relatively small strain. Samples 5 and 6, compacted wet
33
-------
BEFORE
SETTLEMENT
AFTER
SETTLEMENT
Figure 7. Beam representation of a cover systen.
-------
20 22 24 26 28 3O 32 34 38
MOLDING WATER CONTENT. %
S 8 10 12 14 16 18 20
AXIS STRAIN, %
Figure 8. Stress-strain behavior of clay specimens
at different compaction condition.
35
-------
of the optimum water content, show low strength .ind stiffness but exhibit
ductile/pliable behavior.
High strength is seldom required in the cover system of 3 hazardous waste
landfill; therefore, wet-of-optimum compaction of the cover system would be
desirable since it would result in a material which would be more able to
yield and flew without rupture. It thus would be able to conform to nonunl-
forra settlement In the foundation soil underneath. An addition.il "free" bene-
fit of the wet-of-optimum compaction is a lower cover permeability.
Finally Equations 15 and 16 suggest that the ratio h/i should ^t min-
imized. This should be done by maximizing i • A thick cover is necessary to
control diffusion as well as to prevent the intrusion of animals and plant
roots into the landfill. A thick cover also offers the advantage of more
resistance to desiccation due its large mass and thickness.
The beam model shown in Figure 7 is a very simplified model, but is use-
ful in that it is not used for analysis but rather to identify parameters sig-
nificantly affecting the behavior of cover systems. That is, the model is
used in a qualitative rather than a quantitative sense. However, a more com-
plex model consisting of a beam supported by an elastic foundation is worth-
while considering if only to verify that significant parameters have not
been overlooked or omitted by the simpler model. For completeness, three con-
ditions were investigated considering beans supported by a Winkler foundation.
A Winkler foundation is a linearly elastic foundation consisting of springs of
constant sfffness, all in close proximity (adjacent) to each other but all of
which behave independently of the influence of neighboring springs. This rep-
resentation more closely approaches the behavior of soil supported structures
but departs from actual behavior in that soils are not elastic, and elements
of soil are influenced by the behavior of neighboring elements.
Three cases are considered and are shown schematically in Figure 9. They
are a case where the cover beam bridges a zone where interior support is truch
less than that at the edges, a case where the cover beam bridges a zone where
interior support is completely lost under the central span but the bean is
fully supported (clamped) at the edges, and a case where the cover beam
bridges a zone where interior support is lost and the edges have less than
total support.
For all three cases the beams in question have stiffness, El , and
g
density, y • Solutions for cases 1 and 3 are given by Hetenyi . Case 2 is
the case of a beam with no rotation or deflection allowed at the ends.
Case 1 —
For the configuration of case I the maximum moment and shear are located
at points A and B as shown in Figure 9. Values of the maximum moment and
shear are
i«
max
ybh /sinh Xjt - sin Xt\
2.2 Isinh XI + sin \tj
36
-------
1
K <=»
CASE 1
CASE 2
-El
K * <*>
/
1
/ /
I ^^ f • ^-— —
"^s^^ ~^^^^
^^••^^^^ ^^^^
*~*~~- — "^^
V
\
•
•
A K=0 B
«
e
^ «i^
< - o
L
3
(
CASE 3
Figure 9. Models of beams on the elastic foundations.
-------
n Ybh /cosh Xi - cos xA
Snax * X I sinh Xi + sin Xi J
where
Y - density of (soil) beam
b » width of the bean
h « thickness of the beam
X - fylCMEI
K * foundation modulus (frora plate load test)
I * length of beam supported by foundation of modulus K
I - (1/12) bh3
From Equations 17 and 18 the maximum stresses due to moment and shear may
be computed to be
^sinh AI + sin
and
/
\
Xi — sin />*, | (19N
_ y Vbh t/^/cosh Xi - cos Xi\ (7^
°s ' C2 T \r£* ^sinh Xi » sin Xij (20)
where C. and C. are constants. From Equations 19 and 20 It is observed
that distress will be minimized If Y , bh , E and the trigonometric
expression are minimized and K Is maximized. This seems consistent with
intuition since induced stress will increase if the density (unit weight) of
the beam increases over a span with less than complete support. However, the
density of the beam and its depth, h , are largely uncontrollable, density
being essentially constant and h is usually dictated by factors outside the
realn of soil mechanics. E should be minimized as predicted by the simpler
model, and f should be maximized since the greater the foundation support,
the lesser will be the beau distress. The trigonometric expressions in Equa-
tions 19 and 20 are bounded between zero and one. If X£ is zero then the
expression becomes zero. However, X* is generally not equal to zero, so I
must be zero which reduces the problem to a .trivial case. If \t > w , then
the trigonometric expression approaches one. The condition \i > ir repre-
sents the case of a long beam.
The conclusion reached by this analysis is that case 1 is consistent and
compatible with the beam model.
Case 2 —
For case 2, the maximum stress due to shear and moment becomes
and .
a - yt (22)
38
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Case 2 may represent the case of a cover fully supported until it loses sup-
port over a length, I such as if a single drum or series of drums collapsed
within a landfill causing cover support oss, or if settlement occurred in the
waste material underneath the cover. Distress due to both moment and shear
may be minimized by minimizing the unsupported length Z which may be accom-
plished by providing adequate compact^.n of wastes so that foundation support
is not lost over a larger distance «. , or not burying drums which otherwise
would ultimately collapse with the consequent loss of cover support.
Case 3—
In case 3 the maximum stresses due to shear and moment are
(23)
m W W\2A + X'L/
and
a - ^ YL (24)
s 4
Both Equations 23 and 24 surest that Y and L should be minimized for min-
imum cover distress, but these limits lead to trivial examples. From Equa-
tion 23, however, it may be determined that distress due to moment Is
minimized if the product ^L • 6 , which produces the very interesting result
that the relationship between unsupported length L , the foundation constant
KI , and the beam E and geometric parameters b and h for minimum dis-
tress are
(25)
Equation 25 suggests that a certain degree of foundation flexibility may be
desirable because if the foundation modulus becomes infinitely large, case 3
degenerates to case 2, which is that of a cover with fixed ends and represents
a condition of more severe distress than that of case 3. A gradual transition
In foundation support to minimize distress in the beam (cove.') is suggested by
Equation 25 along with :ne comparison of cases 2 and 3 and reinforces the
suggestion of the earlier simple model that distress is aggravated in a cover
system if there is a sudden change in stiffness of the foundation, i.e.,
wastes of great differences in stiffness should not be placed in close proxim-
ity to each other.
Tensile Strain
The cover system will be required to increase in length and therefore
carries tensile strain as differential settlement occurs in a hazardous waste
landfill. The cover will crack if tensile strain becomes excessive. Gener-
ally soils are not able to withstand high levels of tensile strain without
cracking.
s
The average tensile strain developed within the cover may be computed
using the simple beam model. This procedure Involves integrating over the
39
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deflected beam shape to determine the arc length of the bean after deflection.
An expression to compute the arc length of the deformed section of the beam
Model shown in F^ure 7 may be determined by integration, and is
(26)
where
I - length of the deformed cover element
& - differential settlement
I - length of the cover element
x - coordinate along the cover element
r:^r^T,:^^^^
Figure 10 and are presented as the dimensionless q«««tlty A/ *
£ increases .
