ENVIRONMENTAL GEOTECHNICS
Desiccation Cracks Result in Preferential Flow
Eric C. Drumm, Daniel R. Boles, Glenn V. Wilson
Introduction
Recent research related to waste site
closure caps and clay barriers has primarily focused upon conditions and
proceduresfor barrier construction, and
regulatory requirements to assure that
clay barriers are constructed to minimize hydraulic conductivity. Less emphasis has been placed upon changes in
hydraulic conductivity associated with
post-compaction variations in water
content. Little guidance is provided to
assure protection during construction
and maintenance during the facility
service life. It has been generally assumed that desiccation problems are
eliminatedafter placement of the upper
protective layers (Daniel and Benson,
1990). However, soil water storage is
not a static property of a soil, but a time
dependent variablebased upon the rates
of soil water inflow and outflow. Clay
barriers can be expected to experience
cycles of saturation and drying.
Desiccation during construction has
long been recognized as leading to increases in hydraulic conductivity
(Kleppe and Olson, 1985). Field investigations of constructed clay liners have
indicated that high hydraulic conductivities generally result from hydraulic
defects in the clay due to construction
methodologies and/or desiccation
cracking due to inadequate protection
after construction (Daniel, 1984; Day
and Daniel, 1985; Benson and Daniel
1990; Elsbury et. al., 1990; Rogowski,
1990).Daniel and Wu (1993) suggested
a methodology for modification of the
hydraulic conductivity zone of acceptability (water content versus dry density) to minimize shrinkage potential.
To properly assess the long-term integrity of compacted clay barriers and
developappropriate maintenance measures, an understanding of the shrinkage
induced volume changes and the development of preferential flow paths must
be obtained. A laboratory investigation
utilizing Iysimeters was conducted to
22
Geotechnical News,
June 1997
measurement of the hydraulic gradient
across the sample, yet permits the development of preferential flow paths between the shrinking intact clay
aggregates (Phifer et al. 1995).
Although more representative of the
conditions occurring in the field, volumetric strain and density changes are
more difficult to obtain in the Iysimeter
than in the FWP. A series of laboratory
Iysimeter tests was designed to evaluate
the effects of desiccation cycles on the
formation of preferential flow paths
(cracks) in clay.
13
Stone
and Water
~D
§
Base
Filter
Fabric
Sand
Collection Bonle
Figure 1. Plan view of individual
chambers of lysimeters (Top),
Cross-section of lysimeter (Bottom)
evaluate the effects of post-compaction
water content variations on the saturated
hydraulic conductivity of a soil that has
been used for hydraulic barriers.
Laboratory Lysimetersversus
Flexible
Wall
Flexible Wall Permeameter (FWP) tests
for hydraulic conductivity permit precise control over the hydraulic gradient
during testing, and permit measurement
of the volumetric strain due to desiccation.
It has been suggested (Phifer et al.
1995) that the geometry of the FWP
sample limits the formation of preferential flow paths. When subjected to drying events, FWP specimens undergo
shrinkage of the intact clay aggregates.
This results in an increase in density or
decrease in void ratio and a corresponding decrease in hydraulic conductivity
of the clay aggregate. In the field, clay
barriers subjected to desiccation undergo shrinkage which may form preferential flow paths between aggregates.
The geometry of thin laboratory
Iysimeter specimens allows direct
Lysimeter Construction
The Iysimeters 'vere constructed of
welded polyvinyl chloride (PVC), with
a diameter of 0.75 m (29.5 in) and depth
of 0.62 m (24 in), Figure I. To investigate the spatial variation of preferential
flow within the sample, the Iysimeter
outflow collector was divided into 14
sectors, each of which permitted an independent measurement of flow and
calculation of hydraulic conductivity.
The water collection sectors were 80
mm (3.2 in) tall, filled with compacted
Ottawa sand, and covered with a layer
of Supac@16NP-L17800 geosynthetic
filter. The soil specimen was placed immediately above the geosynthetic filter.
