United States
Environmental Protection
Agency
Risk Reduction Engineering
Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-91/022 July 1991
EPA Project Summary
State-of-the-Art Field Hydraulic
Conductivity Testing of
Compacted Soils
Joseph O. Sai and David C. Anderson
The congressionally mandated per-
formance standard for soil liners of haz-
ardous waste management facilities is
a hydraulic conductivity of 1 x 10* m/s
or less. In response to this statutory
requirement, the U.S. Environmental
Protection Agency (EPA) has issued
guidance requiring that facilities dem-
onstrate this hydraulic conductivity in
field tests.
Hydraulic conductivity test methods
currently used on soil liners were evalu-
ated for their ability to meet the mini-
mum requirements for field tests, i.e.,
that the test be capable of measuring
hydraulic conductivities of 1 x 1Q-9 m/s
or less and that the values obtained be
representative of the overall soil liner.
Few methods are capable of meeting
these minimum requirements, and even
fewer are both practical to use and
rarely give false low values. Based on
the advantages of all the methods
evaluated, the best and most practical
currently available technologies for
evaluating hydraulic conductivity are
large single-ring infiltrometers and
sealed double-ring infiltrometers. If cor-
rection factors are needed to bring the
values obtained with single-ring devices
to below 1 x 10-8 m/s, confirmatory tests
should be conducted with sealed
double-ring infiltrometers.
the size of infiltrometers used on
soil liners should be at least 2 m2. In
addition, at least three separate tests
should be conducted on each test fill
to allow characterization of the spatial
variability in the soil liner.
A long-term study is needed to allow
a comparative evaluation of candidate
hydraulic conductivity testing devices.
A large collection lysimeter should be
incorporated into the study to give the
true overall hydraulic conductivity value
with which other values should be com-
pared.
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 back).
Introduction
In the Hazardous and Solid Waste
Amendments (HSWA) of 1984, the Con-
gress of the United States mandated that
where compacted soil liners are used, their
hydraulic conductivity shall be 1 x 10'9 m/s
or less. In response to this statutory per-
formance standard, EPA issued guidance
requiring all proposed soil liners in haz-
ardous waste management facilities to
demonstrate this hydraulic conductivity in
field tests. The intent of this requirement
was to obtain accurate and realistic evalu-
ations of how the compacted soil liner
would perform under field conditions. Nu-
merous studies have demonstrated that
values obtained in laboratory hydraulic
conductivity tests are not reliable indica-
tors of the performance of soil- liners un-
der field conditions. No single study has
been conducted, however, to document
the range of field hydraulic conductivity
test methods capable of adequately evalu-
Printed on Recycled Paper
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ating the field performance of a compacted
soil liner.
Field hydraulic conductivity tests can
damage soil liners in several ways. Dam-
age can occur in the form of holes drilled
or trenches cut in the liner to facilitate the
installation of testing equipment. Also,
because field hydraulic conductivity tests
can take as long as sever.al weeks to
complete, both climatic events and weath-
ering processes can substantially damage
a soil liner. Consequently, the EPA has
recommended that field hydraulic conduc-
tivity tests be conducted on a test fill.
Both the test fill and the hydraulic conduc-
tivity tests used on the test fill should
provide a sound basis for meeting the
following objectives:
1) Accurately measuring hydraulic con-
ductivities of 1 x 10"9 m/s and lower.
2) Obtaining hydraulic conductivity val-
ues that accurately represent
the properties of the full-scale soil
liner.
The hydraulic conductivity values ob-
tained must be representative of the full-
scale soil liner. The representative el-
ementary volume (REV) of a soil liner is
the smallest volume above which the vari-
ance no longer decreases significantly. A
REV will be different for every liner and be
highly dependent on the natural variability
of the soil material and the level of quality
assurance exercised during construction
of the soil liner. At least three field hy-
draulic conductivity tests will be required
to define the REV for a given soil liner.
The available field hydraulic conductiv-
ity test methods for soil liners discussed
herein are currently used and readily avail-
able for determining the hydraulic conduc-
tivity of soils compacted in the field.
Field Hydraulic Conductivity
Methods
A wide variety of field hydraulic conduc-
tivity methods have been published in the
scientific and engineering literature. Many
of these methods are not adequate for
evaluating soil liners because they can
neither measure very low hydraulic con-
ductivities nor evaluate a large enough
area to give values representative of the
overall liner. Other methods are adequate
to obtain representative values on low hy-
draulic conductivity soils, but they are com-
plex and time consuming or require a large
amount of time from skilled equipment
operators.