If the maximum tensile strain which can be sustained by a given soil is
measured, estimated, or otherwise obtained, then the mwimurn value of til
which can be tolerated in a cover system of that soil -ay be estimated from
Figure 10.
Figure 11 is a plot of maximum tensile strain reported by several inves-
tigators9'10 versus soil plasticity Index. Figure 11 also suggests that rhe
capacity for tensile strain increases as plasticity index of a soil increases.
For cc^leteness. more research on th. tensile strain capacity «*••"*•
neededTbut the trend for Figure 11 is clear, showing that, for similar condi-
tion, of compaction (water content and dry density), the tensile capacity of a
soil increases as the plasticity index increases. Therefore, since soil that
Ire able to withstand higher levels of tensile strain are preferred for the
instruction of cover sy«e« of hazardou, waste landfills, the s.lectior , of
soils with higher plasticity indices is indicated if a selection is possible.
Additionally, it should be stated that it would be highly <••*'««•
nerform a laboratorv study of th. tensile properties of potential soils of
Tichthl covtr system of a hazardou. wast, landfill will be constructed.
Inv.,tlg.tio«'should include (for th. -oil selected for the c ov.r) sever a
molding condition, to d.t.rmin. th. condition which °Jf«edi.^e^"' ;°""?
M«n of ten.ll. strain. «cono«v, and .... of placement for the differential
2 sss
be considered.
Effects of Differential Subsidence on th. FML
Th. discussion of subsidence and settl.rn.nt effects has thus far
on d.£r«tion of th. soil portion of the cov.r. Effects of settlement on the
-------
I 00
0.73
0.90
0.23
I i i I i i i i
i i i I I I i I
10 10.0
TENSILE STRAIN, X
100.0
Figure 10. A/I versus average tensile strain.
-------
3.5
LEGEND
LEONARD ( BEAM FLEXURE TESTS )
T T
O TSCHEBOTARIOFF ( DIRECT TENSION TESTS )
D WES DATA ( DIRECT TENSION TESTS )
10 100
PLASTICITY INDEX, %
7
1000
Figure 11. Tensile strain versus plasticity index.
-------
FML of the cover must also he considered. Field data on FML performance In
hazardous waste landfills are not available, but laboratory tests have been
conducted on FML's under simulated fill and embankment conditions11'12.
Flexible membranes can elongate substantially before failing, and little prob-
lem is anticipated for cover FML failure in the case of cover subsidence over
a large ar2a. Locally severe subsidence, however, may produce substantial
differential settlement and much greater elongation of the FML. Several
investigators have shown through multidimensional stress-strain analyses that
allowable strains reported by manufacturers of FMLs may be much higher than
the actual strain at failure of FML's in field conditions. Manufacturers'
elongation data are generally for one-dimensional strain stretch tests wherein
strain is distributed evenly within the grip points of a tensile test device.
In situ conditions can be expected to produce multidimensional stresses and
uneven distribution of strain and cause thinning and possible tearing and
premature failure of FML's.
Steffen11 tested several geomembranes in a pressure vessel designed to
stress the antire surface of a 3-foot diameter specimen of the geomemhrane.
He reported strains at failure of 9 percent for 90 mil HOPE and 15 percent for
80 mil HDPE, which is about 1 percent of the strain reported from manufactur-
ers' one-dimensional stress-strain tests. (His tests on PVC, CPE, EPC, and
EPDM produced higher strains, from 40 to 70+ percent.) Tests conducted on
varying thicknesses of HDPE indi-ated that thicker FMLs were able to achieve
higher strains before failing. Strong showed through tests of membranes
stressed over artificial fissures and hard points in a pressure cell that high
localized elongations could be minimized by using thicker membranes and by
incorporating a geotextile (a woven fabric) into the geomembrane application.
The investigations indicate that failure of FML's In areas of severe differen-
tial settlement may occur at lower strain values than would be expected from
FML manufacturers' test data. Furthermore, thicker FML's may allow greater
strains to occur before failure.
Because the FML is secluded within the cover, it cannot easily be
inspected and its condition determined. Every effort should be made when
placing wastes in the landfill to reduce the potential for differential set-
tlement, particularly in the upper layers where local subsidences of the cover
may severely strain the FML component.
-------
SECTION 5
MITIGATION OF SETTLEMENT AND EFFECTS OF SETTLEMENT
LANDFILL TREATMENT TO REDUCE SETTLEMENT POTENTIAL
Section 4 discussed the philosophy and theory behind controlling the
amount of and total time for settlement of a hazardous waste landfill. Prac-
tices that optimize the variables of Equation 4 reduce the time to maximum
settlement and make the landfill more manageable after closure. This subsec-
tion describes potential ways of treating the landfill contents to reduce aid
hasten ultimate settlement. Because soils and soil-like sludges and other
materials constitute a major part of all hazardous waste landfills, it is not
unreasonable to suggest adaptation of soil stabilization techniques to land-
fills. This subsection presents methods for fill compaction and waste
fixation.
Fill Compaction
The following discussion makes reference to cohesive and .loncohesiv*
soils. Cohesive soils are generally those consisting of grain diameters pass-
ing the No. 200 US Standard sieve, or 0.074 millimetre (silts and clays), and
coarse grained materials are those with substantial amounts of fines in the
matrix such as clayey sands. Cohesionless soils are coarse grained soils such
as sand and gravel, the grains of which are more free to move within the soil
mass than are the grains of cohesive soils.
Standard Compaction Methods—
Standard compaction methods for soils include the use of specially
designed motorized compaction equipment and laboratory and field monitoring
procedures to achieve desired soil density, plasticity, and permeability. The
reader Is referred to the discussion of soil compaction methods and procedures
regarding the application of the methods to landfill cover preparation. The
same methods and equipment are applicable to compaction of some hazardous
waste fills to achieve greater preclosure settlement and to lessen the poten-
tial for postclosure settlement and subsidence.
Vibrocowpaction--
Vibrocompaction methods in use in civil engineering include blasting,
vibrating prob«, and vibratory rollers and have been used for rapid densifica-
tron of saturated cohesionless soils. The range of grain-size distributions
suitable for treatment by vibrocompaction is generally from coarse to fine
sand (noncohesive soils). The effectiveness of the vibratory methods is
greatly reduced if the percent finer than the No. 200 sieve exceeds about '
20 percent or if more than about 5 percent is finer than 0.002 -am, primarily
because the hydraulic conductivity of such materials is too low to prevent
-------
rapid drainage following liquefaction. Only the vibratory roller could he
considered for compacting hazardous waste landfills. The other methods are
considered coo risky or are inappropriate for use in hazardous waste
landfills.