It was assumed that the sand and
geosynthetic have a negligible effect on
the hydraulic conductivity measurements of the barrier, provided the hydraulic conductivity, K, of the barrier is
significantly lower than that of the sand,
yet large enough that the flow through
the system is greater than the water storage capacity of the sand.
Thus, it was assumed that the maximum hydraulic conductivity that could
be measured in the Iysimeter was about
I x \O-4cm/sec,and the smallest detectable conductivity was about 1 x \0-9
cm/s.
The entire Iysimeter was elevated
and erlenmeyer flasks were connected
beneath each sector to collect the flow.
,
ENVIRONMENTAL GEOTECHNICS
The two outer sectors (#13 and #14,
Figure 1)were intended to intercept any
sidewall leakage from water that passed
between the sample and the lysimeter
wall, and were not used in the conduc-
tivity determination. The computed
conductivity through each of the 12 internal sectors could then be computed
and compared withthe overallhydraulic
conductivity.
13
Figure 2. Map of Surface Cracks after First Desiccation Cycle
Table 1
Hydraulic Conductivity by Sector for each Saturation Cycle
Sector
Initial
Measurement,
Saturation 1
(cm/sec)
Saturation 2
(cm/sec)
Saturtion 3
(cm/sec)
1
2 x 10-6
0.0
3 x 10-5
2
1 x 10-7
0.0
>1 x 10-4
3
2 x 10-7
0.0
>1 x 10-4
4
8 x 10-8
6 x 10-7
2 x 10-7
5
2 x 10-5
1 x 10-6
5 x 10-6
6
0.0
3 x 10-5
>1 x 10-4
7
1 x 10-6
2 x 10-6
>1 x 10-4
8
6 x 10-6
5 x 10-6
>1 x 10-4
9
0.0
7 X10-6
>1 x 10-4
10
6 x 10-7
7 x 10-5
>1 x 10-4
11
3 x 10-6
1 X10-7
>1 x 10-4
12
3 x 10-5
2 x 10-7
2 x 10-5
Overall
(based on all
sectors
1.3 x 10-6
7 x 10-6
1.4 x 10-4
Preparation of Clay Barrier
Specimen
The soil was obtained from the West
Chestnut Ridge borrow area in eastern
Tennessee and had been used for cap
construction at the Department of Energy's Oak Ridge Operations site. This
soil is similar to that investigated by
Daniel and Benson (1990). A comparison of the shrinkage induced volume
changes exhibited by this materialunder
flexible wall permeameter conditions
and laboratory lysimeter conditions was
described by Phifer et aI. (1995). The
soil is classified as a high plasticity clay,
with a liquid limit =61 and plastic limit
= 31. The Standard Proctor maximum
dry density = 1.37 Mglm3 and the optimum volumetric water content =33 percent (Boles 1994).Kaolin is the primary
clay mineral (Lee et aI. 1984).
After being passed through a number
4 sieve,the soil was compacted at about
3 percent above optimum water content
in two lifts to obtain a density of about
88 percent standard Proctor density.The
final height of the specimen was about
76 mm (3 in). Toprevent swelling of the
soil and simulate a topsoil overburden,
a layer of gravel was placed on the soil
resulting in a stress of approximately 6
kPa (0.9 Ibslin2). The gravel was separated from the soil by a layer of the
geosynthetic material. Additional details are provided elsewhere (Boles
1994).
Hydraulic Conductivity
Measurements
The specimen was saturated under a
constant head of 0.33 m (13 in), and the
flow from each sector collected in
flasks. An additional flask was maintained to permit correctionfor anywater
lost due to evaporation. The hydraulic
conductivity was calculated by Darcy's
Law based on the hydraulic gradient
across the sample and the area of the
collectionsector.The hydraulicconductivity measurements for each of the 12
sectors are listed in Table 1.Also shown
is the overall hydraulic conductivity of
the entire Iysimeter based on the sum of
the flow and area from all 12 sectors.
