United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-90/025 Sept. 1990
Project Summary
Relationship of Laboratory- and
Field-Determined Hydraulic
Conductivity in Compacted
Clay Layer
A.S. Rogowski
A field-scale research facility was
constructed to evaluate hydraulic con-
ductivity of a compacted clay liner. The
facility was instrumented at a number of
points to measure infiltration, drainage,
and properties of a clay liner. Design and
instrumentation of the facility were based
on results from the prototype studies.
Preliminary small scale studies have
shown that any perforation of the com-
pacted clay may result in a preferentia!
water flow pathway. To avoid this situa-
tion in the field, changes in density and
porosity were monitored horizontally
across the facility. Outflow from below
the compacted clay was collected in 250
evenly spaced drains, and infiltration
rate was measured in an equal number
of buffered infiltration cylinders. Very
small changes in elevation were mea-
sured with a laser beam apparatus to
evaluate the extent of swelling. Data
obtained during the clay liner construc-
tion showed that although the average
water content and density of compacted
clay were close to design specifications,
spatial variability of values was large.
Infiltration and drainage rates observed
following ponding were poorly predicted
by the prototype studies. The experi-
mental clay liner was ponded for 1 yr.
During that time inflow, outflow, and
changes in density were monitored at
250 locations. Flux density values, com-
puted from observed infiltration and
outflow measurements, were compared
with effective flux density values that
were based on breakthrough time distri-
butions for water and tracer. Results
suggested that both water and tracer
can move considerably faster than ex-
pected through only a small fraction of
the total pore space. A ranked distribu-
tion of laboratory values underestimated
field distribution of hydraulic conductiv-
ity by a factor of 5. However, paired
values from the same location differed at
times by several orders of magnitude.
Although the core samples and nuclear
surface moisture density probe data
adequately described spatial distribu-
tions of water and density within the
compacted clay, neither water content
nor density appeared to be correlated in
any way with the spatially distributed
hydraulic conductivity. One possible
reason for the above lack of correlation
appeared to be a rapid breakthrough of
water and tracer through a network of
preferential flow pathways dominating
flow and transport.
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).
Objectives
The purpose of this study was to evalu-
ate spatial distribution of field-observed hy-
draulic conductivity in aclay linerconstructed
to engineering specifications and to com-
pare it with laboratory values determined on
cores.
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The specif ic objectives of this study were:
(1) to documentthe state-of-the-art of the in
situ hydraulic conductivity determina-
tion on compacted clay soils,
(2) to construct a field-scale test plot area of
recompacted clay, install permeability
measuring devices, and determine the
in situ hydraulic conductivity with the use
of selected permeants, verifying that
accurate data can be obtained, and
(3) to compare field hydraulic conductivity
data with those obtained on laboratory
cores to determine whether significant
lack of agreement in values exists, and
if so, to evaluate the factors responsible.
The initial part of this effort included a
general literature survey and methods
evaluation (Phase I) aimed at selecting
appropriate field-scale procedures to mea-
sure the hydraulic conductivity of a com-
pacted clay liner. Particular emphasis was
placed on reports addressing cohesive soils,
undisturbed sample sites, and relatively
large surface areas, and on reports describ-
ing methods that could provide data within
a reasonable time frame. In Phase II, field
apparatus and prototype liner were designed
and tested for accuracy and performance.
In Phase III, which is summarized here, a
field-scale facility was constructed and a
clay liner was compacted according to stan-
dard procedures and ponded. After com-
pleting the field phase, core samples were
removed from the site for laboratory deter-
mination of the hydraulic conductivity.
Methods
The clay linertesting facility consisted of
an elevated, bridge-like platform supported
by reinforced beams resting on compacted
level subgrade. In a crawl space under the
platform, percolate was collected. The floor
of the platform was sealed, and a bead of
bentonite was placed 3 ft away from the
sidewalls on the floor to separate any wall
flow from the rest of the leachate. Three 6-
in. thick layers of the clay material were
compacted to 4-in.-thick lifts following the
procedure developed on a construction test
plot; the finished liner was then 12 in. thick.
Construction test plot trials indicated that if
a sheepsfoot roller were used on the first lift
it could dent the bottom set of aluminum
access tubes imbedded in the platform deck.
Consequently, the first lift was spread very
carefully with a backhoe, moistened with a
known amount of water necessary to bring
it to wet of optimum, tilled with rototillers,
sampled for moisture, and compacted in
place with a dozer and smoothfaced roller.