Where dry or saturated coheslonless fills are being placed, vibratory
rollers are likely to he the best and most economical means for achieving high
density and strength. The effective depth of densifIcation may be 6 feet or
more for the heaviest vibratory rollers (Figure 12a). For a fill placed in
.successive lifts, a density-depth distribution similar to that in Figure 12b
results. A properlv matched system of lift thickness, soil type, ard roller
type can yield compacted layers at a relative density of 90 percent or more
(relative density is a comparison of the existing void ratio of a soil with
the range of possible void ratios for the soil, and is expressed by
(e - e)/(e - e ) where e is the void ratio in its loosest state
max max min max
and e . the void ratio in Its densest state).
mm
Precompression—
Preloading—Earth fill or other material is placed over the landfill
prior to final closure in amounts; sufficient to produce a stress in the soft
soil equal to that anticipated from the final structures (or in the case of
landfills, the final cover). As the time required for consolidation of the
soft soil may be long (months to years), varying directly as the square of the
layer thickness and inversely as the hydraulic conductivity, preloading alone
is likely to be suitable only for stabilizing thin layers and with a long
period of time available prior to final development of the site.
Surcharging—If the thickness of the fill placed for preloading is
greater than that of the expected structure-induced loading (the final cover
and appurtenances), the excess fill is termed a surcharge fill. The amount of
consolidation varies approximately in proportion to the stress Increase. The
preloading fill plus surcharge can cause a given amount of settlement In
shorter time than can the preloading fill alone. Thus, through the use of
surcharge fills, the time required for preloading can be redured signifi-
cantly. Both primary consolidation and most of the secondary compression
settlements can be taken out in advance bv surcharge fills. Secondary com-
pression settlements may be the major part of the total settlement of highly
organic deposits or of old landfill sites. The landfill operator will prob-
ably have to ask the pennitter for an extension of closure time to perform the
surcharging.
Vertical drains—The required preloading time for most soft clay deposits
more than about '0 feet thick will be large. The consolidation time may be
reduced by providing a shorter drainage path by installing vertical sand
drains. Sand drains are typically 10 to 15 inches in diameter and are
installed at spacings of 5 to 15 feet. Perforated risers can also be used as
vertical drains. Horizontal drainage layers facilitate internal drainage. As
discussed previously, any system that removes leachate from within the land-
fill helps reduce the time to achieve maximum settlement and adds to the long
range stability of the landfill. ,
-------
DRY DENSITY, PC F
100 lOi
DRY DENSITY, PCF
NOTE. SINGLE LOOSE LIFT OR
NATURAL DEPOSIT OF
THICKNESS >% FT
60 60
RELATIVE DENSITY. PERCENT
a. DENSITY VERSUS 0£PTH FOR DIFFERENT NUMBERS
OF ROLLER PASSES
1
' ••
k
f "
ki
0
4.1
t.o
IO IOO lOi II
"" — --^,
NOTE: 2-FT LlF
S ROLLE
^N
<
T HEIGHT \
R PASSES. \
)
»o 6o ao 100
RELATIVE DENSITY. PERCENT
b. DENSITY VERSUS DEPTH RELATIONSHIP FOR A
SERIES OF 2-FT LIFTS
Figure 12. Sand d^nsifIcatIon using vibratory rollers.
! I
-------
Waste Fixation
Waste fixation describes a variety of processes bv which fluid or liquid
wastes are strenpth-ned or made solid by mixing with other agents. Similar
terms often used are waste solidification or waste stabilization. The two
processes most commonly used hv operators of hazardous waste landfills to fix
wastes are absorption lor adsorption) and cementation. Some fixation pro-
cesses absorb the liquid waste and make the mixture appear as a solid. The
liquid may or may not be immobilized. Other processes, such as cementation,
produce a chemical and physical change in the mixture and impart considerable
strength relative to the original substance. Two goals of fixation are gain
in compressive strength and binding or retention of liquids. Increase in com-
pressive strength reduces compression of the waste and limits settlement of
the landfill. Retention of liquids prevents the production of leachate within
the landfill but if not completely effective may increase the time to ultimate
consolidation.
Fly ash primarily from coal-fired power plants, kiln-dust from cement
manufacture, and absorptive clays are often-used absorbents in the waste dis-
posal industry because of their relatively low cost and availability. Some
fly ashes and kiln dust have pozzolanic qualities, that is, they react with
calcium hydroxide in the presence of water to form cementitious compounds.
Fly ash and kiln dust serve both as an absorbent and in some cases as
strengthened when mixed with manv liquid hazardous wastes. Their effective-
ness as stabilizers depends both -orT'tnepropertles of the absorbents and of
the materials being stabilized. Tests must be run on potential mixes to
determine effectiveness.
Absorptive clays such as fullers earth are used in more limited quanti-
ties in landfill waste stabilization because of higher materials oost. A com-
mon use is as an additive to drums of liquid wastes to reduce the amount of
free liquids entering the landfill. Adsorptive clays are rarely used in
solidifying large volumes of hulk wastes, whereas fly ash and kiln dust are
commonly used for that purpose. Engineering characteristics of mixtures of
fly ash and absorptive clays and water and oil (simulated liquid wastes), and
of real wastes traated with pozzolanic material, are presented in Section 3 of
this manual.
Chemical grouts and plastlcizers have been introduced to the growing
waste fixation market. The long-term effectiveness of particulate additives
to retain liquids within the waste under consolidation pressures and after
chemical breakdown of the mixture in a landfill environment has not been
determined, but Is suspected.
Boutwell1* reported on a process whereby a small quantity of a polymer is
added to a waste-dust mixture to render the mix less porous by blocking the
pores. Technological advances- will surely be made in the field of waste fixa-
tion in the next few years and should be monitored for application to hazard-
ous waste landfill operations.
-------
DESIGN AND CONSTRUCTION OF COVERS TO ACCOMMODATE SUBSIDENCE
This subsection discusses consldera'ions in designing and constructing
final covers to withstand deformations resulting from settlement and
subsidence.
Compaction of Cover Soils
Coals of Compaction—
The barrier soils of final landfill covers are usually compacted to
achieve desired low permeability to liquids, a primary concern of hazardous
waste landfills. Consideration must also be given to preserving the plastic-
ity and flexibility of the cover soil to protect it when it is subjected to
deformation. Inflexible or stiff cover soils are more likely to crack when
deformed than are soils of low stiffness. The desired flexibility can be
achieved by compacting the cover soils at a water content that is wet of opti-
mum. Figure 13 is a soil's compaction curve. The curve is made up of points
representing the dry densities of soils compacted at increasing water con-
tents. The maximum density that csn be achieved is represented by the peak of
the curve. The water content at the peak is called the optimum water content
for the soil. Any more water added to the soil will produce only lower com-
paction densities. Cover soils compacted wet of optimum water content are
more plastic and less^sJtiff and brittle than they would be if compacted at
lower water content, and are less likely to develop zones of tensile stress
than are soils compacted dry of optimum. Fortunately, soils compacted wet of
optimum also exhibit low permeability, and the goals are compatible.
Standard and Modified Compaction--
The specification of compactive effort to b« performed on a soil Is
determined from laboratory tests conducted on a sample of the soil. Two com-
mon laboratory tests are the standard Proctor and the modified compaction
tests. In the standard Proctor test, samples of the soil at increasing water
contents are compacted by hand in a mold using a 5.5 pound hammer falling
12 Inches per blow and applying 25 blows per layer for 3 layers. The dry
density of the sample after compaction at each water content is recorded and a
curve like that in Figure 13 ts produced. The standard Proctor test was con-
sidered to reproduce compactive efforts similar to those of compaction equip-
ment in use when the test was developed. Compaction specifications were made
based on a percentage of the maximum density achieved in the Proctor test
(say 90, 95, or 100 percent of standard Proctor).