After reaching steady state flow and
determining the hydraulic conductivity,
the water and overburden were removed
June 1997
2:J
ENVIRONMENTAL GEOTECHNICS
from about 1.3 x 10-6em/see to about
7 x 10.6 em/sec. This relatively modest increase in K was accompanied
by a conductivity decrease in 8 of the
Iysimeter sectors, and an increase in
only 4 sectors. This suggests that the
shrinkage-induced
volume change
decreases the conductivity in some
crack-free sections of the barrier and
.
Overall K
RUN t
= 1.3 x 10-. cm/s
RUN 2
OverallK =7.0 x 10-. cm/s
LEGEND
D
Fl
~
F7l
Ed
10-10em/.
.
10-. em;'
.
10-0 em/.
.. .
.
EEillillJ 10-1 em/.
II
.
10-6 em/.
11II 10-0 emf.
RUN:t
Overall
K > 1.4 x 10-' cm/s
10-4 emf.
Figure 3. Measured Hydraulic Conductivity by Lysimeter Sector
and the sample permitted to dry. The
drying resulted in a number of cracks on
the surface of the clay specimen, as
mapped in Figure 2. The overburden
was then replaced and the constant head
flow condition restored. After reaching
steady flow, the desiccation process was
repeated. Swelling of the clay due to the
removal of the overburden was observed
to be negligible.
of the computed hydraulic conductivity
in each of the Iysimeter sectors, for each
of the three saturation (two dry-back)
cycles. The Iysimeter sectors are shaded
in proportion to the computed value of
hydraulic conductivity.
The following observations can be
made:
.
Results
The specimen had an initial overall hydraulic conductivity of about 1.3 x 10-6
em/see, which increased to about 7 x
10-6em/see after the first cycle of drying
(gravimetric water content decrease of
about 4.5 percent). After the second desiccation cycle, corresponding to a water
content decrease of an additional 4 percent, the conductivity increased to more
than 10.4 em/sec. Figure 3 is a schematic
24
GeotechnicalNews,
June 1997
.
Prior to being subjected to drying,
the relatively homogeneous compacted clay barrier yielded measurements of hydraulic conductivity that
varied 3 orders of magnitude across
the sample. This suggests that hydraulic conductivity measurements
are dependent upon the soil volume
through which the flow is measured.
Internal or invisible defects in the
barrier may contribute to the quantity
of flow.
After the first drying cycle the overall hydraulic conductivity increased
.
causes cracks and conductivity
creases in other sections.
in-
After drying, the sections in which
cracks were observed tended to produce conductivities on the order of
that of the entire sample, while the
sections with fewer cracks tended to
have lower values of K. Therefore, it
can be concluded that the regions
with cracks tended to dominate the
conductivity of the entire sample,
and that measurements of conductivity should be made over areas sufficiently large to reflect the overall
barrier properties.
After the second, more severe dryback cycle corresponding to nearly a
9 percent decrease from the compaction water content, the hydraulic conductivity exceeded the upper limit at
which hydraulic conductivity can be
recorded in the Iysimeters (1 x 10-4
em/see). The value of conductivity
increased in all but one Iysimeter
segment. In general, the 4 sectors
with K <10-4 em/see were those with
no visible cracks.
Summary
A series of laboratory Iysimeter tests
was conducted to investigate the variation in hydraulic conductivity of a
compacted clay barrier material due to
cycles of desiccation. To investigate the
spatial variation of hydraulic conductivity within the sample, the water collection chamber of the Iysimeter was divided into sectors. Independent
measurements of flow and calculation
of hydraulic conductivity were obtained
for each sector. After reaching steady
flow and determining the hydraulic conductivity,the samples were permitted to
dry. This resulted in a number of cracks
on the surfaceof the clay specimen. This
saturation/dryback cycle was repeated
twice.
