Geosynthetics October/November 2023 - 40

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By George R. Koerner
Have you noticed lower trial weld
strengths in polyethylene seams?
olyethylene (PE) seaming techniques are used in the field to fabricate rolls of geomembranes
into liner systems. There are two predominant methods used: hot wedge
" fusion " welding and " extrusion " seaming. Over the past year, installers have noticed lower
trial weld strength than in years past. This makes for much discussion on the jobsite between
certified quality auditor (CQA) personnel and installers. No one wants to be below
the GRI-GM19 specification for seam strength at the start of a job. This article presents
one rational for lower peel and shear strength as a result of reduced thickness of a welding
edge. Specification thinning in the area to be bonded can compromise seam strength.
Obviously, this needs to be checked and reconciled prior to job commencement.
The hot wedge method of seaming PE consists of a hot electrically heated element in
George R. Koerner, Ph.D.,
P.E., CQA, is the director of
the Geosynthetic Institute
in Folsom, Pa.
the shape of a wedge that is passed between the two sheets to be bonded. Contacting the
two heated surfaces together causes a fusion of fresh material to create a weld. Immediately
following the melting, roller pressure brings the molten surfaces together to form a homogeneous
fused bond. Single hot wedge and dual hot wedge systems are both available. The dual
hot wedge weld forms a continuous air channel between two welds. The air channel can be
used as a means of testing the bond continuity when air pressure is injected into it. Fusion
welding parameters include speed rate, temperature and roller pressure. Most modern welders
can continuously control, monitor and record these parameters. Adjustments are made
according to environmental conditions such as ambient temperatures and relative humidity.
Extrusion welding is commonly used for PE geomembrane repairs and around appurtenances.
The extrusion welding process starts by hot air tack welding the overlap. This blows
away debris from the area to be welded. Directly following the hot air tack weld, the area of the
geomembrane to be bonded is buffed, cleaned and beveled with a handheld grinder. Finally,
a ribbon of molten polymer (the same composition as the geomembrane being seamed) is
extruded onto the prepared ground surface by a handheld extruder. This fillet bead of the molten
polymer is placed at the edge of the overlap where the two geomembranes are being seamed.
The seams described above are tested via ASTM D6392, " Determining the Integrity of
nonreinforced Geomembrane Seams produced using Thermo-fusion Method " in the field as
startup weld and destructively as quality control and quality assurance tests to determine the
integrity of geomembrane seams produced. ASTM D6392 presents the procedures used for
determining the quality of bonded seams subjected to both peel and shear tests. In both cases,
seams are evaluated for strength, ductility and the type of failure. It should be clearly stated
that a seam needs to exceed all three characteristics to pass the GRI-GM19 specification.
An observation that will have direct consequences of seam strength is parent material
thickness. As can be seen from Figure 1, thickness is linearly proportional to yield strength
in PE. Therefore, if the material is thin, it will be difficult to meet specifications assuming
a film-tear-bond (FTB) response where the seamed geomembrane breaks in the parent
material rather than within the seam.
The Geosynthetic Institute (GSI) has a new test method to check for this inconsistency.
GRI-GM37 standard test method for Determining the Weld Edge Thickness of

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Geosynthetics October/November 2023

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