United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-021 Mar. 1984
SERA Project Summary
Laboratory Studies of Soil
Bedding Requirements for
Flexible Membrane Liners
G.L. Carr and B.C. Gunkel
A two-phase study was conducted to
investigate the performance of membrane
liners during construction of hazardous
waste landfills and to develop a means
for protecting the liners from damage.
Phase I consisted of a series of full-scale
field tests to determine a method of
protecting flexible membranes from
damage during the construction of
landfills. Subgrade soils were selected
to be representative of those typical of
areas in which landfills are constructed.
Four membranes were tested. Each was
placed on top of a subgrade and covered
with various thicknesses of a sand
material. The test items were trafficked
using three different vehicles represent-
tative of the loadings that might be
applied during landfill construction.
Performance of the membrane was
judged by its resistance to puncture and
wear. The lean clay bedding provided
the best protection for the liner and
was effective in preventing puncture
by the subgrade.
Phase II developed three laboratory
tests to simulate field loadings on
flexible membrane liners during construc-
tion of hazardous waste landfills. One
test method used a moving pneumatic-
tire loading, another used a rotating
gyratory load, and the third used a
.cyclic vertical plate load. Loading
conditions and thickness of cover
material over the membrane varied
using Boussinesq equations to produce
vertical stresses on the membrane
similar to those encountered under field
conditions.
Test results showed that the moving
pneumatic-tire load test would be the
most useful for determining cover and
bedding criteria using available site
soils and candidate membranes. Also, a
layer of clay soil effectively prevented
puncture of the membrane by the
subgrade.
This Project Summary was developed
by EPA's Municipal Environmental
Research 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
Many industrial wastes are highly toxic
to the environment if their disposal is not
properly controlled. A common disposal
method is the use of landfills, but
improperly designed landfills could result
in contamination of ground and surface
waters by toxic wastes. Contamination is
the result of various physical, chemical,
and biological processes that occur when
water or fluids percolate through the
wastes and produce a leachate that
pollutes the soil and ground water. The
placement of an impervious, flexible
membrane over the subgrade in hazardous
waste landfills could be one solution to
controlling the leachate. Butearthmoving
equipment used during construction and
underlying angular rock and soil particles
present possible sources of puncture and
other damage. To study the problem, the
U.S. Environmental Protection Agency
(EPA) requested the U.S. Army Engineer
Waterways Experiment Station (WES) to
investigate the requirements for protecting
flexible membranes from damage.
The initial objective of this study was to
investigate the performance of membrane
-------
liners during construction of hazardous
waste landf His and to develop a means for
protecting the liners from damage. This
objective included the development of
laboratory tests that could be used to
determine bedding and cover requirements
for protecting the membranes from
puncture.
The study was conducted in two
phases. In Phase I, the performance of
flexible membranes was investigated
through the construction and testing of
full-scale test sections. A test section
containing 12 test items was constructed
and subjected to three types of vehicle
traffic (tracked, pneumatic-tired, and
cleated). During this phase of the study,
four flexible membranes, six selected
subgrades, three thicknesses of a protec-
tive sand layer, and two bedding materials
were investigated.
In Phase II, three laboratory tests were
developed to simulate field loading
conditions on flexible liners during
construction of hazardous waste landfills.
The three tests included the use of a
moving pneumatic-tire loading, a rotating
gyratory load, and a cyclic vertical plate
load. Tests were conducted using a
gravelly sand or limestone subgrade
under the membrane liner and a gravelly
sand cover. In some tests, a lean clay or a
fabric was placed between the liner and
subgrade to protect the membrane liner.
One type and thickness of cover and three
membrane liners were used in the tests.
Other special tests were conducted to
develop a test that could possibly be used
as a screening test for membrane liners.
Methods and Materials
Phase I: Full-Scale Test
Section Studies
Construction of Test Sections
A 16- x 240-ft section was constructed
under shelter at the WES. The test
section consisted of 12 test items, each
20 ft long and 16 ft wide (Figure 1).