Some prolerts required higher compaction efforts. A modified compaction
test was developed using greater laboratory compaction effort. The modified
test uses a 10 pound hammer falling 18 inches per blow, with 25 blows applied
to each of 5 layers. A job specification of 95 percent modified compaction
would then produce an in-place soil of higher density than one compacted at
">5 percent of standard Proctor.
Compaction specifications further indicate the water content relative to
optimum at which the soil should be compacted, because soils have different
characteristics at water contents above, at, or below the optimum water con-
tent (Figure 13). Clays compacted on the wet side of the optimum water con-
tent are less parmeable than those compacted on the dry side. Clays compacted
48
-------
MAX DRY DENSITY
(J
&
>"
K
{/)
8
X
Q
WATER CONTENT. X
Figure 13. Soil compaction curve. Each point on the curve represents
a sample of the soil compacted to a particular dry density
at a given water content.
-------
wet of optimum are more compressible at low stresses and less
high stresses than are clays compacted dry of optimum. Cl*s ccr d f
optimum are stronger and have a higher stress-strain modulus thaTdo cUvs
compacted wet of optimum. Because the characteristics desired in clavs of ch*
covers of landfills are primarilv low permeability and low stif ^ss compac-
tion of cover clays should logical lv be specified at lower companion e?fort
at wet or optimum (generally not to exceed 3 percent wet of optimum).
Control of the compaction effort of soils in the field consists of con-
ducting in-place or laboratory tests to determine the field density and water
content of the soil after compaction to assure compliance with specifications.
Soils that are too loose require additional compaction effort which can be
accomplished by increasing the weight of ballast or the number of passes of
the compacting unit or by reducing the thickness of the spread layer. Compac-
tion effort can also be increased by using heavier or different types of
equipment.
If the water content of the soil is above the desired value, it can be
reduced by aerating the soil through scarifying or tilling. If the water con-
tent is too low, water can be added to the fill and distributed or mixed with
the soil in the borrow area.
Soils of the final covers of hazardous waste landfills require special
precautions and considerations. Care oust be exercised in placing soils over
an FML to prevent damage to the FML. A buffer layer of granular material like
sand should be placed over the FMT, to protect it (see Figure 2) . The in-place
density and water "content of the compacted soil should b«s carefully checked to
ensure compliance with compaction specifications. If subsidence or differen-
tial settlement of the cover is expected or predicted, the soil portion of the
cover should be flexible and of low stiffness to withstand the deformations.
Compaction Equipment —
The principal types of compacting equipment are the smooth wheel roller,
the rubber-tired roller, the sheepsfoot roller, and the vibratory compactor.
Vibratory rollers are the least effective compactors for cohesive soils, the
kind of soil used in the barrier portion of landfill covers. Rubber-tired
rollers with high tire pressures and sheepsfoot rollers are effective for
cohesive soils. Sheepsfoot rollers are particularly effective at bonding of
lifts during compaction of cohesive soils. Footed rollers were in use at
several RCRA landfills inspected in a previous investigation . Table 4 sum-
marizes the capabilities and characteristics of compaction equipment.
Compaction Characteristics of Soils —
Suitability of soils for embankments is similar to that for fill covers
because the desired characteristics for both applications include accommoda-
tion of deformations and low permeability. The clay-rich soils (SC, CL, and
CK) yield the lowest permeabilities and highest plasticities when compacted
and are the soil types commonly used to construct the soil barrier portion of
landfill covers. Reference 16 discusses soil types and compaction
characteristics.
50
-------
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51
-------
TAKE *. it***!•*•«>
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52
-------
Other Construction Considerations
The RCRA hazardous wast* landfill cover Is made up of layers which
include materials other than the soil barrier, as illustrated in Figure 2.
This subsection discusses considerations in the design and construction of the
cover as a layered unit, following the basic component arrangement shown in
Figure 2.
Suitability of Various Soil* as Covers—
LuCton et al. evaluated and ranked soils for their performance as land-
fill covers. Table 5 lists the rankings for selected performance character-
istics. Rankings are 1 (best) through 13 (poorest). Since this report
concerns itself with the soil barrier portion of the cover of a hazardous
waste landfill, some of the characteristics in Table 5 are not directly
applicable. Soils with fines in the matrix and clay soils perform well in
impeding percolation of water and migration of gases (columns A and B). Ero-
sion control (column C) is not a primary consideration for the barrier portion
because the barrier is not normally exposed.
Rankings for column D (crack resistance) are based on expansion and con-
traction with accompanying cracking controlled by the clay mineralogy of the
soils. Fine grained and clayey soils accordingly rank low in resistance to
that kind of cracking. Final cover barrier-soils, "however, are covered
Immediately after emplacement and are not allowed to undergo change in water
content. Cracking by expansion/contraction is not normally a problem. From
the standpoint of resistance to cracking during deformation from settlement '
and subsidence, the clay soils rank high, as discussed in earlier sections.
If the cover layers overlying the barrier portion are compromised, and the
clay portion is exposed to the atmosphere, drying or water infiltration can
occur, and the barrier may well be subject to cracking by desiccation and
shrinkage as suggested in column D of Table 5. The table might best be used
as a guide to selection of other layers that make up the cover and less to
evaluate soils for the construction of the cover soil barrier.
Use of Soil Additives and Soil Stabilization—
Where appropriate soils for use as cover are not available on site, it
may be necessary to bring in clay rich soils or to add bentonite (a swelling
clay) to available soils to achieve the desired characteristics of low perme-
ability and plasticity in the cover. Table 6 presents recommended application
^•ates for sodium bentonite to reduce permeability of soils in farm ponds. It
can be used to estimate the amount of bentonite required for soils of landfill
covers. The use of soil stabilization techniques such as addition of lime or
pozzolanic materials or grouts is not anticipated to be of use in cover soils
because the techniques tend to great-^y stiffen the treated soils, an undesired
quality in landfill covers.
CORRECTIVE ACTION FOR SUBSIDENCE
Corrective actions for subsidence events in a hazardous waste landfill
are beyond the scope of this report, but seme points are worthv of mention'.
Cover damage caused by subsidence will require repair and will likely
require correction of the cause. Damage is expected to be corrected hy
53
-------
TABLE 5. RANKING OF USCS SOIL TYPES BY PERFORMANCE OF COVER FUNCTIONS
13
Soil Type
CW
GP
CM
GC
SW
SP
SM
SC
ML
CL
OL
MH
CH
OH
Pt
Column A
Impede Water
Circulation
10
12
7
5
9
11
8
6
4
2
—
3
1
—
—
Column R
lupede Gas
Migration
10
9
7
4
8
7
6
5
3
2
—
—
1
—
--
Column C
Water Erosion
Control
1
1
4
3
2
2
'I
'
"1
12
11
10
9
8
5
Column D
Crack (
Resistance
1
1
3
5
1
1
2
4
6
8
7
9
10
9
—
Column E
Reduce Frost
Heave
1
1
4
7
2
2
5
6
10
8
8
9
3
—
—
-------
TABLE 6. SOIL CONSERVATION SERVICES RECOMMENDED SODIUM BENTONITE
APPLICATION RATE FOR FARM PONDS
Application Rate
Soil Application Method psf
Clay Pure membrane or mixed layer 1.0-1.5
Sandy silt Mixed layer 1.0-1.5
Silty sand Mixed layer 1.5-2.0
Clean sand Mixed layer 2.0-2.5
Open rock or gravel Clay or sand mixed layer 2.5-3.0
55
-------
excavation and exposure of the barrier layer, removal of the damaged part,
refilling of the underlying foundation, and replacement of the barrier layer.