Even before the initial desiccation,
,
ENVIRONMENTAL GEOTECHNICS
the conductivity determined in the individual sectors varied by three orders of
magnitude. After desiccation, the development of preferential flow paths in the
clay barrier resulted in a conductivity
increase in 4 sectors, and shrinkage of
the intact clay matrix resulted in a decrease in 8 sectors. In general, the high
hydraulic conductivities corresponded
to the sections of clay barrier where
cracks were observed on the surface.
After the second dessication event, the
overall K was determined to be greater
than 1 x 10-4 cm/sec, and K increased in
all but one lysimeter sector. Under these
conditions, the use of Darcy=s law and
a continuum porous flow model may not
be appropriate.
Acknowledgments
This work was supported, in whole or in
part, by a DOE cooperative agreement
DE-FC05-920R22056. This support
does not constitute an endorsement by
DOE of the views expressed herein. The
authors are grateful for this support. The
authors are also grateful to Phillips Fibers for providing the Supac@ 16NPL17800 filter fabric, and to Jennie
Ducker for preparing the figures.
References
Benson, C. H. and Daniel D. E. (1990).
"Influence of Clods on Hydraulic
Conductivity of Compacted Clay,"
Journal of Geotechnical Engineering,ASCE, Vol.116,No. 8,pp.12311248.
Boles, D.R. (1994) "The Effects of
Water Content Variation on Laboratory Samples Simulating Low Permeable Clay Barriers," thesis
presented to the University of Tennessee, Knoxville in partial fulfillment of the requirement for the
degree of Master of Science.
Daniel, D. E. (1984). "Predicting Hydraulic Conductivity of Clay Liners," Journal of Geotechnical
Engineering, ASCE, Vol. 110,No.2,
pp. 285-300.
Daniel, D. E. and Benson, C. H. (1990).
"Water Content-Density Criteria for
Compacted Soil Liners," Journal of
Geotechnical Engineering, ASCE,
Vol. 116, No. 12, pp. 1811-1830.
Daniel, D. E. and Wu, Y.(1993). "Com-
pacted Clay Liners and Covers for
Arid Sites," Journal of Geotechnical
Engineering, ASCE, Vol. 119,No.2,
pp.223-237.
Day, S. R. and Daniel D. E. (1985).
"Hydraulic Conductivity of Two
Prototype Clay Liners," Journal of
Geotechnical Engineering, ASCE,
Vol. 111,No.8, pp.957-970.
Elsbury,B. R., Daniel, D. E., Sraders, G.
A., and Anderson, D. C. (1990).
"Lessons Learned from Compacted
Clay Liner," Journal of Geotechnical
Engineering, ASCE, Vol. 116, No.
11, pp. 1641-1660.
Kleppe, J. H. and Olson, R. E. (1985).
"Desiccation Cracking of Soil Barriers," Hydraulic Barriers in Soil and
Rock, ASTM STP 874, pp. 263-275.
Lee, S.Y, Kopp, O.C., and Lietzke,
D.A. (1984) "Mineralogy of West
Chestnut Ridge Soils" Report to Oak
Ridge National
Laboratory
ORNLffM-9361.
Phifer, M. A., Drumm, E. C., and Wilson, G. V. (1994). "Effects of Post
CompactionWaterContentVariation
on Saturated Conductivity," ASTM
STP 1142 Hydraulic Conductivity
and WasteContaminant Transport in
Soils, David E. Daniel and Stephen
J. Trautwein,Eds., AmericanSociety
for Testing and Materials, Philadelphia.
Phifer, M., Boles, D., Drumm, E., Wilson, G. (1995) "Comparative Response of Two Barrier Soils to Post
Compaction Water Content Variations," Geotechnical Specialty Publication No. 46 and Proceedings,
ASCE SpecialtyConference-Geoenvironment 2000: "Characterization,
Containment, Remediation, and Performance
in Environmental
Geotechnics, New Orleans, LA, pp
591-607.
