United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S-92/024 Sept. 1992
EPA Project Summary
Construction, Monitoring, and
Performance of Two Soil Liners
Ivan G. Krapac, Keros Cartwright, Bruce R. Hensel, Beverly L Herzog, Timothy
H. Larson, Samuel V. Panno, James B. Risatti, Wen-June Su, and Kenneth R.
Rehfeldt
A prototype soil liner and a field-
scale soil liner were constructed to test
whether compacted soil barrier systems
could be built to meet the standard set
by the U.S. Environmental Protection
Agency (EPA) for saturated hydraulic
conductivity (< 1 x 10'7 cm/s). In situ
ponded infiltration rates into the proto-
type liner were measured with the use
of two large, (5-m diameter) sealed
double-ring infiltrometers. The satu-
rated hydraulic conductivity of the liner
was estimated from the infiltration data
to be no more than 3.6 x 10-" cm/s.
Fluorescein and rhodamine WT dyes
were allowed to infiltrate the prototype
liner for 46 days. Dye patterns observed
during excavation of the prototype liner
indicated that lateral flow occurred be-
tween lifts and along the interface be-
tween soil clods. Although the proto-
type liner met the EPA requirement for
hydraulic conductivity, the dye flow
paths indicated a need for better bond-
ing between lifts and for reduced soil
clod sizes to eliminate preferential flow
paths in the liner.
The field-scale liner (7.3 x 14.6 x 0.9
m) consisted of 6 compacted lifts each
15-cm thick. Full-scale equipment was
used for compaction. This liner was
compacted at an average moisture con-
tent of 11.5%, 1.5% wetter than the op-
timum moisture content as determined
by the Standard Proctor test. The mean
dry density of the liner was 1.84 g/cm3,
93% of the maximum Standard Proctor
density.
Based on 1 yr of measurements of
water infiltration into the liner, estimates
of saturated hydraulic conductivities
were 3.3 x 10* by large-ring infiltrom-
eters, 5.3 x 10-8 by small-ring infiltrom-
eters, and 6.7 x 10"8 cm/s by a water
balance analysis. Measurements of soil
tension using pressure transducer ten-
siometers indicated that the wetting
front had reached a depth greater than
20 cm.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at the
back).
Introduction
In 1985, the Illinois State Geological
Survey began a multi-year study to evalu-
ate the procedures used in constructing
and testing soil liners at waste-contain-
ment facilities. To fulfill the study objec-
tives, the movement of water and solutes
through two test liners was monitored. The
project was divided into three phases.
Phase 1, which began in 1985, included
(1) an evaluation of the properties that
make a soil suitable for constructing a
liner and (2) the selection and character-
ization of a soil for use in this project.
Phase 2, which began in 1986, included
the construction of a prototype soil liner to
test construction practices and to deter-
mine if a hydraulic conductivity less than
1 x 10'7 cm/s could be measured in situ
using the soil selected. Phase 3, begun in
1987, included the construction and long-
term monitoring of a field-scale soil liner.
This extensively instrumented liner contin-
ues to be monitored.
n?3) Printed on Recycled Paper
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Procedures
Soil Selection Criteria
Qualitative selection criteria were es-
tablished to compare the construction char-
acteristics of three glacial tills (the Snider,
Plat, and Batestpwn Members of the
Wedron Formation). Representative
samples of the tills were collected from
five locations in Illinois. Numerical criteria
were assigned to hydraulic conductivity,
Atterberg limits, particle-size distribution,
natural moisture content, and the dry bulk
density. One potential material failed the
hydraulic conductivity criterion. Other prop-
erties tested (dispersivity, clay mineral-
ogy, specific gravity, cation exchange ca-
pacity) were not significantly different in
the three tills to serve as selection criteria.
Because of the similarity of their proper-
ties, the final selection of a material to
construct the liner was based on eco-
nomic factors related to the cost of obtain-
ing and transporting the soil. The
Batestown Till, an illitic glacial till with a
(oam texture, was the material selected to
construct the soil liners.
Prototype Liner
A small, 3- x 9- x 0.9-m prototype liner
was built in six lifts. Each lift was con-
structed by spreading a 23-cm thick layer
of foose soil and compacting it to 15 cm.
The loose soil contained some clods and
stones up to 15 cm in diameter, although
an effort was made to remove any clods
and stones larger than 10 cm in diameter.
A padfoot compactor with feet 10 cm long,
capable of delivering 222.4 KN (50,000
tb) of force in the vibratory mode, was
used to compact the soil. Lift surfaces
were scarified after compaction to improve
lift bonding. The liner was compacted at
an average moisture content of 11.7%,
and a dry density of 2.02 g/cm3.