Correction of the cause of subsidence mav require Increasing the strength
of the underlying waste materials. Of the measures expected to be applicable,
grout Injection to increase the compresfive strength may be the most cost-
effective. However, various methods of deep compaction may also be applica-
ble, such as vlbrocowpaction, vibrodisplacement compaction, and heavy tamping.
Excavation and replacement of the waste itself is not a feasible measure at
this time.
56
-------
REFERENCES
•LCTW cF> W' C" "R°le of Flxntion Processes in the Disposal of Wastes,"
ASTM Standardization News, 1984, pp 23-25.
2. Hagerty.^j., Ullrich. R., and Thacker, B. "Engineering Properties of FGD
Sludges, Proceedings of the Conference on Geotechnical Practice for Dis-
posal of Solid Waste Materials. June 13-15, Univ. of Michigan, Ann Arbor,
Specialty Conference of the Ceotechnical Engineering Division, ASCE,
1977.
3. Murphy, W. L., and Gilbert, P. A. "Settlement and Cover Subsidence of
Hazardous Waste Landfills," Report No. EPA-600/2-85/035, U.S. EPA Munic-
ipal Environmental Research Laboratory, Cincinnati, Ohio, 1985.
4. Lambe, T., and Whitman, R. Soil Mechanics, John Wiley and Sons, Inc.,
New York. 1969. p 522.
5. Taylor. D. W. Fundamentals of Soil Mechanics. John Wiley and Sons,
New York, 1948.
6. Horz, R. C. Geotextiles for Drainage, Gas Venting and Erosion Control at
Hazardous Waste Sites, I' S. Army Engineer Waterways Experiment Station,
Vicksburg, Mississippi, 1986.
7. Harr, M. E. Groundwater and Seepage. McGraw-Hill Inc., New York, 1962.
8. Hetenyi, M. Beams on Elastic Foundations, University of Michigan Press,
Ann Arbor, Michigan, 1946.
9. Leonards, C. A., and Narin, J. "Flexibility of Clay and Cracking of
Hams," Journal of the Soil Mechanics and Foundations Division, American
Society of Civil Engineers, Vol 89, No. SM2. 1963.
10. Tschebotarioff, G. P., Ward, E. R.. and DePhillippe, A. A. "The Tensile
Strength of Disturbed and Recompacted Soils," Proceedings of the Third
International Conference on Soil Mechanics and Foundations Engineering,
Vol 1, Zurich, Switzerland, 1953.
11. Steffen, H. "Report on Two Dimensional Strain Stress Behavior of Geomem-
bran-ss With and Without Friction," in Proceedings. International Confer-
ence on Geomemhranes. Vol 1, Denver, Colorado, 1984, pp 181-185.
12. Strong, A. G. "Longevity Aspects of Polymeric Linings for Water Contain-
ment, " in Proceedings, International Conference on Geomembranes. Vol 1,
Denver, Colorado, 1984, pp 281-285.-
13. Headquarters, Departments of the Army and the Air Force. Soils and Geol-
ogy, Procedures for Foundation Design of Buildings and Other Structures
(Except Hydraulic Structures), Army TM 5-818-1, Air Force AFM 88-3,
Chap 7, pp 16-1 through 16-17, 1983.
57
-------
14. Boutwell, G. P., .'r. "Fixation in Land Disposal," paper preprint pre-
sented at Air Pollution Control Association Workshop on Hazardous Waste,
Baton Rouge, Louisiana, 1985.
15. U.S. Department of Che Army, Office, Chief of Engineers, Earth and Rock
Fill Dams. General Construction Considerations, Engineer Manual EM 1110-
712300. 1971.
16 U.S. Department of the Anny, Office, Chief of Engineers. "The Unified
Soil Classification System," Technical Memorandum No. 3-357, U.S. Army
Waterways Experiment Station. Vicksburg, Mississippi, 1960.
17. Lucton, R. J.. Re*an. G. L., and Jones. L. W "Design and Construction
of Covers for Solid Waste Landfills," Report No. EPA-600/2-'9-t65,
Municipal Environmental Research Lab. U.S. Environmental Protection
Agency, Cincinnati. Ohio, 1979.
58
-------
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(Arllv.)
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lapvt*n4Mnl li*v« In 1. 2. ot 1 lf*o* lank lift*. • II. (tn*t i\
i.\ (1 cl«y vail 4i«tni U»*i» cnr> 4ru*« In Ar*« 1
w/l ll topvull) at In lr*nrh«« "mtnl**i. ci«t.«d.
Art«» :. i K(tt« i
Will u»* •tl|y»
2 |»«f Illl
O ll «l «4«« I4fw«* un-«lu««| C*H». run- tutt«r*» I1***)
•n4 \ 1 •« lna>l««l«4 ••i. 1 fl N|>n .illt*4 Ll^MlJ. • la 1? In MA
NUM. -1/2 ll fU.it. 4fuM pUkllv k«*« »44t !••• «nd •olUIIUd *.U«l.d IA>i|v«
lopt.tll «n4) c|| | w*«it«
• Ion . ••!)••.
I-.I,.,I (, .,• Ill)
-------
TA1II A-1. «) />
(conl IguouM c«ll«>
llilchact*
40 II '
4« II. M..
11% ll t.luw
loun4>
41 ll XI
IK II k.lo.
fruund)
HA
Imirlar
1 o« I Cl.cl.l Illl
•n4 Ucu«-
Irlnr clay
1 on / I.IUUI Illl
Irlo* clay
1 on 1 Silly ll«4,
grcvtl, •!••
cl.y
lii;:" i,1'-;,"-;-.
•A two ,-,„ k..,..,,,. ,.„,..
n .oM.rl. 1. .l.c. 1. H | Ml
o « ia fi ,..,.i
fl loMUlc 1 II l.vu.k.tf l.-.l... ,|«y.
0 o 10
-------
fit* L**cb>*t*
«•, Collaciloa Ijalaa,
l«*i C*«*r Spat,*
IM«* llMK. 4-
1-1/2 1C 1} U.
|r*v*| *« |«*
I*. tfrala plaa la ttaala layar.
caaclau fl>
I/I I../(I
t ull t ••MM
>4 U. laaaal
It *«li~.k .1 4 I* Ml coa»acn
4-u. aioiK. rvc ci.r. arcikai
U ll-U. r U«ltlv«*
.1 LllJi!«.
10 N.I. IUI ll«< Ij-ll
i llquU (I In. kwlt .
•I*n4ln|
liquid) In
dry**
••< «.
<>llli
U « i./
It/*
•• i
• Ill«l
lack call, llooc 1 I
aloaaa II la clar. 20-all >VC.
(-la. vllilll.4 It-la. wuo>-
clay callacclaa aaclal clay.