Construction began with excavating an
area of the subgrade floor of the shelter to
a depth of 6 in. and a width of 16 ft. The
last 40 ft at the north end of the test
section was excavated to a depth of 12 in.
to accommodate a 6-in. layer of coarse
gravel that was overlaid with 6 in. of
sandy silt. This fine-grained sandy silt
was used as a bedding material to protect
the flexible membranes from puncture
during traffic tests. The remainder of the
240'
Item 1
11 12
Shoulder-
Lane - 1
0-7
Bulldozer Track
J_
Lane - 2
_L
_L
_L
J_
Pneumatic - Tired Tractor
JL
-L
_L
_L
Lane - 3
Cleated Landfill Compactor
Lane - 4
Sample Membrane During Construction
t Shoulderl
16'
Plan
Item 1
8
10 11
Shoulder
2"
Crushed
Gravel
Protective Layer of Sand- 6-18"
Sand
_J 1 L_ L_
I Gravelly B Coarse
l_ Sand —SSL-Gravel
'Shoulder
6-18"
Gravelly
Clayey
Sand
Flexible
Membrane
Profile
Figure 1. Plan and profile of test section.
test section was backfilled with the
selected subgrades, which were then
compacted with pneumatic-tire and
vibratory rollers. After the six subgrades
were placed, each of the 12 test items in
the test section was covered with flexible
membranes. Shoulders were then con-
structed on both sides of the test sections
using material that had been excavated
previously from the floor of the shelter.
Next, sand was dumped between the
shoulders at each end of the test section.
Then a bulldozer pushed the sand toward
the center of the test section. At all times
during the placement operations, care
was taken to maintain at least 6 in. of
sand between the bulldozer tracks and
the flexible membranes.
Subgrade Soils
Six subgrade materials were selected
and used for the 12 items of the test
section. These materials were classified
according to the Unified Soil Classification
Designation Thickness, mils
System (USCS) as follows:
Item No. Classification
1-2 Crushed gravel(GP)
3-4 Gravelly clayey sand (SP-SC)
5-6 Sand (SP)
7-8 Gravelly sand (SP)
9-10 Coarse gravel (GP)
11-12 Sandy silt (ML)
Note that the sand in items 5 and 6 was the
same type used for the protective cover
layers. The sand was a local (Vicksburg,
Mississippi) sand usually used as the fine
aggregate in concrete.
Membranes
The four flexible membranes and one
fabric used as a bedding material were as
follows:
Type
M1
M2
M3
M4
F1
20
20
30
36
30
Elasticized polyolefin (31 10)
Polyvinyl chloride (PVC)
Chlorinated polyethylene (CPE)
Reinforced chlorosulfonated polyethylene (CSPE-R)
Nonwoven polypropylene and nylon
-------
Traffic Vehicles
The vehicles used to apply traffic to the
various test programs were (1) a D-7
bulldozer equipped with 22-in. -wide
tracks weighing approximately 44,000 Ib
and having a contact pressure of 9 psi; (2)
a pneumatic-tired tractor weighing
37,190 Ib and equipped with two 29.5 x
29, 22-piy tires (each tire had a contact
area of 574 in.2, which produced a
contact pressure of 32 psi); and (3) a
model 816 landfill compactor weighing
40,900 Ib and equipped with four cleated
steel wheels with a contact pressure of 18
psi.
Traffic Pattern
Traffic tests were conducted on each
test item to simulate actual heavy equip-
ment operations during the construction
of landfills. Traffic was applied with
both the tracked bulldozer and the cleated
landfill compactor in the same manner.
Soil Data
Except for the crushed and coarse
gravel material, laboratory compaction
tests and unsoaked California bearing
ratio's (CBR's) were performed on the
selected subgrades. Field tests were also
performed to determine moisture content,
density, and CBR value on the in-place
material of the test section.
Membrane Evaluation
Only the after-traffic condition of the
flexible membranes was considered.
After 10 passes of tl\e traffic vehicles, a
trench was excavated 'across each traffic
lane in all 12 items. A sample of the
membrane was removed from each traffic
lane in each item, marked for identifica-
tion, and inspected. After the membrane
was patched and the protective layer of
sand in the trenches was replaced, traffic
was continued on the membrane that showed
only a few or no punctures. After 30 passes,
traffic was stopped and a final inspection
was made.
Failure Criteria
Each sample of membrane was placed
over a light table and inspected for
punctures. A 5-ft area within the wheel
path was marked on the membranes, and
from this area the number of punctures
noted was recorded. A membrane was
considered to have failed if any punctures
were noted.
Phase II: Laboratory Studies
After completion of the full-scale tests,
laboratory tests were developed to
determine bedding and cover requirements
for protecting membrane liners from
punctures. One criterion for equipment
was that it be readily available to most
commercial laboratories and that it be
adaptable for testing membrane materials.
The three types of test equipment
selected were a gyratory compactor, a
plate-loading machine, and a moving
pneumatic-tired wheel. The initial tests
were conducted using the plate-loading
equipment since this method has been
used to test fabrics used as reinforcement
in pavements. The gyratory tests were
conducted next since they required a
small sample and were easy to conduct.