The experimental configuration of the
prototype liner consisted of two, large-
diameter, sealed, double-ring infiltrometers,
with inner rings 1.5-m in diameter. The
Infiltrometers measured in situ infiltration
rates that were subsequently used to esti-
mate the hydraulic conductivity of the liner.
Soil-water monitoring instruments were
also tested in the liner. Monitoring devices
included tens'tometers and gypsum blocks
to measure soil-water tension and pres-
sure-vacuum lysimeters to collect soil-wa-
ter samples for solute concentration analy-
sis. Horizontal and vertical installation tech-
niques for the monitoring devices were
compared to determine which method was
more reliable. Horizontally installed moni-
toring devices were positioned during con-
struction of the liner. A tensiometer and a
gypsum block were placed on the surface
of compacted; lifts 1, 3, and 5, which cor-
responded to, depths of 75, 45, and 15
cm, respectively. In addition, lysimeters
were placed o'n the tops of lifts 4 (a 30-cm
depth) and 5. The wires and PVC tubing
connected to these devices were laid
across the top of the compacted lift in
shallow trenches cut into the compactor
foot pattern and covered with loose soil
before compaction of the next lift. Verti-
cally installed monitoring devices were
positioned between the inner and outer
rings of the infiltrometer after the liner was
constructed. Additional, vertically installed
instruments were placed near each hori-
zontal instrument. For vertical instruments,
holes slightly larger in diameter than the
instruments were cored to the tops of lay-
ers 1, 3, 4, and 5 and the holes backfilled
with a bentonite slurry after installation of
the instrument.
Dyes were added to the water in one of
the infiltrometers to reveal preferential flow
paths in the liner caused by endemic soil
properties and compaction processes. The
dyes were also used to reveal preferential
flow paths around monitoring devices re-
sulting from the installation methods.
Field-Scale Liner
The field-scale soil liner was built to
assess the a|real variability of a liner's
hydraulic properties, to determine the tran-
sit time of water and tracers through the
liner, and to address the feasibility of con-
structing a liner with the EPA's hydraulic
conductivity specifications. Before con-
struction of the liner, the clod size of the
Batestown till was reduced to less than 5
cm in diameter and rocks larger than 5 cm
in diameter were removed. Water was then
added to the soil so that the soil moisture
content would; meet the design specifica-
tion of 1% to 2% wetter than the optimum
moisture content as determined by Stan-
dard Proctor itests. The soil was then
stored and thp soil moisture content al-
lowed to equilibrate for 3 wk.
A static padfoot compactor, with a rated
operating weight of 20,370 kg, compacted
the six lifts of the 7.3- x 14.6- x 0.9-m
liner; each foot was 20 cm long. The soil
for each lift was spread, tilled, and then
compacted to a thickness of 15 cm. The
surface of each lift was scarified to facili-
tate bonding between lifts. The liner was
compacted at.an average moisture con-
tent of 11.5%; 1.5% wetter than the opti-
mum moisture content as determined by
the Standard Proctor test. The mean dry
density of the liner was 1.84 g/cm3, 93%
of the maximum Standard Proctor density.
The liner is enclosed in a shelter to
allow year-round monitoring and to pre-
vent the liner from freezing. A 31 -cm-deep
pond, contained above the liner, was filled
in 1988. An underdrain system consisting
of geomembrane and gravel was built be-
neath the liner to collect any water dis-
charging from the liner. The underdrain
system was built so that the amount of
discharge from each quadrant of the liner
can be collected and measured. In addi-
tion to the underdrain system, pan lysim-
eters were installed beneath the liner in
the center of each quadrant as another
method of measuring liner effluent. Four
large-ring (1.5-m diameter) and 32 small-
ring (0.3-m diameter) infiltrometers moni-
tor infiltration rates at various locations on
the liner surface. The infiltration rates are
used to estimate the spatial variability of
the liner's hydraulic conductivity and pos-
sible scale effects of measurement. In ad-
dition, a different tracer (bromide, o-tri-
fluoromethyl benzoic acid, m-trifluoromethyl
benzole acid or pentafluorobenzoic acid)
was added to each of the large-ring
infiltrometers. We use tensiometers in-
stalled in 12 nests with 6 instruments in
each nest (1 tensiometer in each lift of the
liner) to monitor changes in the moisture
and soil-water tension in the liner. Soil
water samples are collected with the use
of pressure-vacuum lysimeters located in
10 nests of 6 lysimeters. Evaporation pans
are used to measure evaporation rates
from the pond above the liner surface so
that a water balance for the liner can be
determined.