• laaa al laala« 4-la. ia|>aall
laa. la J»-la.
OriM* *nd bulk
I'lua* vpoi- 4-ln. awlk
^••* aa aolI* auMi
SI* Ho. II !><>. .1 !...,
10 * a»i« a«c-
(•na. kilcka. aoaMllata alllfa.
»ell. alaallcl «lifa «llh kui aluJi«a
fo lilt
laclr loan
Ucllxl
Il«4af4iala
•aalal acliMirh
ol 4-la. W la
I II al aaa«>
•oil. aialna la
4-11 caavlala
rlaar al canlar
II aaaay rlay
ullar. lit all
, I II
lar. II la.
oakwll
UIH| aallal*.
lhl»"
layara
ll'luld* kwa«
Nurbania I ll tmihcd
till
Kail
-------
or\
Ln
Ilia
II lacauelef all
II lacavalaa*
liaagkaa
1) lice»elee
If erKhea
J» Iaca«ala4 fll
NullUall
:i«|la-cell
(eailllple
Slatla-cell
Irene heel
Slna.le-t.-ell
Irene hea)
Sln.le call
lane)!1!!! lauaJIM
Surface Surface l*eulh/
U 1 to \ at 10 II
•41 > cell
lull • I'll II 1 a> 1* ll
(wne irancfcl
100 • 100 ll la. !.'-!/ ll
(oa« lianch)
IM • 100 li -I ac 11 II
>«0 • (00 ll a.V ac i: II
Interior
Sla^a Sluaea auagra^e
(V w« H) Halailela
1 -a l.t Clap, ellip
•aaV
1 »« 1 i flay, allly
rlay. allly
aan4
1 on 1 M lly clay
over line
allly aaaj
1 i>n 1 f^lay. aandy
i lay. allly
aani!
1 UH 1 Ml IK. eanj.
Thick clay
at aaae
tealh I*
»•••' llaar la.ce
« 4-/I itmaUtt clar rea*a«tea' la
»-la Mile
rieioaM-rlc » ll ol li-pla>e or 1 I. al
airrlaca al leawJaVal rlay-rlth e-/l
2-) ll
rieloMlrlc t ll ol la-alaie clay rick aall
eurlace al
1-) ll
10 ll
' l«*fc 4*ltt 1 IOA ^ ft uf r l«p
HUrE, ihln t«y*( ul il«T (but-
It DA
II «*
Trtnchaa
ralla
Irtntliaa
calla
Tranchaa or
calla
Tranchaa Of
c.lla
lf> • lib ll.
lypUal call
or I reach
I. fa> • loo 11.
lyeltal cell
or I rent a.
I.OUO * 12% ll.
typical
1. /Ul - 1W 11 .
typical
4U II
II II
Ml II
Natural clay
-------
o>
Ill* t«Mll«la •!•.! C*«f S.«c.
•*>. Collacfla.) Syrian lAacandlnf Uidar)
21 ft-lH. PVC .If* NajM ataMMd laa-
•••ry IM ll a* aMtlM !• «b*««-
.Ida. Mil «.!!«• *,Ild*. rvc *oll. |i*4.«4
l*allla a* flllvr
It Moo* HA
J) Bc.laplpa In «.co«.«cl.d >l*y,
11 lli.lnplp* In NA
aauiaal 1 1* <
It Uialnplf" In a*cu^*«.l«d clay.
M/aw-f dral. layir.
c«co*ja.cl*d clay.
Una; Waal* layai rri»aa.aa Hindi ln| laoraxdlil •
liquid* and bul» (diuaa *l*c««d llfla pi far liquid! dual . - .d •/ kulb .ulld
•vllda )-da*p «li.r* bulk waaia llcjlda waalt b«l««*n
HA tulk al.bllli*d fl.allr. t*ui>4. ~l) ll Und HA Liquid. «l*.d Nona
llqulda and varluua 4:on- duaiaad on «/nal l*« aull
aolld «••!• lalnvl*. «l**p Ja<*> and apr**d
Cluahad diuaja Inlu fill
liquid* *nd ful liquid* alctd v/i^.*i(l
•olid vaala kiln dual ui
lly **r, In
pit. lh*n
*M,W*d 10
Uiulllll
;-M Druai* of .!•• lula v*al** HA NA liquid* .(«- NA
blllt*d na.l.a bllli.d by
pl*c* *oll.
/| kula *labllliad. baal* *nl 1 , / 1 All «**ta. 1 Iqtildn at* l^'tb In.
U«* * b*foi* dla- .lalnfi pic* bulb wa*l*
l*y f* pontttuo with c*Minc
«du nctd kiln dull
•cr •• 01 fly *ah
c« 1 1 on
IUI pU»««d tulk <•.««• HA HA HA NA II In.
H Ucu». of nolld fpiy dru*w HA HA Vlou.l In- Hona
>*M*. bulb ap*cilon.
„,.,, llquldt
d*c*n'«d
flo. drua*
.'I tol. U..I. HA HA HA »« '•' '••• •»"
HA Uiu.. ol .olid K» HA HA »'— ••*' * '"' • ""
...la. bulk , f •';
liquid*
F«l«cl«d
laal
lIllliM
HA
I9MI
IMII
Kill
I'll!
( At 1 1 v
IMk
-tin
If «0
IMO
ItW
-------
A-I. Hum UuW)
—Ki^Ki—
S.'J« HOM<
IV o> U)
IK*
JSi
Surf«c«
ll.fl I./
M.I.,
In4lvl4u»l Illu
1*0 • IM II
.IIIW
11 IU
11 M
1* IU
Tr«t»ch*« or
c.ll.
l«0 • ISO ll
%00 • 100 11
t)»l»l lilt
HA
10 II
-------
Ml* tMclMt*
M •r*l«»lM I* •*•*. •*
Hul
Ce»«f il«
II
•Mtxr
<••!
lai«r*«4U(« C«a«lrMCt
C»»«c ltll«»
(•Ik »•>•
-WM
II •»!•»If* •• 10-11
c«M«r«. Ill gr«v«l
U
II
M
IJ-ln.
••II
II
M
II
MI
t In x.11 -l».>
I fa* drtw* «n<
UMIIIUO
lei
-------
APPENDIX R
CLASSIFICATION OF CEOMEMBRANES (reprinted from Proceedings,
International Conference on Geomembranes,
Denver, Colo., June 20-24, 1984)
Geonembrane:
Synthetic membranes, polymeric membranes, flexible membrane liners, plastic
liners, and Impervious sheets are n. few examples of the many names given to
these relatively new materials. Although m-iny users of these materials often
prefer to use trade names, this practice is deeded inappropriate because it
creates considerable confusion.
Geomembrane is Che generic term proposed to identify these liner and barrier
materials. Geomembranes are impermeable membrane liners and barriers used in
civ^l engineering for geotechnical projects. They can be either sprayed on a
surface or prefabricated and transported to the construction site. Sprayed-on
geomembranes are composed predominantly of asphalt. They are either sprayed
directly on a surface (earth, concrete, etc.) or onto a geotextile. Prefabri-
cated geomembranes are usually composed of synthetic polymers, elastomers
(rubbers), or plastomers (plastics); some are reinforced with a fabric. There
are also prefabricated asphaltic geomembranes.
