United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-89/057 Sept. 1990
EPA Project Summary
Stability of Lined Slopes at
Landfills and Surface
Impoundments
D. H. Mitchell, M. A. McLean, and T. E. Gates
This report describes research con-
ducted at the Pacific Northwest Labora-
tory to provide the technology for deter-
mining the stability of soils In contact
with flexible membrane liners (FMLs) on
covers and Interior side slopes of haz-
ardous waste landfills and surface im-
poundments. Included In the research
were analyses of slope stability, labora-
tory tests to measure the f rlctlonal prop-
erties of various types of interfaces in
landfill and surface impoundment con-
tainment structures, and larger-scale
tests to verify the data from the labora-
tory tests.
Instability (sloughing) of soils cover-
Ing FMLs has not been documented at
hazardous waste landfills and surface
Impoundments. However, sloughing has
occurred In canals and tailings ponds.
The sloughing at tailings ponds occurred
during heavy rainfalls, leading to the
conclusion that sloughing was caused
by a buildup of pore-water pressure In
the soil, thereby reducing stability. The
same effect has been demonstrated In
tests on a 100-ft*, "engineering-scale"
test stand constructed for the research
described In this report. Stability analy-
ses, incorporating the effect of pore
water, were developed.
Friction angles determined by
direct-shear tests, the typical method
used to measure Interfacial and matrix
soil properties, were compared with fric-
tion angles measured on the larger test
stand. In general, the direct-shear
method produced friction angles less
than those measured with the larger
engineering-scale system. Therefore,
friction angles determined by the
direct-shear method are probably con-
servative for use In stability analyses.
This Pro/get 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
The U.S. Environmental Protection
Agency (EPA) has issued guidance de-
scribing technologies that will meet RCRA
minimum technology requirements for haz-
ardous waste landfills and surface im-
poundments. The guidance recommends
that these facilities be lined and covered
with combinations of soils and flexible mem-
brane liners (FMLs). Thus, interfaces will be
created between synthetic and soil materi-
als.
Instability abng the interfaces between
synthetic and soil materials could lead to
slippage and loss of integrity of the contain-
ment system. Before issuing a permit, the
EPA should assess, among other things,
this potential for instability. The full report
describes the technology needed to make
this assessment.
Background
Double-liner systems for a landfill and
for a surface impoundment are illustrated
schematically in Figures 1 and 2. Slopes of
the landfills and impoundments may have
several layers of geosynthetic materials
and soils. The geosynthetic materials may
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Optional Soil Protective Cover
Filter Layer (Soil or
Geosynthetic)
Compacted Soil — _ _j-
Primary Liner (FML)
Secondary Liner (Composite
FML & Soil)
Native Soil
Primary & Secondary
Leachate Collection
Systems (Soil or
Geosynthetic)
Figure 1. Cross section of landfill double liner system.
Protective Soil Cover
FML
Composite Liner
Leak Detection/Collection Layer
Figure 2. Cross section of surface impoundment double liner system.
include FMLs, geotextiles (those perme-
able synthetic materials that provide filtra-
tion orseparation), and drainage nets (those
synthetic materials that provide paths for
leachate or other liquid flow).
As indicated in Figures 1 and 2,
geosynthetic materials are in direct contact
with soil lining materials or with an overlying
layer of soil placed as protection for the
geosynthetic material. This can lead to in-
stability at the interface, particularly if the
geosynthetic material is a FML and the
interface is sloped. The side slopes of the
landfill or impoundment structure are espe-
cially susceptible. Soil may slide down the
FML surface, or it may drag the FML with it.
Sliding of the soil is term edsfoug/j/ngf (Figure
3).
Operators of landfills and surface im-
poundments want steep side slopes to
maximize the containment capacity of their
facilities. EPA has stated that field experi-
ence indicates a maximum side-slope ratio
that will hold an earth cover on a smooth
liner (FML) is 3 horizontal to 1 vertical, or
18.3°. This project has found that instability
can occur even on this relatively low slope.
Cases of interfacial instability have not
been documented for hazardous waste
landfills and surface impoundments. How-
ever, the potential exists, as evidenced by
several cases of sloughing in similar struc-
tures used for other purposes such as WE
storage and containment of mine tailing;
is this potential, and the further potential
loss of containment capability, that hi
inspired the research described in this
port.
