Geoysynthetics August/September 2023 - 12

Q & A GMA Techline
THE CHALLENGE OF GEOMEMBRANE
WELD EDGE THICKNESS
Q: Over the past several years, we have seen
a reduction in thickness of the weld edge
(smooth edge) of textured geomembranes.
This is presenting a challenge for installers
when trying to achieve passing trial seams
in the field prior to production welding.
Destructive seam tests are passing, but just
barely (10% above GM19a or less). Can you
reconcile this quandary?
FIGURE 1 Relationship between
thickness and tensile strength of
smooth HDPE geomembranes
A: GRI GM13 and GM17 specifications
allow for thickness measurements to be
less than the nominal geomembrane
thickness for smooth and textured
geomembranes. Specifically, one of 10
thickness measurements for a textured
geomembrane can be 15% less than the
nominal geomembrane requirement, and
this value could be recorded anywhere
across the geomembrane material. GM19a
seam strength specification values are
based on the actual, rather than the
nominal thickness of geomembranes.
This can present a challenge for installers
when trying to achieve passing trial seams
in the field prior to production welding,
based upon GM19a values. Destructive
seam tests are passing, but just barely (10%
above GM19a or less). The Geosynthetic
Institute (GSI) has been given the task to
reconcile this quandary, or at least try to
explain it. We consulted with five major
Geosynthetic Accreditation InstituteLaboratory
Accreditation Program (GAI-LAP)
accredited test labs and found that none of
them take thickness measurements of the
weld edge. However, both ASTM D5199 and
ASTM D5994 stipulate " that samples are to
be taken within 6 " of the edge of the roll. "
It is the Institute's opinion that thickness
measurements should be taken at 6 inches
(15.2 cm) from the edge of the roll on both
sides. The smooth edges of textured sheets
should be checked via ASTM D5199 test.
The remainder of the textured portion of
the geomembrane should be checked via
ASTM D5994. Labs should stay away from
measuring the immediate texture to smooth
transmission zone. However, the smooth
edge absolutely needs to be checked
and must agree with the stated nominal
thickness of the delivered geomembrane.
As you can see in Figure 1, thickness is
linearly related to strength. A reduction
of thickness at the smooth edge will be a
significant factor affecting geomembrane
strength and also the seam produced using
the geomembrane.
Relationship between thickness and strength
100
150
200
250
300
350
50
20
40
60
Thickness (mil)
12
Geosynthetics | August September 2023
80
100
120
y = 2.9562x
R2
= 0.9944
We believe that thickness control (and
for that matter all GRI GM13 properties)
of the geomembrane weld edge is vitally
important. This weld edge portion of the
geomembrane needs to be controlled
through testing. The thickness and visive
tensile strengths are critical to the overall
success of the project. Arguably, the seam is
the most critical part of the installation due
to stress concentration and workmanship
issues. Peel and shear values per ASTM
D6392 contained within GRI GM19a were
determined from years of field data from
thousands of case histories. The case studies
are worldwide and include both public- and
private-sector jobs. Promulgated in 2002,
GRI GM19a seam specification for HDPE
geomembrane was based on the " true "
nominal thickness of geomembranes. Using
Figure 1 as an example, if a geomembrane
has a roll edge of 51 mils (1.3 mm) instead
of 60 mils (1.5 mm), the installer would
be starting the project with a 27 Pounds
Per Inch (ppi) (4728 N/m) handicap.
Superimposed on this recent turn of events
with thinning of the roll edge thickness,
HDPE definitely has a cure time. This can be
explained by rheology and blooming of the
light molecule weight fraction away from
the core of the geomembrane or extrudate
over time as it cools. When HDPE goes from
a liquid to solid state, it cross-links. The
effects of various cross-linking times on the
mechanical properties can be significant.
It has been observed that tensile strength
results can be enhanced up to 13% after
Yield Strength (ppi)
Geoysynthetics August/September 2023

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