Two models, SOILINER and CHEMFLO,
were used to estimate the time required
for water and tracers to break through the
field-scale soil liner. The ability of these
models to predict transit times through the
liner was evaluated by comparing model
predictions to field measurements. To es-
timate saturated hydraulic conductivity of
the liner, the conductivity value required
by each model to produce a flux value
equal to that measured in the liner was
determined.
The numerical codes of both models
use Richards equation to predict one-di-
mensional flow and transport of a
nonreactive tracer through unsaturated
soils. Input requirements for both models
include (1) a mathematical approximation
of a soil-moisture-characteristic curve, (2)
a mathematical relationship between hy-
draulic conductivity and soil moisture con-
tent, (3) values for saturated hydraulic con-
ductivity and moisture content, (4) upper
and lower boundary conditions, and (5)
initial moisture conditions. In addition,
CHEMFLO allows input of chemical trans-
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port parameters such as dispersivity and
diffusion.
SOILINER can simulate flow and trans-
port in a layered soil system; however,
this code does not incorporate adsorption,
degradation, dispersion, or diffusion into
its particle tracking algorithm. Instead, it
tracks the movement of a particle of tracer
through the system by advection only. In
essence, it tracks the point where the
relative concentration (C/C0) of a
nonadsorbed, nondegraded, nondiffused
tracer is 0.50.
CHEMFLO simulates flow and transport
for one soil layer. The effects of disper-
sion, diffusion, and degradation on trans-
port may be incorporated in this model.
CHEMFLO computes the concentration
profile of the tracer in the soil at regular
intervals.
Three simple analytical solutions were
also used to predict water and solute
movement through the soil liner. The three
methods are a simple, transit-time equa-
tion; a modified, transit-time equation; and
the Green-Ampt wetting-front model. The
input parameters required for each equa-
tion include saturated hydraulic conductiv-
ity, porosity, initial soil moisture content,
and depth of the wetting front.
Results and Discussion
Prototype Liner
The infiltration of water and dyes into
the prototype liner was measured. An av-
erage steady-flux of 1.5 x 10-7 cm/s was
achieved 2 to 3 wk after the infiltration
experiment began. The saturated hydrau-
lic conductivity of the liner was estimated
from the infiltration data to be no more
than 3.6 x 10'8 cm/s, which met the EPA
hydraulic conductivity requirement for soil
liners. Transit time for the wetting front to
reach the liner bottom was calculated to
be about 3 yr.
Water containing fluorescein and
rhodamine WT dyes was allowed to infil-
trate into the prototype liner for 46 days.
Dye patterns observed during excavation
of the liner indicated that flow occurred
between lifts and along the interface of
soil clods. Although the liner met the EPA
conductivity requirement, the dye flow
paths indicated that preferential flow paths
existed in the prototype liner suggesting
the need for better bonding between lifts
and smaller soil clod sizes to eliminate
these paths. These observations sug-
gested that soil processing before liner
construction and rigid adherence to con-
struction QA/QC requirements are neces-
sary if soil liners are to perform according
to design specifications.
Dye observed in the seals of the verti-
cally installed instruments showed no evi-
dence of preferential flow; this suggests
that the technique used to install the in-
struments was adequate for at least short-
term monitoring. Dye movement at lift in-
terfaces was, however, enhanced by pref-
erential flow paths around horizontally in-
stalled instruments. Also many of the hori-
zontal instruments were damaged and ren-
dered inoperable during construction of
the liner. We concluded, therefore, that
installing instruments vertically through a
liner after its construction is the most reli-
able method of monitoring a soil liner.
Field-Scale Liner
Only data collected during the first year
of monitoring the field-scale liner are re-
ported here. Monitoring of the liner will,
however, continue until water breakthrough
occurs at the base of the liner. Analysis of
the first year of monitoring has provided
the following information.
Infiltration properties:
• Average infiltration fluxes were 7.9 x
10-" cm/s, 5.0 x 10'9 cm/s, and 1.0 x
10'7 cm/s for the small-ring infil-
trometers, large-ring infiltrometers, and
pond water balance, respectively.
• Flux data measured by the large-ring
and small-ring infiltrometers formed
two statistically distinct populations.
The small-ring infiltrometer fluxes cal-
culated from cumulative infiltration
curves formed a log normal distribu-
tion; the large-ring infiltrometer fluxes
consisted of four widely-scattered data
points.