Classification of Geomembranes
Geomembranes can be classified according to production process and
reinforcement:
1. Made In situ, non-reinforced geomembranes are made by spraying or other-
wise placing a hot or cold viscous material directly onto the surface to
be lined (earth, concrete, etc.). The non-reinforced geomembranes made by
spraying are called "sprayed-on (or spray-applied, or sprayed in situ)
non-reinforced geomembranes." Typical materials used are based on
asphalt, asphalt-elastomer compound, or polymers such as polyurethane.
Due to the spray application, the final thickness of such geomembranes is
not eas.y to control and may vary significantly from one location to
another. Typically, required thicknesses range between 3 and 7.5 mm
(120 .-md 300 mils).
2. Made in situ, reinforced geomembranes are made by spraying or otherwise
placing a hot or cold viscous material onto a fabric. The reinforced geo-
meatbranes made by spraying are called "sprayed-on (or syray-applied, or
sprayed in situ) reinforced geomembranes." Typical materials used are the
sa-ne as for the made in situ non-reinforced geomembranes described above.
Typical fabrics used are the needle-punched nonwoven geotextiles because
they can absorb viscous materials. As discussed Above, the final thick-
ness of such geomembranes is not easy to control. Typically, required
thicknesses range between 3 and 7.5 mm (120 and 300 mils).
Giroud, J. P. and Frobel, R. K. "Geotnembrane Products" Geotechnical Fabrics
Report, Vol I, number 2 (1983).
69
-------
3. Manufactured, non-reinforced geomembranes are made in a plant by extrusion
or calendering of a polyneric compound, without any faSrlc reinforcement,
or by spreading a polvwer on a sheet of paper removed at the end of the
manufacturing process. Typical thicknesses range from 0.25 to 4 mm (10 to
160 rails) for geomembranes wade by extrusion and 0.25 to 2 mm (10 to
80 mils) for geotnembranes made by calendering. Typical roll width for
geomembranes made by extrusion is 5 to 10 m (16 to 33 ft), although some
are narrower. Tynical roll width for geowembranes made by calendering is
1.5 m (5 ft), with some manufacturers producing 1.8 to 2.4 m (6 to 8 ft)
vide rolls.
4. Manufactured, reinforced geomembranes are made in a plant, usually by
spread coating or calendering. In spread-coated geomerabranes, the rein-
forcing fabric (woven or nonwoven) is impregnated and coatee on one or
both sides with the compound, either polymeric or asphaltic. In calen-
dered reinforced geomembranes, the reinforcing fabric is usually a scrim.
Calendered geomembranes are always made with polymeric compounds and are
usually made us of three plies: compound/scrim/compound. Sometimes they
are made of five plies: compound/scrim/compound/scrim/compound. Geomem-
branes with additional plies can be made on a custom basis. Typical
thicknesses of asphaltlc spread-coated geomembranes ars 3 to 10 mm (1/8 to
3/8 inch). Typical thicknesses for polymeric spread-coated and three-ply
calendered geomembranes are 0.75 to 1.5 mm (30 to 60 mils). Typical
thicknesses for five-ply calendered geomembranes are 1 to 1.5 mm (40 to
60 mils).
5. Manufactured reinforced geomembranes laminated with a fabric are made bv
calendering a manufactured geomembrane (usually a non-reinforced geomem-
•brane previously made by calendering or extrusion) with a fabric (usually
a nonwoven) which remains apparent on one face of the final product.
Classification of Geomembrane Polymers
(National Sanitation Foundation):
1. thermoplastics' Polyvinyl Chloride (PVC); Oil Resistant PVC (PVC-OR);
Thermoplastic Nltrlle-PVC (TN-PVC); Ethylene Interpolymer Alloy (EIA);
2. Crystalline Thermoplastics'- Low Density Polyethylene (LDPE); High Density
Polyethylene (HOPE); High Density Polyethylene-Alloy (HDPE-A); Poly-
propylene; Elasticized Polyolefin;
3. Thermoplastic Elastomers' Chlorinated Polyethylen (CPE); Chlorinated
Polyethylene-Alloy (CPE-A); Chlorosulfonated Polyethylene (CSPE), also
commonly referred to as "Hypalon;" Thermoplastic Ethylene-Propylene Dlene
Mono««r (T-EPDM);
4. Elastomers'- Isoprene—Isobutylene Rubber (IIR), also commonly referred to
as Butyl Rubber; Ethylene-Propylene Diene Monomer (EPDM); Polychloro-
prene (CR), also commonly referred to as "Neoprene;" Epichlorohydrin Rub-
ber (CO).
70
-------
APPENDIX C
FIELD EXPERIMENTAL EXAMPLE OF SETTLEMENT ANALYSIS
BY STANDARD CONSOLIDATION THEORY
RETAINING STRUCTURES AND FILL PLACEMENT
An experimental paper-mill sludge landfill was cons.ructed and monitored
for a 2-year period to obtain engineering information essential to developing
procedures for the design and operation of pulp and paper-mill waste land-
fills.* The landfill site was an old gravel pit. The experimental fill con-
sisted of two sludge layers, initially 10 feet thick, with 1-foot-thick sand
drainage blankets at the top, middle, and bottom. An earth dike provided lat-
eral confinement of the sludge, and a surcharge load consisting of 3 feet of
natural soil was used. A lysimeter study provided information on changes in
quality of the leachate when passed through selected natural soils. Fig-
ures C-l and C-2 show the landfill in a plan view and typical cross section,
respectively.
SLUDGE MATERIAL
The dewatered sludge used in the landfill had the physical properties
shown in Table C-l. The Consistency Limits are the water contents at the
liquid and plastic limits, respectively. These properties were determined
from -sample!; taken at various elevations as the sludge was placed. Therefore,
the properties represent the initial, as-placed sludge conditions.
CONSOLIDATION AND SETTLEMENT
Figures C-3 and C-4 give th«j initial average effective stress, P'
-» o
- 138 pound/foot' for each 10-foot-thick layer. The total load acting on the
lower sludge layer, AP, , is calculated as follows:
lower
Weight of sludge (design thickness) above lower layer » 10 ft
x 70 lb/ft3 - 700 lb/ft2
Top sand layer weight - 1 ft * 100 lb/ft3 - 100 lb/ft2
Surcharge weight - 3 ft * 130 lb/ft3 - 390 lb/ft2
AP. w - (700 + 100 + 390) lb/ft2 - 1,190 lb/ft2
Average effective stress P}ower " P0 + APiower " (I38 + 1'190) lb/ft;2
- 1,328 lb/ft2 - 0.664 ton/ft2 s: 0.64 kg/cm2
* From Ledbetter. Richard H., "Design Considerations for Pulp and Paper-Mill
Sludge Landfills," EPA 600/3-76-111, December, 1976.
71
-------
The total load acting on the upper layer, PUpper » is tne weight of the sand
blanket and surcharge. The sand blanket weight Is Included In P' . There-
32°
fore, AP - 3 ft « 130 Ib/foot - 390 Ih/foot - surcharge weigh: . The
upper 7
average effective stress is P' - P1 + ^P „ * (138 + 390) Ib/foot~
upper o upper
2 ' 2
« 528 Ib/foot - 0.264 ton/foct" 0.264 kg/cm .