The pneumatic-tire tests were conducted
last and required the development of test
equipment to simulate the effects of a
moving tire load. In all laboratory tests,
selected parameters were adjusted to
approximate field conditions by modeling
the stress on the membrane.
Results
Phase I: Full-Scale Test
Section Studies
The four membranes investigated
during this study received numerous
punctures when subjected to the subgrades
containing gravel-sized material. But a
considerable decrease in the number of
punctures was observed when the
membranes were trafficked on the items
containing the sand and sandy silt
subgrades. After some traffic operations
were completed on these test items,
no punctures were detected in several
of the membranes. When the fine-grained
sandy silt soil was used as a bedding
material and 6 in. were placed over the
coarse gravel subgrade in items 11 and 12,
fewer punctures resulted.
Another type of bedding material, a
nonwoven polypropylene and nylon-type
material, resulted in a small reduction in
the number of punctures in the M2
membrane but not in the M1 membrane.
Inspection of the trafficked membrane
also revealed that most of the punctures
detected occurred from the bottom in an
upward direction. Thus a bedding and/or
cushioning material would be required to
prevent punctures for subgrades contain-
ing angular gravel and coarse soil par-
ticles.
The three types of vehicle loadings
(tracked, pneumatic-tired, and cleated)
used to apply traffic to the membranes
produced similar degrees of damage.
Phase II: Laboratory Studies
The pneumatic-tire tests conducted in
the laboratory showed that separating
the membranefromthegranular material
by a lean clay will prevent or reduce
punctures in the membrane. In the
laboratory tests, the fabric bedding
material prevented or reduced the
number of punctures in all tests. The field
tests also indicated that the use of a
geotextile under the liner might protect it
from puncture, but not all field tests
indicated this.
A limited number of field tests could be
used for direct comparison with the
laboratory tests since the latter were
conducted to produce a stress on the
membrane liners equal to the stress of
the pneumatic tire on the field liners
under 6 in. of cover. Since the laboratory
pneumatic-tire load is the same type of
load as applied by construction equipment
in constructing landfills, it is considered
to be the most applicable test for
determining bedding and cover require-
ments for membranes used in landfills.
Conclusions
Based on the study results, the follow-
ing conclusions are warranted:
1. The three traffic vehicles used in the
full-scale tests produced similar
amounts of damage to each mem-
brane.
2. The 6 in. of bedding material placed in
the full-scale test section reduced
the number of punctures in the
membranes.
3. The pneumatic-wheel load test is
the most useful laboratory test for
determining cover and bedding
criteria using available site soils and
candidate membranes.
4. Both the cover material placed
above the liner and the bedding
material should consist of a soil
classified as a clay, silt, or sand with
a gradation similar to the clay or
concrete sand used in this study.
Material should have no particles
larger than 3/8 in.
5. The 1 in. of lean clay bedding
material effectively prevented punc-
ture of the liner material by the
gravel subgrade during laboratory
tests.
6. Use of the geotextile as a bedding
material reduced the number of
punctures, and the use of a thicker
geotextile may prevent punctures
from occurring.
-------
Recommendations
The tests reported here show strong
performance trends for liner materials
placed in landfills and need to be
continued to develop a complete range of
design criteria. The following recom-
mendations therefore apply:
1. Additional laboratory tests should be
conducted using other subgrade
materials and methods for protecting
the liners.
2. Further analytical work should be
accomplished on the field and
laboratory data to extend criteria.
3. Criteria obtained from laboratory
tests should be validated by conduct-
ing full-scale field tests.
4. Additional series of tests should be
performed with the laboratory gyra-
tory equipment and the artificial
rocks (barbs) to develop a laboratory
test method for screening membranes.
5. Compaction requirements should be
established for bedding and cover
materials.
The full report was submitted in
fulfillment of Interagency Agreement No.
EPA-86-R-X0937 by U.S. Army Engineer
Waterways Experiment Station under the
sponsorship of the U.S. Environmental
Protection Agency.
G. L. Carr and R. C. Gunkel are with the U.S. Army Engineer Waterways
Experiment Station, Vicksburg. MS 39180.
Robert E. Landreth is the EPA Project Officer (see below).
The complete report, entitled "Laboratory Studies of Soil Bedding Requirements
for Flexible Membrane Liners," (Order No. PB 84-141 498; Cost: $11.50,
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:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
U S. GOVERNMENT PRINTING OFFICE, 1984 — 759-015/7623
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
LOU w TILLEY
KtblUi\' V EPA
L18KARIAIM
230 S DtAKHORN
CHICAGO IL
Si
------- |