Geosynthetics October/November 2020 - 9

MEMBRANE-ENCAPSULATED
SOIL LINERS

SINGLE- AND MULTI-INTERFACE
STRENGTH TESTS

Q: For expansive soil subgrade in pavement,
can you suggest a geotextile that can function
to control swelling and shrinkage in addition
to reinforcement? Also, can you suggest a
sensor that can capture the effectiveness of
the geosynthetic being subjected to traffic
in this application?

Q: Can you give your opinion on the
applicability of multilayer large-scale direct
shear box testing compared to the results
obtained from conventional tests described
in ASTM D5321 and D6243?

A: Please consult Robert M. Koerner's
Designing with Geosynthetics, 6th ed., Xlibris
(2012) for a list of references on "membraneencapsulated soil liners (MESLs)" that are
used often in the southwestern part of the
U.S. with great success. One can impregnate
a needlepunched nonwoven geotextile with
bitumen to establish an MESL rather than
using a ready-made barrier or geomembrane.
In regard to a sensor that can monitor this
situation, I would strongly recommend you
read GRI-GS3 "Selecting In-Situ Monitoring
Methods and Devices for the Evaluation
of Geosynthetic Performance" and John
Dunnicliff's Geotechnical Instrumentation for
Monitoring Field Performance, J. Wiley & Sons
(1988). In addition, TenCate has a GeoDetect
monitoring system based on geotextiles
equipped with optical fibers, which may also
be of interest and a great help to your effort.

WELD STRENGTH
Q: Our client is asking us if there is a
formalized consensus when welding 60-mil
(1.5-mm) high-density polyethylene (HDPE)
geomembrane to 40-mil (1-mm) HDPE in
regard to achieved target weld strength.
A: When welding 60-mil (1.5 mm) HDPE
to 40-mil (1-mm) HDPE, one expects that
only the weld strength of the 40-mil (1-mm)
HDPE would be achieved, per GRI-GM19a:
"Seam Strength and Related Properties of
Thermally Bonded Homogeneous Polyolefin
Geomembranes/Barriers." I have witnessed
this criterion in plans and specifications for
many sites and believe that it is the norm in
our industry for both fusion and extrusion
seaming of HDPE geomembranes. Please note
that this detail (i.e., situation) is called for on a
regular basis whenever liner geomembrane
(generally 60 mil [1.5 mm]) is welded to capcover geomembrane (generally 40 mil [1
mm]) to envelop the waste mass around
the perimeter of a landfill cell.

A: Comparison of single- and multi-interface
strength tests shows relatively good
agreement, T. Stark and F. Niazi (2015) or Z.
Yuan and R. Swan (2008). Single-interface test
results appear to be a bit more conservative
but have less uncertainty than multi-interface
testing. There are several things you need to
know about multi-interface testing:
1.	 The procedure will appear in the next
revision of ASTM D5321 in the appendix.
This means that it is not required but an
option for interface shear testing.

NOMINAL VALUE
Q: Are the thickness specification values in
GRI-GM13 and GRI-GM17 based on minimum
average or nominal values? I will highlight my
question around 60-mil (1.5-mm) textured
geomembrane values and assume it would
apply to the other thicknesses and materials
in the specifications.
Scenario 1 is based on the percentages
below the nominal value.
Scenario 2 is based on the percentages
below the minimum average.
A: Scenario 1 is what the values in GRI-GM13
and GRI-GM17 specifications are based on.
Percentages below the nominal value are
represented by nom. -5% in Table 1. We will
certainly highlight this in the GRI specifications
in the very near future to avoid confusion
going forward. G

2.	 Multi-interface testing requires special
equipment and techniques to search out
the weakest point within a landfill liner
or cover system cross section.
3.	 Multi-interface testing requires significantly
more large-displacement strain to mobilize
peak or residual stresses than in a singleinterface test. The test apparatus (direct
shear box) used for the test needs to be
equipped for this extra displacement
(i.e., well beyond 25% strain).
4.	 The multi-interface testing setup is much
more complicated, especially when dealing
with different hydration and consolidation
requirements of different soil types and/or
geosynthetic clay liners (GCLs).
5.	 The analysis of the failure mechanism
involved with multi-interface tests requires
experience with soil and geosynthetic
forensics. Each side of each layer needs
to be observed and evaluated for signs
of deformation.
Given these criteria, it appears that we will be
seeing more multi-interface testing, especially
for projects designed in seismically active
zones. This is a significant departure from the
defendable boundary condition of forcing the
Mohr-Coulomb failure envelopes between a
single interface at a time for the two half-box
conventional direct shear test.

Nominal
Scenario 1

Minimum Ave.
Scenario 2

Notes

reduction value reduction value

(mils)
-5%
(mils)
-10%
(mils)

-15%
(mils)

60
3

6

9

57

54

51

2

6

9

57

Bases

55

All

51

2 values can
be this low
and meet the
specifications
(8 out of 10)

48

1 value can
be this low
and meet the
specifications
(9 out of 10)

TABLE 1 GM-13; Thickness (mils)
(textured geomembrane)

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Geosynthetics October/November 2020

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