Laminated GCLs: a stronger case than the peel number suggests

Design the complete barrier.

I favour laminated geosynthetic clay liners built around a separately manufactured, properly specified geomembrane. When the polymer is expected to form part of the containment barrier, that is a logical way to engineer the product.

Choose the membrane for its barrier performance. Choose the bentonite and geotextiles for their roles. Then design the bond and the joints so the completed system works.

My concern is that a higher peel number can overshadow all of those decisions. A laminate may offer better evidence of polymer quality, controlled texture and useful joint details, yet still be dismissed because another product is harder to pull apart.

If the bond already meets the demands of construction and service, the value of further adhesion needs to be explained. Asset owners should know what the extra requirement achieves.

A thicker polymer layer deserves a proper geomembrane specification.

Polymer faces can help limit moisture loss and bentonite erosion. But when a supplier offers a thicker polymer layer and asks the designer to rely on it for liquid or gas containment, the discussion has moved beyond protecting the bentonite.

Published coated-product literature makes explicit claims for an additional hydraulic and gas barrier. In an extrusion-coated GCL, molten polymer is applied directly to the geotextile. In a laminate, the membrane is manufactured separately and then attached. Published coated construction and barrier claims, published laminated construction.

If we are considering a polymer layer around 1 mm or thicker, why would we stop at coating mass and peel strength? We should be asking about the same properties we would investigate in a separately supplied geomembrane.

The principle also applies below 1 mm. The function assigned to the polymer determines the evidence required; thickness does not create an exemption.

For an LLDPE layer used as a geomembrane, GRI-GM17 is a relevant manufacturing benchmark within its 0.5–3.0 mm scope. It addresses physical, mechanical and durability properties. HDPE requires the corresponding GRI-GM13 assessment. These are manufacturing benchmarks; project chemistry, service conditions and the installed system still need assessment. GRI-GM17.

A label saying “PE” is not enough. I would want a defined resin grade and formulation control, traceable production records, thickness tolerances, mechanical properties and ageing data appropriate to the proposed duty.

Calling the layer a coating should not reduce that expectation.

Show the ageing results for the finished polymer.

Oxidative induction time, or OIT, measures resistance to oxidation under defined test conditions. HP-OIT is the high-pressure test. An initial result and a retained result after ageing answer different questions.

We need to know what manufacturing leaves in the finished polymer, then how much of that measured resistance remains after specified ageing. A resin certificate alone cannot answer both questions.

Protecting the antioxidant system requires attention to the whole production process. Extrusion-coating literature describes how high processing temperatures can change the polymer, and how oxidation, antioxidants and adhesion interact. I would require melt temperature, time at temperature, air exposure and cooling conditions to form part of a qualified process. Controlled cooling matters, but it cannot establish the oxidative stability left in the finished layer. That needs testing. Extrusion-coating process and stabilisation.

Published literature for enhanced coated GCLs gives OIT of 200 minutes and HP-OIT of 600 minutes, and states that the required residual values remain after oven ageing. The technical note identifies ASTM D5721, but does not disclose the numerical aged results, retained percentages or specimen preparation. The product page repeats those coating values, but its test-report section does not provide the underlying report. These are specific initial declarations accompanied by a general assurance about ageing. Published coating stabilisation and oven-ageing declaration, published coating properties and test-report section.

In the public documents reviewed, I have not found a product-specific data sheet or laboratory report showing measured aged results traceable to specimens from the finished enhanced coating. That is the evidence gap. A data sheet can state the requirement; the supporting report must show what was tested and what it achieved.

For a polymer layer relied upon as a geomembrane, I would require qualification reports identifying the material, formulation, thickness, production process and specimen preparation, with initial and aged results, retained percentages, test methods and exposure conditions. The evidence should represent the finished barrier layer supplied, including the effects of coating or lamination.

The initial result should identify the unaged, manufactured polymer being tested. Retention should be reported against that baseline using the same test method. Raw-resin data, finished-sheet data and finished-coating data should be clearly distinguished. A manufacturer's separately supplied geomembrane does not qualify its GCL coating, and matching OIT values do not establish equivalent mechanical or joint performance.

ASTM D8117 expressly allows OIT testing of incoming formulated resin before extrusion. The test name and a number therefore do not identify the production stage represented by the specimen. OIT assesses stabilisation; it does not establish that the chemical formulation remained unchanged. OIT interpretation and resin testing.

For LLDPE, GM17 includes retention requirements after 90 days of oven ageing at 85°C using standard OIT or HP-OIT, and a separate UV-ageing requirement using HP-OIT. A high initial value does not replace those checks. Neither initial OIT nor retained OIT alone establishes service life. GRI-GM17 durability requirements.

This strengthens the case for a separately qualified membrane with clear production records, followed by verification that lamination preserves its properties. The same evidence should be expected from a coating. Peel strength cannot establish oxidative durability.

Stress-crack resistance needs its own evidence.

The polymer must resist cracking under the stresses it will carry. For HDPE, stress-crack resistance, or SCR, is a critical part of that assessment. GRI-GM13 includes an SCR requirement using the single-point notched constant tensile load procedure in ASTM D5397. OIT and factory peel results cannot replace it. HDPE manufacturing and SCR requirements.

LLDPE also needs appropriate evidence of performance under sustained loading and deformation, but the same test cannot automatically be applied. GM17 does not include HDPE's routine SCR requirement. ASTM D5397 excludes materials without a well-defined yield point, including some LLDPE formulations. The test method and acceptance criteria must suit the actual polymer and its intended service. LLDPE manufacturing benchmark, SCR test applicability.