Rogowski, A. S. (1990). "Relationship
of Laboratory- and Field- Determined Hydraulic Conductivity in
Compacted Clay Layer." Report No.
EPA/600/2-90/025, United States
Environmental Protection Agency,
Cincinnati, Ohio.
Eric C. Drumm, Department of Civil
and Environmental Engineering, The
University of Tennessee,Knoxville, TN
37996-2020 Tel: (423)974-7715 Fax:
(423)974-2608
e-mail: edrumm@utk.edu
Daniel R. Boles, Law Engineering and
Environmental Services, 1725 Louisville Drive, Knoxville, TN 37921
Glenn V. Wilson, Desert Research Institute, WRC, 755 E. Flamingo Road, Las
Vegas, NV 89119
Assessment of Barrier Containment
Technologies: A Comprehensive
Treatment for Environmental
Remediation Applications, edited by
Ralph R. Rumer and James K.
Mitchell, National Technical
Information Service, Springfield, VA,
Publication #PB96 -180583, 437
pages, 1995 (prepared under the
auspices of U.S. Department of
Energy, U.S. Environmental
Protection Agency, and DuPont
Company)
Because of mutual interests and similar
needs in determining what was known
and understood about containment technologies and the level of information
needed to support consistent decision
making relative to their application in
remediation, the U. S. Department of
Energy and the U. S. Environmental
Protection Agency collaborated with
DuPont Company in organizing and
sponsoring the International Containment Technology Workshop, held in
Baltimore, MD, August 29-31, 1995.
This publication is a result of that workshop.
The workshop was organized into
small working sessions, each dealing
with a particular aspect of barrier containment technologies, to provide opportunity for the invited participants to
share information, exchange opinions,
and achieve consensus on what was
known, what was not known, and what
was needed concerning the application
of containment technologies in remediation. The working sessions dealt with
a range of topics, including: design and
construction of vertical barriers (including sheet piles), barrier floors (indigenous and artificial), caps, geomembrane
applications, barrier materials (soilbased and chemical-based), permeable
Geotechnical News,
June 1997
25
ENVIRONMENTAL GEOTECHNICS
reactivebarriers, contaminanttransport
modeling,performancemonitoring,and
emplacement verification. Each working sessionwas chairedby a recognized
leader(s)in the field who was responsibleforpreparingthe summaryreport for
his or her working session.
This publication contains the edited
summary reports from each working
session.Althoughparticular individuals
have been credited with the preparation
of each section, other significant contributors are also identified. Each section contains extensive references. A
full listing of all participants can be
found in an appendix to the publication.
The design of this publication, the artwork,and the index were all professionally prepared. A listing of each section
with principal author(s) is given below:
"Soil- and Cement-Based Vertical
Barriers with Focus on Materials": J.
C. Evans (41 p.)
"Design, Construction, and Performance of Soil- andCement-Based Vertical Barriers": G. M. Filz and J. K.
Mitchell (33 p.)
. "Vertical Barriers: Sheet Piles": D.
R. McMahon (19 p.)
. "Vertical Barriers: Geomembranes":
R. M. Koernerand 1.L. Guglielmetti
(25 p.)
"Caps": D. E. Daniel and B. A. Gross
(23 p.)
"Floors and Bottom Barriers: Indigenous": G. P.Boutwell andT.Hueckel
(45 p.)
"Artificially Emplaced Floors and
Bottom Barriers6: M. E. Peterson
and R. C. Landis (27 p.)
"Chemical-Based Barrier Materials": 1.M. Whang (37 p.)
"Contaminant Transport Modeling":
A. J. Rabideau (55 p.)
"Permeable ReactiveBarriers": S. H.
Shoemaker, 1. F. Greiner, and R. W.
Gillham (55 p.)