Air-Entry Permeameters
Air-entry permeameters give rapid field
measurements of vertical hydraulic con-
ductivity in initially unsaturated soil. This
method uses Darcy's fundamental law for
water flow through soil to determine soil
hydraulic conductivity above a water table.
The air-entry permeameters method can
rapidly measure field hydraulic conductiv-
ity values as low as 1 x 10'9 m/s. One
hour or less is needed to run the test,
depending on the temperature of the wa-
ter and the hydraulic conductivity of the
soil. A major shortcoming of this method,
however, is that it tests only small areas
of soil (0.3 m2) and therefore may not
reflect the effect of soil macropores on
hydraulic conductivity. Soil rnacropores
that are widely spaced in a soil liner can
have pronounced effect on hydraulic con-
ductivity.
Air-entry permeameters can be used to
obtain the vertical hydraulic conductivity
of one lift in a compacted soil liner or the
entire thickness of soil liner. The data
may be inaccurate, however, either be-
cause of inadequate sealing between the
cylinder and the soil or because of errors
in determining the exact locations of the
wetting front. Air-entry permeameters are
easy to transport and to use. Also, the
method is not labor-intensive (two men
can set up the equipment and make the
measurements).
Borehole Methods
Several borehole methods have been
used to measure the hydraulic conductiv-
ity of soils. Borehole permeameters are
essentially in-hole constant-head or fall-
ing-head permeameters. The methods
involve measuring the steady-state infil-
tration rate of water into unsaturated soil
from a cylindrical borehole. Two borehole
methods that have been used to test the
hydraulic conductivity of soil liners are
Guelph and Boutwell permeameters. The
Guelph permeameter uses a Mariotte si-
phon to maintain a constant head of water
within the borehole. The Boutwell
permeameter involves an evaluation of the
flow into a cased borehole and an evalua-
tion of subsequent flow after an uncased
extension has been added to the hole.
Guelph permeameters have not been
widely used to determine hydraulic con-
ductivity on compacted soil liners.
Studies conducted on low hydraulic con-
ductivity soils indicated, however, that the
instrument is capable of measuring values
as low as 2 x 10'9 m/s. The Guelph
permeameter only evaluates a small area
of soil (3 x 10-1 to 2 x 10'3 m2). For this
reason, the method is unlikely to give val-
ues that are representative of the overall
field hydraulic conductivity.
Boutwell permeameters can measure
hydraulic conductivities of 1 x 10'9 m/s
and less. The volume of soil tested in a
single borehole, however, is relatively
small. Because of the small size of the
test area, soil macropores and other flaws
in soil liner construction may be missed
with this method. As a result, hydraulic
conductivity determined by this method
may not be representative of the actual
field value.
Multiple tests could be conducted to
evaluate a larger aggregate'area. For sev-
eral reasons, however, running many small
tests may not achieve the desired test
objective of obtaining a representative
measure of the field hydraulic conductiv-
ity. For example, even if many tests were
conducted, the small scale of each indi-
vidual test would greatly increase the prob-
ability that through-going macropores
would be truncated. Such truncation of
through-going macropores would signifi-
cantly reduce the hydraulic conductivity
below the actual field value. Other prob-
lems include the large amount of potential
smearing per unit volume pf soil and the
difficulty that may be encountered in dis-
tinguishing between defective test results
and results reflecting the value of a small
area of liner with a macropore.
Porous Probe Permeameters
Porous probe permeameters typically
consist of a cone-shaped porous probe
that is either pushed or driven into the
soil. One such commercially available
porous probe, the BAT permeameter,' can
measure hydraulic conductivities of 1 x
10"9 m/s or less. Because of the small
volume of soil tested :by the BAT
permeameter, the method is unlikely to
yield values consistently representative of
the overall hydraulic conductivity of the
soil being tested.
Ring Infiltrometers
In ASTM Method D 3385, double-ring
cylinders are used to determine the rate
of infiltration of water into soils. This
method has been widely used for more
than a decade for the evaluation of both
percolation rate in septic fields and infil-
tration rate in soils to be irrigated. Two
open cylinders, one inside the other, are
driven into the soil and partially filled with
water. A constant water level is main-
tained on the soil by continuously adding
water. The volume of water added in a
given period is used to determine the rate
of infiltration.
• Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
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The ASTM double-ring infiltrometers
should not be used to measure infiltration
rates on compacted soil liners. This
method is both difficult to use and unreli-
able in soils with a hydraulic conductivity
less than about 1 x 10'8 m/s. Also, loss of
water due to evaporation may be higher
than the quantity of water permeating the
soil.
Modifications have been made to the
basic ASTM double-ring infiltrometer to
make it more sensitive for measuring low
infiltration rates. These modifications in-
clude the use of a large outer ring and
sensitive devices for measuring water
level. With these modifications, the ASTM
double-ring device has been used to mea-
sure infiltration rates of 9.7(± 8.1) x 10'10
m/s. The ability of the test to obtain repre-
sentative values is questionable, however,
because of the relatively small size of the
inner ring (0.3 m in diameter). Macropores
that can affect the infiltration rate by or-
ders of magnitude could have a wider
average spacing than the diameter of the
inner ring.
Box infiltrometers use a sealed box and
a relatively complex standpipe arrange-
ment to measure infiltration into soil in a
0.36-m2 area. They have been used to
measure hydraulic conductivities as low
as 5 x 10'10 m/s.
A single-box infiltrometer coverage (0.36
m2) is not sufficient to determine the over-
all field hydraulic conductivity. One study
of box infiltrometers used arrays of four to
allow definition of both the overall field
hydraulic conductivity and the spatial vari-
ability of the liner. The aggregate area
evaluated by the infiltrometers (1.44 m2)
appeared to be sufficient to obtain repre-
sentative hydraulic conductivity values.
The test site was small (8 m2), however,
and it required greater effort per unit area
to produce a uniform soil liner than is
typical for full-scale liners. Consequently,
although an aggregate area of 1.44 m2
may have been sufficient to characterize
the spatial variability of the test site, a
larger area probably would have to be
tested under conditions more typical of
full-scale liners. The box infiltrometer is
relatively difficult to use and would defi-
nitely require skilled personnel to install,
monitor, and collect and analyze data.
A single-ring infiltrometer consists of a
metal cylinder (20 to 60 cm in diameter),
which is pressed or driven into the soil
(Figure 1). Infiltration is measured by
ponding water inside the cylinder and then
(1) measuring the rate at which the free
surface falls, or (2) measuring the rate at
which water must be added to maintain a
constant depth in the cylinder.
Large single-ring infiltrometers can mea-
sure hydraulic conductivity values as low
as 1 x 10'9 m/s. If the ring is of sufficient
diameter, the hydraulic conductivity val-
ues obtained should be representative of
the overall soil liner. Until there is a
definitive data base documenting the mini-
mum ring diameter needed to obtain these
representative values, a total area of at
least 2 m2 within the ring is recommended.
Errors can be introduced into the hy-
draulic conductivity values as a result of
lateral flow and evaporative losses. There
is no danger of falsely concluding that a
soil liner meets a specific performance
standard, however, as long as the
uncorrected hydraulic conductivity values
are less than 1x1O'9 m/s. If a correction
factor must be used to reduce hydraulic
conductivity below 1 x 10/9 m/s, additional
tests should be conducted with either a
double-ring infiltrometer or by using mea-
sures to eliminate evaporative losses.
Single-ring infiltrometers have the ad-
vantage of being simple and inexpensive.
Care should be taken, however, to make
sure that thermal expansion of the water
does not yield false low values. Other
possible sources of false low values are
the interception of rainfall by the open ring
or incorrect calculation of the hydraulic
gradient. These sources of error can be
avoided by using a sealed ring and a free-
draining layer beneath the known thick-
ness of a soil liner, respectively.
Sealed double-ring infiltrometers (SDRI)
use a sealed ring or box that measures
infiltration into a relatively large area. The
SDRI has a sealed inner ring that permits
measurement of low infiltration rates and
minimizes problems with temperature fluc-
tuations and evaporation. It is relatively
easy to operate, but as with all infiltration
ring devices, it must be carefully installed
to ensure that no leakage occurs around
the edges.
The SDRI consists of an inner ring and
an outer ring (Figure 2). The fiberglass
inner ring has a sloped top that extends
only 12 cm above the liner surface, and
the outer ring is used to pond water around
the inner ring to ensure only vertical per-
colation is measured. Flow is measured
during an infiltration test by using a flow-
measurement bag attached to the
infiltrometer. Any water flow out of the
infiltrometer into the ground is replaced by
water from the bag. Flow measurement is
initiated by filling the bag with a known
weight of water, connecting it to the
infiltrometer, and periodically retrieving and
reweighing it.