• Gedstatistical analysis (Kriging) of the
small-ring infiltrometer fluxes 'esti-
mated a mean infiltration flux for the
entire liner of 7.1 x 10'8 cm/s. Kriged
estimates of infiltration fluxes for each
quadrant of the liner ranged from 6.7
x 10-8to7.1 x 10-8cm/s.
• An isotropic exponential variogram
was found to best model'the spatial
relationship of the small-ring infiltrom-
eter fluxes. Flux data were spatially
uncorrelated at measurement dis-
tances greater than 1.3 m. This analy-
sis, and the small variances exhibited
by the flux data, suggested the liner
was homogeneous with respect to in-
filtration fluxes.
• Hydraulic gradients in the field-scale
liner have fluctuated between 1.1 and
1.7. When steady state conditions are
achieved in the liner, the gradient
should be approximately 1.3.
Saturated hydraulic conductivity:
• Hydraulic conductivities calculated us-
ing Darcy's law were 5.3 x 10'8 cm/s,
3.3 x 10'9 cm/s, and 6.7 x 10'8 cm/s
for the small-ring infiltrometer, large-
ring infiltrometer, and liner water-bal-
ance data sets, respectively.
• Hydraulic conductivities calculated us-
ing the Green-Ampt infiltration model
were 3.8 x 10'8 cm/s, 2.4 x 10'9 cm/s,
and 4.7 x 10'8 cm/s for the small-ring
infiltrometer, large-ring infiltrometer,
and liner water-balance data sets, re-
spectively.
• All saturated hydraulic conductivities,
regardless of the method of calcula-
tion or data set used, were below the
EPA maximum of 1.0 x 10"7 cm/s.
The consistency and reproducibility
of these data among the four quad-
rants of the liner indicate that the
regulatory requirement for the satu-
rated hydraulic conductivity was
achievable.
Predictive methods (modeling):
• The numerical code of SOILINER was
used to calculate the relationship be-
tween flux and hydraulic conductivity.
When observed flux was inserted into
the model, a corresponding hydraulic
conductivity of 5.1 x 10'8 cm/s was
obtained. This value is similar to the
hydraulic conductivity value of 5.3 x
10'8 cm/s (calculated by using Darcy's
law) and 3.8 x 10'8 cm/s (calculated
by using the Green-Ampt) based on
the small-ring infiltrometer data set.
• Transit times were calculated by three
analytical methods provided in the
EPA Technical Resource Document
on liner design, construction, and
evaluation.1 The results estimate the
earliest time at which water will exit
the bottom of the field-scale liner. The
simple transit-time equation, which as-
sumes steady-state saturated condi-
tions, predicted the transit time to be
5.5 yr. The modified transit-time equa-
tion, which adds suction at the base
of the liner to the simple transit-time
equation, predicted water break-
through to be 3.7 yr. The Green-Ampt
model predicted a transit time of 1.3
yr. All these predictions assumed ef-
1 U.S. Environmental Protection Agency, I988a, De-
sign, construction, and evaluation of clay liners for
waste management facilities: Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, EPA/530/SW-86/
007F, 502p.
-------
fectiva porosity equals total porosity
and ignored dispersion and diffusion.
• SOILINER predicted chemical break-
through at 12.6 yr. The model does
not consider the effects of effective
porosity, dispersion, diffusion, attenu-
ation, and reaction. Therefore, mean-
ingful contaminant transport results
were difficult to calculate with
SOILINER.
• CHEMFLO predicted breakthrough of
the tracers between 2.5 and 4.6 yr.
Other findings:
• Tension/head data in the liner ap-
peared to be affected by atmospheric
pressure and temperature fluctuations.
Even after correcting for barometric
pressure variation, we observed a cy-
clic pattern of pressure head: pres-
sure head is greatest in the summer
and lowest in the winter. An increas-
ing time lag with depth in the liner
indicated that the cyclic rise and fall
of pressure head was at least par-
tially caused by changing tempera-
tures In the liner.
• The effects of temperature and atmo-
spheric pressure on the tension data
made exact measurements of a wet-
ting-front depth impossible. The ap-
parent reaction of head values to
changes in temperature suggested
that the liner was saturated to a depth
greater than 20 cm, tension-saturated
to a depth of a least 70 cm, and
unsaturated at its base.
• Tracer data suggested that no prefer-
ential, lateral flow paths exist in the
field-scale liner, either because they
were eliminated during liner construc-
tion or were not intersected by the
sampling devices.
Conclusions and
Recommendations
Standard engineering geology practices
are adequate for sampling and selecting
borrow materials for use in construction of
soil liners. The measured properties of the
soil used to construct the field-scale liner
deviated from initial predictions (based on
field sampling) by less than 10%; densi-
ties were slightly less than estimated; and
plasticity indexes were slightly higher than
estimated by the material selection pro-
cess.