"Performance Monitoring and
Evaluation": H. I. Inyang (47 p.)
.
.
.
.
.
.
.
.
.
Summary of the 1997
International Containment
Technology Conference and
Exhibition
The 1997 International Containment
TechnologyConference and Exhibition
was conducted on February 9-12, 1997
26
GeotechnicalNews.
June1997
in St. Petersburg,Florida. Over 500 participants from 22 countries attended the
conference, including individuals from
industry, government, and academia.
Many participants were from the U.S.
Environmental Protection Agency due
to the growing importance of containment systems as a cost effective regulatory technology. This highly successful
conference resulted from an intensive
workshop on containment technology
systems conducted in August of 1997 in
Baltimore, Maryland. The primary
sponsoring organizations of the conference were the U.S. Department of Energy,the DuPont Company and the U.S.
Environmental Protection Agency. The
co-sponsoring organizations were the
Florida State University,U.S. Air Force
Armstrong Laboratory - Environics Directorate,American Societyof Civil Engineers and the Society of American
Military Engineers.
Following the opening plenary session, there were 19concurrent technical
sessions, 30 poster presentations and 10
student poster presentations, addressing
issues related to state-of-the-art approaches and innovative technologies
for containment systems designed for
contaminated sites, both surface and
sub-surface. The technical sessions and
poster presentationsaddressed topics on
geomembranes, sheet piles, modeling,
grouting, performance criteria and
monitoring, slurry walls, caps, permeable reactive materials and walls, slurry
walls, and other related topics.The closing plenary session utilized a panel of
eight experts from U.S. and non-U.S.
industry, academia and the National
Academy of Sciences, to provide a review of the technical sessions and insights as to the future of containment
technology. The exhibition included
vendor and agency booths and provided
a venue for commercialization of containment technologies worldwide.
Two well attended training seminars
were conducted. These seminars focused on Barrier Emplacement Quality
Assurance & Monitoring Strategies and
Quality Control for Vertical Barriers,
both of which provided information on
topics pertinent to containment technology systems.A sitetour of the U.S.DOE
Pinellas Plant wasconducted. These ex-
tracurricular events were extremely effective and well attended. An important
component of the 1997 conference was
the Business Commercialization Center
which provide a venue for business negotiations and marketing opportunities
for conference delegates.
As a result of the overwhelming success of the 1997conference, the Second
International Containment Technology
Conference and Exhibition will be conducted in 1999.For more informationon
on, or to be added to the mailing list for
the 1999 conference, please contact
Loreen Kollar, Conference Coordinator
at Florida State University, 2035 East
Paul Dirac Drive, 226 Morgan Building,
Tallahassee, Florida, 32310-3700, 904644-5524 (voice), 904-574-6704 (fax),
or ICTCE@mailer.fsu.edu (e-mail).
In Situ Remediation '97
In Situ Remediation '97 is a key element
in ASCE's Annual Convention. The
conference to be held October 5-7, 1997
in Minneapolis, MN will bring together
more than 40 paper presentations and
two plenary speakers. James K.
Mitchell, Virginia Polytechnic Institute
& State University,will open the conference with his plenary topic: "Waste
Containment Barriers: Evaluation of the
Technology." Michael Kavanaugh of
Malcom Pirnie will follow with his plenary topic "Treatment Technologies for
In Situ Remediation." The following list
of session topics will be covered in the
two and one-half day conference:
Permeable Reactive Barriers
Soil Vapor Extraction & Air
Sparging
In Situ Containment Technologies
. Non-Aqueous Phase Liquids
Integrated In Situ Remedial Technologies
Stabilization/Solidification
Collection Technologies
Case Studies Including Bioremediation
Developing Technologies: Electrokinetics and Radio-Frequency Heating
..
.
.
..
.
.
For further information, contact ASCE
at 1-800-548-2723 ext. 6300 (USA) or
703-295-6300 (International), email:
conf@asce.org.