The SDRI can measure hydraulic con-
ductivities of less than 1 x 10'9 m/s. The
SDRI also is available commercially in a
relatively large size (the inner ring covers
an area of 2.3 m2). Tests conducted with
SDRI's appear to confirm that the equip-
ment can yield values representative of
the overall soil liner.
Although SDRI tests require more in-
stallation time than many methods, peri-
odic recording of time and measuring bag
weight are the only tasks performed after
the equipment is installed. The SDRI
appears to yield high quality data with few
possibilities for yielding false low values.
Collection Lysimeters
Collection lysimeters are placed beneath
the compacted soil liner to collect liquid
percolating through the liner. A pipe is
often installed at the low end of the lysim-
eter to collect and move any accumulated
liquid to an access point where the liquid
can be measured. Lysimeters have been
used to monitor the quantity and quality of
the leachate from landfills in Wisconsin
and Canada.
Collection lysimeters can measure hy-
draulic conductivities of less than 1 x 10'9
m/s. If large enough, lysimeters can also
yield hydraulic conductivity values that are
representative of the overall soil liner. One
study showed a 19.4-m2 collection lysim-
eter to be large enough to yield represen-
tative values. The main drawback of this
method is that it can take months to ob-
tain steady-state hydraulic conductivity val-
ues. Unlike infiltrometers, which give ini-
tially high values that decrease to a low
steady-state value, collection lysimeter val-
ues typically begin very low and gradually
build up to a higher steady-state value.
Consequently, the testing period may well
interfere with facility construction and the
adequacy of a particular soil liner design
may not be known until the end of a long
test period.
Tests comparing collection lysimeters
and large infiltrometers have shown that
values are relatively close. Therefore, if a
collection lysimeter is desired, concurrent
testing of the soil liner with large
infiltrometers is suggested. This proce-
dure could save months of waiting for
results definitive enough to begin construc-
tion of a facility.
Constructing a collection lysimeter is
time-consuming and requires skilled per-
sonnel. Special care must be taken to
avoid damaging the material from which it
is constructed underlying the collection
field. Because such damage could result
in false low hydraulic conductivity values,
concurrent testing of the soil liner with
large infiltrometers is recommended. Ulti-
mately, a large collection lysimeter has
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V V
Figure 1. Open Single-Ring Infiltrometer.
-OUTER RING WALL
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the potential for yielding the most accu-
rate values for quantification of the vol-
ume and rate of liquid that moves through
a compacted soil liner. In practice, how-
ever, the response time of a lysimeter
limits its utility for short-term tests.
Other Apparatus
The applicability of both velocity
permeameters and porous plate
infiltrometers for measuring the hydraulic
conductivity of compacted soils is being
studied. Current data are not sufficient to
determine the adequacy of either method,
but both methods show promise.
Various methods are also available for
obtaining the hydraulic conductivity of soil
cores in the laboratory. Laboratory meth-
ods generally do not provide a reliable
indicator of field performance of clay lin-
ers.
Laboratory hydraulic conductivity tests
can measure very low hydraulic conduc-
tivity values; however, using these values
to represent field conditions presents sev-
eral problems. Because laboratory
samples used to determine hydraulic con-
ductivities are typically too small to have a
representative distribution of the
macropores present in the field, the val-
ues derived are one to three orders of
magnitude lower than the actual field val-
ues.
Conclusions
Widely varying methods are currently
being used to evaluate the field hydraulic
conductivity of soil liners. Only a few of
these methods, however, can reliably meet
the following requirements for evaluating
soil liners:
1) A method should be capable of ac-
curately measuring hydraulic conduc-
tivities of 1 x 10'9 m/s and lower.
2) The values obtained should be rep-
resentative of the overall hydraulic
conductivity of a soil liner.
Of the few methods capable of meeting
these requirements, even fewer are rela-
tively simple to use, rarely give false low
values, and provide definitive results in a
reasonable time frame. Key findings of
each method evaluated are summarized
here.