Stability Analysis
Sloughing of soil on FMLs is simila
shallow failure of the surface stratum
slopes, as found in natural soils. The dif
ence in permeability between the surf.
and the underlying stratum allows an
crease in pore pressure in the surface s
turn when rainwater percolates into it.'
surface stratum may collapse as a re:
-------
FMLs
Cover Soil
Leak Collection Layer
Cover Soil
Top FML Pulled from
Anchor Trench
Leak Collection Layer
Figure 3. Examples of cover instability (sloughing).
The effect of pore-water pressure on the
stability of soils placed over FMLs can be
significant and should be incorporated into
the stability analysis.
The frictional force resisting sloughing
at the interface (see Figure 4) is:
f - (ye cosB - u.)(tano)L (1)
where
y= soil density
z = soil depth (perpendicular to the
slope
B = slope angle
L = length of slope
H = pore-water pressure
0= friction angle of the failure plane
Another force resisting sloughing is ad-
hesion (c) of the surfaces on each side of
the interface plane.
The force promoting sloughing (see
Figure 4) is gravity, represented by:
g = Cyz sinB) (2)
Thus, the factor of safety (FS). the ratio
of resisting forces to promoting forces, can
be determined by:
FS • (vz cos 8 - uWariBl + c (3)
•yzsinB
By examining the equation, one can see
that, as pore pressure increases, the effec-
tive stress on the liner lessens and, there-
fore, the frictional force lessens. Decreasing
the resisting forces (numerator) decreases
the factor of safety (FS).
The value for pore-water pressure is not
something that typically would be measured
at an actual disposal site. Instead, one would
expect that maximum pore-water pressure
would be determined by modeling or by
worst-case assumptions.
Figure 5 illustrates the importance of
pore-water pressure in determining the fac-
tor of safety in the stability analysis. The soil
depth is two feet in this case. The factor of
safety is also very sensitive to the slope
angle and to the interfacial friction angle. It is
less sensitive to soil depth over the FML,
except when the pore-pressure head ap-
proaches the depth of the cover.
Interface adhesion (c) does not appear
to be an important factor in the analysis and
apparently can normally be neglected.
If the analysis results in a factor of safety
of less than 1.0, it is possible that the toe of
the slope may provide the required stability.
The full report includes the methodology for
calculating the resisting force of the toe. The
report also includes consideration of
anchor-trench resistance for those cases
where the geosynthetic component may be
providing sliding resistance.
Direct Shear Tests
Direct shear tests were performed to
measure interfacial properties of soils and
geosynthetic materials, to demonstrate the
effects of compaction and moisture on fric-
tion angle and adhesion, and to provide
direct-shear results for comparison with
engineering-scale tests.
The direct-shear test procedures are
detailed in the full report. The test apparatus
was a Soiltest model D-124A direct-shear
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Plane of
Weakness
Figure 4. Forces acting on soil mass on infinite planar sloped surface.
\\NN\\NN\
\N\\S\N\\
\\NN\\NS\
\\N\\N\N\
\\ \ \ N • \ N \ \
\\\\\\\\\
\ \\NSNSSN
\ \\NN\\\\
\\S\\S\\
\\\\\
Partially
Saturated Soil
Low-Permeability Liner
Figure 5. Pore-water pressure in soil on sloped system.
device, designed specifically fo
shear-testing in soil samples. The shea
box could accommodate 2 x 2-inch (5.1;
5.1-cm) soil and geosynthetic specimens.
For soil alone, the shear plane wa
through the mass of the specimen. Fo
soil-geosynthetic combinations and fo
combinations of geosynthetics, the spec
mens were carefully prepared and emplaca
in the shear box so that the shear plan
coincided with the interface of the material:
Tests were done with soils that were corr
pacted, where appropriate, to specifie
densities, and at specified moisture cor
tents. All tests were performed under sati
rated conditions except for the FMl
geotextile tests that were d ry. Table 1 show
the direct-shear test matrix.