Surface probe readings of water content
and density at 36 locations indicated that
the desired compaction had been achieved.
The first lift was scarified with dozer treads,
and the next lift was spread in a similar
manner. Moistening, tilling, and sampling
were followed by compaction with six to
seven passes of the dozer and sheepsfoot
roller and with a pass of the smoothfaced
roller before moisture and density were
sampled. After the surface was scarified,
the same procedure was used for the third
and final lift. During the construction phase
on lifts 2 and 3, the dozer tended to compact
the clay during spreading and make it diffi-
cult to rototill; compaction equipment could
not approach any closer than a foot from the
sidewalls. These problems were addressed
by a more intensive but slower rototilling
and by the use of a small vibrating roller
nearthe sidewalls and an electric jackham-
mer right next to the sidewall. Although the
degree of compaction near the sidewalls
was judged to be near that obtained over
the remainder of the area, a detailed analy-
sis of core samples indicated considerably
lower values of density next to the sidewalls.
Subsequent examination of the completed
liner cross section revealed no obvious flaws
or planes of discontinuity among the three
lifts. A grid of collection drains under the
compacted clay was complemented by a
grid of 11-in.-diameter buffered infiltration
cylinders at the surface.
Twenty-four horizontal aluminum tubes
were embedded in the floor of the platform
to provide access for the transmission-
gamma-probe used to measure density.
After the liner was compacted, 24 upper
access tubes were positioned on the clay
surface exactly 1 ft above the lower ones.
The density was computed from gamma
attenuation made with a source (Cs137) in the
lowertubeandthedetectorintheuppertube.
A wooden walkway resting on upper
access tubes was constructed, and the fa-
cility was covered over with a building; heat
and light were installed. To correct infiltra-
tion for evaporation, 35 11-in.-diameter
evaporation pans and one large, class-A
evaporation pan were placed on the clay
surface. In addition, 35 square metal ped-
estals were also positioned on top of the
liner to monitor swelling. The floor of the
platform was covered with burlap and a thin
layer of coarse sand. After the liner was
installed, a 1-in. layer of coarse sand was
placed on the surface to minimize evapora-
tion. Clay soil used for the liner was a B-
horizon of a commercially available cherty
silt loam. It is classified as a CLtype brown
till with laboratory permeability of less than
1 x 107 cm/sec. Prototype studies sug-
gested that some swelling was to be ex-
pected after ponding. The clay was pur-
chased from a supplier and trucked to the
site where the experimental liner was to be
constructed. The clay materials were of
variable quality and water content and con-
tained some very large clods (> 6-in.). Stan-
dard Proctor test results at 18% water con-
tent gave a projected maximum bulk density
of 110 Ib/cu ft(pcf). Collection of density
data began immediately after the liner was
constructed and the upper set of access
tubes was installed. The liner at 18% water
content and compacted to 110 pcf was far
from saturated (60%). Monitoring the den-
sity before and after ponding was designed
to account for evaporative losses before the
ponding and the rate at which the liner
wetted after ponding. Highest density was
observed in the central portion of the site.
The dual probe density values before pond-
ing differed by a factor of 0.9 from surface
probe density values observed during the
construction of individual lifts. A matrix of
constantly changing, spatially distributed,
dual probe density was used to compute a
matrix of total available pore space and a
matrix of the amount of water needed to
saturate the liner before and during the
different times after ponding. Observed
changes were indicative of the proportion of
infiltrating water either diffusing into the clay
matrix or passing through the larger pores.
The water level on the liner was maintained
with an automatic constant head tank and
the infiltration in the individual (250) rings
was monitored by measuring water loss
from constant head bottles. Tubing from in-
dividual drains was routed to the perimeter
of testing facility platform, where percolate
was collected in conveniently sized contain-
ers. Readings of water level change in
evaporation pans gave the corrections to
infiltration data.
Results
Collection of infiltration, leachate, and
evaporation data began immediately after
ponding (3/85). Initially, the data expressed
as flux were collected on a daily basis, later
on a weekly or longer-interval basis. Soon
after the start it. became apparent that the
infiltration cylinders and leachate drains
situated next to the sidewalls in the lower
density zones were responsible for a large
portion of the infiltration and drainage. The
leachate from all drains near the sidewalls
was therefore isolated, combined into one,
and measured separately from the central
matrix of 184 individual rings and drains that
represented the area compacted with
sheepsfoot roller.