Soil properties must be strictly specified
and quality control rigidly maintained to
ensure that a soil liner will be constructed
to perform according to design criteria.
Specifications for an acceptable soil must
include not only a maximum laboratory
conductivity or in situ hydraulic conductiv-
ity, or both, but should also include mois-
ture content at time of compaction, maxi-
mum clod size, and minimum density and
plasticity requirements.
Soil moisture should be 1% to 3% wet-
ter than the optimum value determined by
a Standard Proctor test. Liner materials
should be processed before liner construc-
tion to ensure a uniform moisture content,
a clod size less than 5 cm in diameter, no
stones greater than 5 cm in diameter and
as few smaller stones as practical.
Construction equipment must be large
enough to fully compact the entire thick-
ness of each lift, and compactor feet must
be at least as long as the compacted
thickness of each lift, preferably as long
as the loose lift plus the thickness of the
loose material resulting from scarifying the
surface of the previously compacted lift.
Compaction 'should continue on each lift
until a prescribed minimum density is mea-
sured at a reasonable number of loca-
tions before proceeding to the next lift.
Transport rates through the liner can be
affected by the physical state of the liner.
In our experiment, tensiometer results sug-
gested that air is entrapped throughout
the field-scale liner. The presence of this
entrapped air can significantly affect wa-
ter movement through the liner. When two
fluids such as air and water occupy the
pores of the :soil, the effective permeabil-
ity of the soil to each is decreased. Effec-
tive permeability to one fluid may be zero
if no interconnected pores contain that
fluid. Thus, the permeability to air may be
zero, not allowing the air to escape, yet
reducing the effective permeability to wa-
ter. This condition can exist until the air is
totally dissolved. This phenomenon can
result in reduced water-transport rates. In-
creased transport rates could, however,
result if the air is trapped in small isolated
pores and water occupies the large pores.
In this case, the reduction in effective per-
meability to water will be insignificant, but
the reduction in effective porosity will in-
crease transport rates. The liner will not
reach "true" steady state until all entrapped
air is dissolved. The effect of these phe-
nomena on the performance of a soil liner
needs to be evaluated.
Liners with low hydraulic conductivities
can contain preferential pathways through
which fluid flow is concentrated. The pro-
totype liner had an estimated hydraulic
conductivity of 3.6 x 10~8 cm/s, yet showed
significant preferential paths; dyes pen-
etrated 30 cm into the liner during the 50-
day test, suggesting that breakthrough
could have occurred at the bottom of the
liner in less than 6 mo. The main path-
ways were horizontal along lift interfaces.
Infrequent fine fractures or other pathways
can carry significant amounts of fluid
through a liner; the occurrence of these
pathways can be reduced only by strict
design, construction, and quality control
standards.
Questions regarding methodologies to
collect in situ infiltration data have arisen
from this research. Differences have been
noted in infiltration fluxes, as measured
by different types of infiltrometers. Pertur-
bations in measurements of infiltration
rates and soil tensions have been corre-
lated with barometric pressure fluctuations,
or temperature changes in the liner, or
both. Continued monitoring of the liner
and further laboratory and field research
may explain these observations.
Land burial of wastes is a commonly
used waste management strategy. Soil
liners are and will continue to be an inte-
gral part of many waste management pro-
grams. When properly applied, designed,
and constructed, soil liners can effectively
contain contaminants so that human health
and the environment are protected.
The full report was submitted in fulfill-
ment of Cooperative Agreement No.
CR812650 by the Illinois State Geological
Survey under the sponsorship of the U.S.
Environmental Protection Agency.
•U.S. Government Printing Office: 1992— 648-080/60057
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Ivan G. Krapac, Kerns Cartwright, Bruce R. Hensel, Beverly L Herzog Timothy H.
Larson, Samuel V. Panno, James B. Risatti, and Wen-June Su are with the Illinois
Stata Geological Survey, Champaign, IL 61820. Kenneth R. Rehfeldtis with the
Illinois State Water Survey, Champaign, IL 61820.
Michael Roulleris the EPA Project Officer (see below).
The complete report, entitled "Construction, Monitoring, and Performance of Two
SoU Liners," will be available from:
(Order No, EG-141; Cost: $4.00, subject to change)
Illinois State Geological Survey
615 E. Peabody Drive
Champaign, IL 61821
or
(Order No. PB92-124049; Cost: $26.00, subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Off leer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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