Air-entry permeameters, velocity
permeameters, porous plate infiltrometers,
borehole methods, and porous probe
methods, can give rapid field measure-
ments of vertical hydraulic conductivity,
but they may not test a large enough area
to give values representative of the over-
all soil liner. The ASTM double-ring
infiltrometers use an outer ring to ensure
that the inner ring measures only the ver-
tical hydraulic conductivity of a soil. This
method cannot measure hydraulic con-
ductivity values of less than 1 x 1 Cr8 m/s
or obtain values representative of an over-
all soil liner. The primary disadvantages
of box infiltrometers are that they are diffi-
cult to use and that skilled personnel would
definitely be required for installing, moni-
toring, and collecting and analyzing data.
Single-ring infiltrometers should cover
an area of at least 2 m2 and care should
be taken not to rely on correction factors
to reduce the hydraulic conductivity val-
ues below 1 x 10'9 m/s. Sealed double-
ring infiltrometers have received substan-
tial field testing and have a demonstrated
capability of measuring hydraulic conduc-
tivity values below 1 x ICr9 m/s and of
obtaining values representative of the over-
all soil liner. The method requires more
installation time than many methods, but
it has the advantages of few ambiguities
in the experimental method and few pos-
sibilities for yielding false low values. Col-
lection lysimeters can be time-consuming
and may require skilled personnel. Ulti-
mately, this method has the potential for
yielding the most accurate values for hy-
draulic conductivity of a soil liner.
Recommendations
To maximize the probability of obtaining
a hydraulic conductivity value that is rep-
resentative of the overall soil liner, a test
method should have the following quali-
ties:
1) The ability to measure hydraulic con-
ductivities of less than 1 x 10'9 m/s.
2) Minimal requirements for skilled per-
sonnel during installation, operation,
data acquisition, data reduction, and
interpretation of results.
3) Few ambiguities in the experimental
method and few possibilities for yield-
ing false low values.
4) Inexpensive enough to allow at least
three replicate tests to determine
spatial variability of field hydraulic
conductivity.
5) Sufficient area of coverage for each
replicate test to ensure that a statis-
tically sound average number of
macropores per unit area is covered.
6) Sufficiently short time required to
conduct each test to ensure that it is
practical for use.
Not all of these qualities are quantifi-
able, given the current state of knowledge
on hydraulic conductivity test methods and
soil liners. It is possible, however, to
pinpoint where additional study is needed
and to suggest the best currently avail-
able methods.
For a better definition of the test meth-
ods that would maximize the probability of
obtaining representative hydraulic conduc-
tivity values for soil liners, a long-term
soil-liner study is recommended that would
incorporate a large collection lysimeter (to
give true overall hydraulic conductivity) and
arrays of candidate hydraulic conductivity
testing devices. The results could be used
to define the reliability of the testing de-
vices and to characterize their sensitivity
to spatial variability within the soil liner.
Ideally, several sized versions of each test
method should be used in each array to
obtain a better understanding of the mini-
mum size requirement for the test de-
vices.
Based on the advantages and disad-
vantages of the methods reviewed in this
document, the following are recommended
as the best and most practical currently
available technologies for evaluating field
hydraulic conductivity:
1) A single-ring infiltrometer covering
an area greater than 2 m2.
2) A sealed double-ring infiltrometer
with an inner ring covering an area
greater than 2 m2.
Until conclusive studies are completed,
correction factors (such as those com-
monly used with single-ring devices)
should not be relied on to reduce hydrau-
lic conductivity values below 1 x 1Cr9 m/s.
Also, correction factors for evaporative
losses should be avoided. If the
uncorrected hydraulic conductivity values
are higher that the maximum allowable
value (1 x 10'9 m/s), confirmatory tests
should be conducted with sealed double-
ring infiltrometers.
Spatial variability of the soil liner must
be characterized to ensure that the hy-
draulic conductivity values obtained are
representative. At least three field hy-
draulic conductivity tests should be con-
ducted, and the values obtained from each
of the three should be 1 x 10-9 m/s or less.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3413 by PEI
Associates, Inc., under the sponsorship of
the U.S. Environmental Protection Agency.
•&U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40049
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Joseph O. Saiand David C. Anderson are with K.W. Brown & Associates, Inc.,
College Station, TX 77840.
Walter E Grube, Jr., is the EPA Project Officer (see below).
ThQ complete report, entitled "State-of-the-Art Field Hydraulic Conductivity Testing of
Compacted Soils, "(Order No. PB91-206243/AS; Cost: $17.00, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer 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
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