Figure C-5 shows the consolidation characteristics for the sludge used i;.
the experimental landfill. Using the settlement equation
(C-l)
the primary settlement for each layer can be calculated as follows:
Lower layer properties.
C - 1.65
c
HC - 10 ft
e - 4.85 at P*
o o
P' - 138 lh/fr2
o
.:P, - ;ir- ib/ft2
lower
/ •>'
(1.65)(10 ft) /, 1328 lb/ft~
"Blower " l^-85 P10 138 Ib/ft2
- 2.82 ft * 0.9833
* 2.77 ft - 33.28 in.
Upper layer properties.
C -
c
H -
t
e m
o
P1 -
o
AP
upper
1,65
10 ft
4.85 at P'
o
138 Ib/ft2
2
390 Ib/ft
-------
AH _ . n.frsuio ft:)
upper
Pri I -t- 4.85~
A:s ih/ft2\
\13S Ih/ff/
- 2.*2 ft « 0.582
• I.h4 ft - 19.72 in.
Secondary settlement, defined as
*H,ec - CaHt
vrtiere C » coefficient of secondary compression, from lab
t - time for which settlement is significant
t" « tine to completion of 100 percent primary consolidation.
can be calculated ns follows:
Lower layer C - 0.018 from Figure C-5 laboratory tests con" spending to
3
P.' - 0.664 ton/ft2
1 owe r
H - !0 ft
For one cycle of log *lme
AH « C H
sec. a t
lower
0.018 x 10 ft - 0.18 ft
• 2.16 in.
I'pper .'aver C » 0.016 from Figure C-5 laboratory tests corresponding to
a
P' « 0.264 ton/ft2.
upper
HC - 10 ft
For one cvcle of log time
AH « C H
sec a t
upper
» C.C16 « 10 ft - 0.16 ft
- 1.92 in.
Total settlement for the landfill is calculated as follows:
73
-------
Lower Inver AH - AH . •»• AH . 33 2a <„ A , ,
lower P sec 16 In>
- 35.44 In.
Upper laver ^^ , ,9.72 in. * 1.92 in. . 21.M lp>
upper
Total for the landflJ1
'"total * ^total. + ^total
lower upper
- 35.44 in. + 21.64 in. - 57.08 in. - 4.76
ft
74
-------
TABLF C-l. PHYSICAL PROPERTIES OF PAPER-MILL SLUDGE
Sludge Sample
Elevation Consistency
ln Lav°r Limits
N'o. ft (LL-PL)
1-0 5 325.4-141.6
L-t* 2.5 257.3-102.7
L-2* 7.5 247.7-105.6
U-l** 2.5 184.5- 86.0
U-2** 4 218.5-101.6
U-3** 5 297.5-133.0
U-4** 7.5 287.4-122.1
U-5** 10 302.8-138.6
Ash
Content
percent
35.7
42.2
43.3
59.4
46.5
36.5
34.2
32.2
Solids
Content,
Percent
bv Weight
28.5
27.2
28.2
34.4
31.9
26.9
29.0
28.4
Specific
Gravity
of Solids
2.01
2.05
.2.07
2.24
2.07
1.91
1.87
1.92
* Average of three samples.
** Average of three tests per sample location.
Sludge unit weight as placed, y * 70 pcf.
Soil surcharge unit weight, Y a: 130.4 pcf.
Laboratory test sample locations.
Laboratorv test
.:• •• - -
^^ss
XX" i-'cper sluice
$$^^
sample locations.
x^
layer V\
^N>
, 'f. U-5
! X U-2
X. U-2
' -f. u-i
\
v\
'
-------
EXISTING GRAVEL
PIT WALL
' /L—J~—GRAVEL POCKET \
LIMITS OF BOTTOM
SAND BLANKET
LEGEND
* INSTRUMENT GROUPS
EARTH SURCHARGE LIMIT
BOTTOM SANO BLANKET LIMIT
• ELEVATIONS
Figure C-l. Experimental landfill, pl?.n view.
76
-------
INSTRUMENT GROUP 4 ^ INSTRUME A T GROUP 5~
EARTH SURCHARGED \
14-OH 20-
1
UPPER SLUDGE LAYER
(INITIALLY 10'THICK)
WWIDE GRAVEL DRAIN ^»v«N
NOR TH SIDE ONL YJ
I LOWER SLUDGE LAYER
(INITIALLY 9.6* THICK)
PIPE
v...
« 0A4//V />//>f WOA r« SIDE ONL Y)
LEGEND
SETTLEMENT FLAST
PIEZOMETERS
SAND DRAINAGE BLANKETS
GRAVEL
Figure C-2. Typical cross section of experimental landfill.
-------
00
to
TOTAL VERTICAL
1 FT SAND. 100 PCF STRESS
s ' '
10 FT SLUOG
70 PCF
' / y >
/ y / y
' ; / ;
,' ] $\>-""2
^ y 4SOPSF\.
J) » \
' SOOPSF \
1 FT SAND. 100 PCF
Figure C-3. Load-depth d
LOAD INCREMENT. AP
Mil 11 111 111
DEPTH Z. FT
J W 0
1 FT SAND. 100 PCF
/ y / ^ y ) f
10 FT SLUDGE
y 70 PCF ' ^ ^
/^ ;ol
y x y
/v/^,1
1 FT SAND. IOO PCF
0
lagr
/OO
138
PS'F
176
•"•• •
HYDROSTATIC EFFECTIVE STRESS.
PRESSURE SLUDGE
\ U = 7W « 2 >00 PL - P0 - 0
\ PSF
312\ 13A
PSF \ >SF\
624, "IF \ 176
am.
EFFECTIVE STRESS. P'0
AP
AP
*-»>
Figure C-4. Load Increment added to a sludge layer.
-------
5.o
45
4.0
I 3.5
o
>
3.0
7.5
2.0
0.015 —
0.005
a
o
0.03
5£
iw 3
9 g o.ois
ik
ik >
o 0.010
••
f0' 138PSF » 0.069 T/FT*
•o " 4 85
Ce • 1.65
i iii it11
OM 0.1 0.2 0.4 04 1 23
0.06 0.1 OJ 0.4 0.6 1 23
0.05 0.1 0.2 0.4 04
PRESSURE*. T/Fr2
2 3
Figur» C-5. Consolidation characteristics of sample sludges.
-------
APPENDIX D
CONSOLIDATION EQUATION
The consolidation equation may be expressed as
THc2
: " ~Cv~ (D-D
where
t - time required for consolidation
T • a dimensionless time factor
He • length of the longest drainage path
Cv - coefficients of consolidation
The coefficient of consolidation may be expressed in the form
(D-2)
•VTW
where
k - coefficient of permeability.of the soil medium
e - void ratio of the soil medium
-de
3v * "do ' n"8*tlve slope of the void ratio versus pressure relationship
for the soil in question
Substituting Equation D-2 into Equation D-l ,
>• e)
Differentiating the well known weight volume equation
G rw
where
GS - specific gravity of the soil solids
>d -' dry density of the soil
yields
YW
/D ox
(D~3)
de . -c
Finally substituting Equations D-4 and D-5 into Equation D-3 gives
80
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