Table 2 summarizes the direct-she:
test results for soil and FML combination
As expected the soils alone showed tr
highest friction angles, and the soil con
bined with the harder FML (high-densi
polyethylene [HOPE]) showed the lowe:
Table 3 shows the results of the direct-she
tests for FMLs in combination wi
geotextiles and drainage nets. Frictic
angles are low, substantially lower th<
soil-FML combinations. This suggests th
interfaces between geosynthetics and FM
(instead of FMLs and soils) may limit tl
angle of slopes unless the various comp
nents are well-anchored.
Engineering-Scale Tests
The "engineering-scale" (large-sea
tests were conducted to compare with t
laboratory direct-shear tests in determini
whether the latter can provide reliable d;
for interfacial stability analyses. In additk
the engineering-scale tests were design
to verify the effect of pore-water pressure
the stability of FML-soil combinations.
The test stand (Figure 6) was f abrical
specifically for the purposes of this proje
It could accommodate a mass 10 x 10 f
(3.05 x 3.05 m) in horizontal dimensic
and up to 50 inches (127 cm) thick. 1
bottom 14 inches (35.6 cm) was filled v
soil, providing the base for the geosynthi
material. The geosynthetic material co
then be loaded with up to 36 inches (9
cm) of soil. During the tests only 12 incl
(30.5 cm) of soil was applied. One sid<
the test stand could be raised to ere
various slopes of the test materials. '
geosynthetic material being tested \
mounted securely to the sides of the
stand at the 14-inch (35.6-cm) level.
The test stand was equipped with
linear variable displacement transduc
(LVDTs) arranged within the 12 inches ({
cm) of soiloverlying the FML to measure
downslope soil displacement. Press
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Table 1.
Direct-Shear Test Matrix
Soil only
HOPE
HDPE/geotextile
CSPE/geotaxtile
HDPE/drain net
SoiH
SP
X
X*
X
X
X
Soil 2
SP
X
X
Soil3
SM
X
X
Soil 4
SC
X
X
SoilS
CL
X
X
' determined at 3 different densities.
Table 2.
Direct-Shear Test Results for Soil and FMLs
so//r
Soil only
HOPE
CSPE
0'
38
36
SP
c"
1.6
0.4""
0.1
Soil 2
0
33
21
28
SP
c
0.9
0.4
0.5
SoilS
0
25
22
31
SM
c
2.2
0.7
0.5
Soil 4
0
»**»
25
20
SC
c
****
0.1
0.8
SoilS
0
24
18
10
CL
c
2.4
0.1
0.8
0 = interfacial friction angle.
c = cohesion for soils and adhesion for soil/FML tests.
average values from tests at 3 different densities.
measured friction angle was well above values typically measured on this type of soil; errors in data or procedures are
suspected.
Table 3. Direct-Shear Test Results for FMLs, Geotextile, and Drainage Net
HOPE CSPE
0 c)psi 0 c)psi)
Mirafi 140N
PN4000
9
12
0.3
0.2
18
not performed
0.8
not performed
transducers were also placed within the
overlying soil to measure the pore-water
pressure. Water was added to the system
by an adjustable rainfall simulator.
In the tests of soil on a FML, 12 inches of
soil was placed on the FML at the specified
density. While applying "rainfall," the slope
was increased periodically until soil slippage
was noted by the LVDTs and by observation.
Pore-water pressure was also monitored
and recorded. For the tests of geotextile or
geonet on a FML, the geosynthetic material
was placed directly on the FML. Measure-
ments of slippage were made with the
LVDTs, but water was not applied. Each
engineering-scale test was performed at
least three times.
The following four tests were made at
engineering scale:
poorly graded sand on HOPE
poorly graded sand on geotextile on
HOPE
* poorly graded sand on geotextile on
CSPE
poorly graded sand on drainage net
on geotextile on HOPE
The engineering-scale test results of
poorly graded sand on HOPE are shown in
Table 4. The friction angles were calculated
from the equation:
tan0= 72 sin B
derived from equation (3), with a factor of safety
equal to 1.0 and adhesion equal to zero.
Note that two friction angles (27°)jire
significantly lowerthan the others and lower
than the friction angle measured in the
direct -shear test of the same materials. It is
believed that the two low values are spuri-
ous and may have resulted from movement
of the the test stand during the test.