A rapid increase in density after ponding
followed by a more gradual rise over the
next 9 mo was indicative of progressive
saturation. Although at the time of ponding,
the average water content was 18.1% by
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weight, the water content afterthe liner was
drained and covered with plastic averaged
18.5%. A gradual increase in density con-
tinued through the first 9 mo, and about the
time the drains were vented, the readings
stabilized.
Infiltration rings were activated in stages.
Changes in infiltration were first measured
with a form of a hook gage and Mariotte
constant head bottles—first by volume and
ultimately, as the infiltration rates became
slower, by weighing. Faster infiltration rates
were observed near the side walls in lower
density material. Before the liner was
drained, fluorescein was introduced into all
the rings to check for leaks, and nontoxic
daylight fluorescent water color was added
to mark potential flow pathways when rings
were excavated or when the surrounding
area was cored.
The assumption underwhich the results
were analyzed was that a distribution of
infiltration rates was represented by indi-
vidual ring inflow rates, and the distribution
of outflow flux was given by the individual
drain outflow. The observed variability was
assumed to be a function of soil properties
as well as spatial distribution of the prefer-
ential flow pathways. Hydraulic conductivi-
ties based on inflow and outflow data were
not significantly different from one another;
both, however, were significantly lowerthan
hydraulic conductivity based on the initial
infiltration values. The outf lowflux appeared
to be the best description of clay liner perfor-
mance. Tracer tests with Br- were carried
out towards the end of the study, whereas
water breakthrough times, given as first
arrival of water at the respective drains,
were recorded immediately following pond-
ing. Bromine waschosen as atracer because
of its conservative behavior and low back-
ground concentrations.
Tracer breakthrough, given as the first
arrival of tracer at the principal drain or one
of the surrounding drains, originated from a
centrally positioned infiltration ring. Water
breakthrough, however, was a result of
ponding the entire facility, and outflow was
from a much larger area surrounding each
drain. In either case, the clay was not fully
saturated. Initial water content distribution
corresponded to a 100 Pa tension, and
water content after the water was drained
was no more than 3% higher throughout.
Under these conditions, observed break-
through times would most likely be a result
of short-circuiting flow through the macro-
pores since breakthrough times based on
laboratory hydraulic conductivity values for
a 1 -ft thick clay liner would be expected to
be several years.
The considerably shorter breakthrough
times observed for water and tracer suggest
relatively low effective porosity. Results
showed that over 44% of the site, flow took
place through less than 1 % of the area. At a
few locations, however, flow may have oc-
curred through 10% or more of the local
cross sectional area despite the assumed
uniform compaction and water content of the
clay liner material. Results suggest that a
seemingly uniform clay liner is, in fact, a
highly variable one with an effective porosity
that can range from a low of 0.1% to more
than 5%.
Subsequent coring (with a Veihmeier
tube*) of the tracer application area and
surrounding sites on the 0.3-m grid and
qualitative tests for the tracer corroborated
preliminary observations. On sites where
little tracer was lost as leachate, strong evi-
dence of tracer showed in corings as a tight,
well-defined "plume" surrounding the infiltra-
tion ring to which tracer had been added. For
sites where much tracer was lost, however,
the distribution plume was quite extensive
but rather diffuse throughout the area of nine
drains. Percolate quality appeared some-
what related to hydraulic conductivity, de-
gree of saturation, and number of pore vol-
umes of percolate.
One possible source of fluctuations in
inflow, outflow, and bulk density as well as
tracer concentration could be the swelling of
randomly distributed montmorillonite clay
minerals within the liner matrix on wetting
and possibly moderate shrinkage as a result
of consolidation and piping in zones of lower
compaction. The mineral composition of the
clay liner material was primarily illite and
kaolinite with some montmorillonite. Thus,
although swelling could not be ruled out, little
was expected.
After the excess ponded water was re-
moved and the clay liner drained, several
studies were initiated to check or corrobo-
rate observations made during the ponded
stage. To complement detailed inflow/outflow
data for all ring/drain combinations, 3-in.-
diametercoreswere removedfromthecenter
of each ring infiltrometer by using a standard,
split-tube sampler. The central portion, or
the most homogeneous portion, of each was
trimmed for use in laboratory analysis of
saturated hydraulic conductivity.