The results of the tests of geotextile
(Mirafi 140N) on HOPE and on CSPE are
presented in Table 5, and the results of
geotextile on drainage net on HOPE are
shown in Table 6. In the latter tests, the
geotextile was Mirafi 140N and the drain-
age net was PN4000. Table 5 shows that
the direoY-shear test produces a more con-
servative friction angle than the engineer-
ing-scale test for geotextiles on HOPE and
CSPE (compare Tables 5 and 3). Table 6
compared with Table 3 shows a similar
relationship.
Recommendations
The results of the study indicate that the
designer can use direct-shear tests to pro-
vide data for interfacial stability analyses of
landfill and surf ace impoundment structures.
Direct-shear tests should be carried out
using the same materials and simulating
the conditions of the field application. That
includes the orientation of the synthetic
materials and direction of applied stress.
The effect of pore-water pressure in a
protective cover soil should also be consid-
ered in the design. The designer should
account for the uncertainty of key variables
to assure that the desired factor of safety is
maintained under worst-case conditions.
Jn order to maximize the slope of an
FML-soil system, several options may be
chosen. The pore-water pressure may be
minimized by using coarser materials for
the cover soil. Coarser material may be
used over finer soil to gain the same effect
while increasing the mass. The combina-
tion of soil and FML may also be selected to
increase the friction angle.
Geosynthetic materials are evolving
rapidly, and attention is being given to the
effects of surface texture, combinations of
materials, design configuration, and other
factors on interfacial slope stability. The
designer should be alert to these changes in
his choice of materials for stability analysis,
design, and use.
Thefullreportwassubmittedinfulfillment
of Interagency Agreement Number
DW89930846-01-0 with the U.S. Depart-
ment of Energy under the sponsorship of
the U.S. Environmental Protection Agency.
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rainfall simulator
slope
adjuster
fulcrum
slope toe stop
Figure 6. Engineering-scale test stand.
Table 4.
Engineering-Scale Test Results with HOPE and Poorly Graded Sand
Test Date
9/16/86
9/31/86
9/25/86
10/27/86
11/06/86
11/13/86
Dry Soil Density
Ib/cf
104
101
102
99
106
106
Incipient Failure
Angle
(degrees)
18.9
17.2
22.5
14.8
>30
13.9
Pore-Water
Pressure in.
HŁ>
6.3
6.5
5.0
6±"
0
fit"
Calculated
Friction Angle*
(degrees)
38
35
37
27
>30
27
* Calculated friction angle assumes no adhesion. The wet density of the soil was 123.5 Ib/cf, from separate measurement of
saturated soil at 104 Ib/cf dry density.
" The pore-water pressure transducer output was erratic; the pressure was estimated from a backup manometer system.
Table 5. Engineering-Scale Test Results with a
Geotextile on HOPE and CSPE
FML
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
HOPE
CSPE
CSPE
CSPE
Test
10/09/86
10/09/86
10/10/86
10/15/86
10/16/86
10/23/86
10/24/86
10/24/86
4/11/87
4/14/87
6/04/87
Date of Incipient
Failure
Angle
(degrees)
18.4
21.1
17.5
23.8
20.1
<15.1
12.9
13.6
28.5
28.5
25.8
Table 6. Engineering-ScaleTest
Results with a Drainage
Net on HOPE
Date of Test Failure Angle (degrees)
10/17/86
10/20/86
10/21/86
22.0
20.4
17.4
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D.H. Mitchell, M.A. McLean, and T.E. Gates are with Battelle, Pacific Northwest
Laboratories, Richland, WA 99352.
Robert P. Hartley is the EPA Project Officer (see below).
The complete report, entitled "Stability of Lined Slopes at Landfills and Surface
Impoundments," (Order No. PB 90-251877/AS; Cost: $23.00 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 Rese
Information
Cincinnati. OH 45268
US.OFFiCiALMAiL"
U.S.P8STAlrt
VO'rl
SO.3 5
Official Business
Penalty for Private Use $300
EPA/600/S2-89/057
»GE.C,
CHICAGO
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