In addition, a large number of 2-, 3-, and
6-in.-diameter cores were taken for compari-
son with and calibration of the dual gamma
measurements of density, for evaluation of
changes in density with depth, and for the
assessment of the extent and distribution of
coarse fragments within the clay liner. With
Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
the use of nuclear surface probe, a set of
surface moisture measurements, as well
as surface and direct transmission density
measurements at 2, 4, 6, and 8 in. were
made to be compared with the initial sur-
face probe readings made during the con-
struction. When the water had drained
from the liner, vertical access tubes were
installed at 45 locations from which 2-in.
cores had been previously removed, and
45 gypsum moisture blocks were placed 3
in. below the clay surface. Monitoring
change of the liner water content began
and continued on a regular basis for about
18 mo until the liner was broken up, in-
spected, and removed from the site.
The laboratory hydraulic conductivity
analysis was carried out on 3-in.-diameter
cores varying in length from 2-7/8 to 6 in.
depending on the number of sections the
original core was divided into and on how
well a refrigerated core segment could be
trimmed to fit in the apparatus. Because of
stones imbedded in the clay matrix, this
was, at times, difficult. Initially, only the
most homogeneous portions of the original
core were selected for analysis.
Conclusions and
Recommendations
A clay liner was constructed from the B-
horizon of a typical soil that met normal
engineering specifications. In the course
of analysis, the soil was found to contain a
larger than expected number of coarse
fragments.
The considerable variability, which ex-
isted in the spatially distributed flow in the
compacted clay matrix, may have affected
the rate, quality, and pathway of leachate
flow through the clay liner.
A sand layer on top of the clay liner
acted as a moisture barrier and prevented
rapid drying.
Higher flow rates, which originated in
apparently unsaturated areas, suggested
the presence of the preferential flow path-
ways.
There was no correlation between
laboratory and field-derived values of hy-
draulic conductivity on the point-to-point
basis when values from the same locations
were compared. When, however, the dis-
tribution of laboratory values was com-
pared with the distribution of field values,
the results appeared to be linearly corre-
lated.
Laboratory hydraulic conductivity val-
ues were not a good indicator of the clay
liner behavior. Ring infiltrometers, despite
problems, appeared to provide better esti-
mates of potential outflow from below the
compacted clay. Best estimates of clay
frU. S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20104
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liner performance were obtained by follow-
ing the conservative tracer (Br-) movement
and by breakthrough history.
Little or no change in clay liner density
with time suggested a very limited move-
ment of water into the clay matrix after the
clay was compacted. The largest amount of
change, which occurred just after flooding,
could be indicative of the extent of
macroporosity. A surface moisture-density
probe in a direct transmission mode was
found to be a quick and satisfactory method
of determining field distribution of moisture
and density in shallow lifts. No relationship
was observed between density and water
content of the clay and the values of hydrau-
lic conductivity.
The flow regime consisted of concurrent
alternating, filling, and draining episodes
distributed throughout the space occupied
by the compacted soil mass.
Results from preliminary studies have
shown that any perforations of the com-
pacted clay can result in preferential water
movement along these access points. Infil-
tration and drainage rates from field scale
facility were poorly predicted by the small-
scale studies.
Because the amount of available pore
space in well compacted clay is very small,
even a small change may have dispropor-
tionately large effects, i.e., minimal swelling
of about 2.4 mm could account for as much
as a 20% increase in available pore space.
About one-tenth as many samples were
needed to characterize the compacted clay
liner density and water content as were
needed to characterize hydraulic conduc-
tivity with the same degree of precision.
Considering that hydraulic conductivity per
se did not appear to be the primary con-
trolling factor in the flow and breakthrough
of water and tracers in the compacted clay
liner, more effort is needed to characterize
possible distribution of preferential flow
pathways.
The full report was submitted in fulfill-
ment of Interagency Agreement No. DW
12930303 between the Agricultural Re-
search Service, U.S. Department of Agri-
culture and the U.S. Environmental Protec-
A. S. Rogowski is with the Northeast Watershed Research Center, ARS-USDA, Univer-
sity Park, PA 16802.
Walter E. Grube, Jr. is the U.S. EPA Project Officer (see below).
The complete report, entitled "Relationship of Laboratory- and Field-Determined Hydrau-
lic Conductivity in Compacted Clay Layer," (Order No. PB 90-257 775: Cost $31.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The U.S. 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
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-90/025
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