SCR is not created by the extrusion process alone. Polymer structure, the additive package, processing history and thermal history all influence the result. Resin selection and manufacturing control belong in the same assessment. Factors affecting SCR, ASTM D5397.

A controlled blown-film or flat-die process gives us a defined membrane to qualify. I would require its extrusion and cooling conditions to form part of the qualified production process, with records linking the tested material to the sheet supplied. Naming the process is not proof of performance; the finished membrane still has to meet the specification.

This is another reason to favour a separately manufactured membrane for lamination. We can assess its mechanical and ageing performance before attachment, then verify that assembly preserves the properties required in service. Where an HDPE coating is assigned the same barrier duty, I would expect coating-specific SCR evidence using an applicable method and representative specimens. For LLDPE, I would require the appropriate formulation-specific assessment of sustained loading, deformation and durability.

If that evidence is missing, the property has not been demonstrated. A high peel number cannot fill the gap.

Lamination lets us select and verify the membrane before assembly.

A separately manufactured membrane gives the designer a clear component to specify. Its formulation, thickness, texture and test records can be assessed before it reaches the GCL production line.

This is already reflected in published laminated-product data. One example specifies a nominal 0.5 mm textured HDPE membrane, minimum average and individual thickness limits, texture measurements and testing frequencies. It states that the membrane is tested by its manufacturer before delivery to the GCL factory, with a GM13 declaration adjusted for the stated thickness. Published laminated-component specification.

That is a useful model for procurement: a defined membrane, with its own acceptance requirements, incorporated into a composite product.

It also gives us a clear way to control the PE formulation independently of the attachment process. I would specify the resin grade, antioxidant and stabiliser formulation, relevant OIT and HP-OIT limits, and oven-ageing retention criteria before manufacture. Qualification should cover the agreed formulation and production process, with traceable quality records for the membrane supplied. Changes to the resin, additive package or process should trigger review and, where they could affect performance, requalification.

That is the practical advantage of a separately manufactured sheet: the barrier can be selected and qualified for its own job before the GCL bond is developed. The evidence should then show that lamination preserves the required polymer properties, including oxidative durability. With adhesive lamination, the membrane formulation can be chosen for the barrier duty while a separate adhesive provides the attachment.

By comparison, a coating mass tells us how much polymer is present per square metre. It does not establish the minimum continuous barrier thickness, particularly where polymer enters the geotextile structure or forms surface texture. Published coated-product tables include coating mass per unit area; designers still need evidence of the local barrier thickness and continuity relied upon in the design. Published coating data.

The engineering advantage of lamination is the ability to select a membrane for the required containment duty and verify it separately. The completed laminate must then retain those properties.

Adhesive lamination is my preferred development route.

A suitable adhesive system allows the membrane and its attachment to be specified separately. The membrane can be chosen for barrier performance, while the adhesive is chosen to provide the necessary bond to the GCL.

The attraction is that the barrier sheet itself does not have to be softened to form the attachment. Some adhesives are applied hot, so the process still needs temperature control. The aim is to preserve the membrane's verified thickness, texture and properties through assembly.

Coating and heat-bonding processes can also use controlled resin formulations. Their process names do not prove inadequate quality. Heat-bonded laminates can start with a separately specified membrane, but need evidence that the bonding heat and pressure preserve its required properties.

My preference for adhesive lamination is therefore a design choice: specify the membrane we want, then develop a compatible bond around it. It is a strong route for product development, provided the adhesive demonstrates the required resistance to water, chemicals, temperature, ageing and sustained loading.

That approach may produce a lower peel result than a deeply integrated coating. I would accept that trade-off where testing demonstrates adequate, consistent attachment and the laminate delivers better overall barrier performance. Lower adhesion is not an objective in itself, and it should never conceal a bond that is inadequate for the application.

Controlled texture is a practical advantage.

A membrane made by flat-die extrusion with controlled profile or embossing rollers can have a defined, repeatable surface pattern. Asperity height—the height of the surface projections—can be measured against an agreed specification before lamination. Structured geomembrane manufacturing.

That gives us something useful to purchase and check: surface height, pattern, distribution and permitted variation, supported by production records. We can then assess whether the material supplied represents the specimens used for design testing.

Published enhanced coated-product data describe the surface as embossed and structured. Embossing can produce a repeatable pattern, but the process description alone does not demonstrate consistency in the supplied rolls. I would require measured surface-height distributions, sampling locations and frequencies, acceptance limits and the minimum continuous polymer thickness beneath the texture. Claims of consistency should be supported by those records. Published coated-surface description.

A published coated-surface criterion specifies a minimum nodule height for 80% of nodules. That criterion alone leaves the lower height limit and distribution of the remaining nodules unspecified. A designer seeking tighter control should require those details. Published surface criterion.

GM17 also uses a minimum average asperity requirement. Tighter individual limits and pattern requirements must be added where the design needs them. Repeatable manufacture supports the use of representative test data; it does not make a shear result transferable to different soils, stresses or hydration conditions. GRI-GM17, Table 2(b) and Note 8.

This is a reason to give a well-specified laminate credit for its surface control, rather than allowing a peel comparison to dominate the assessment.

Peel strength needs a reason behind the number.

A sound bond matters during handling, cover placement and service. But the specification should explain the required resistance and how it relates to those demands.

Drainage geocomposites offer a useful comparison. Published design literature records a historical recommendation of approximately 180 N/m, or 0.18 kN/m, for geotextile-to-geonet peel strength. The same literature also warns that average results can hide weakly bonded areas and describes the need to consider construction loads and variability. Geocomposite lamination design and testing.

That 180 N/m value is not a GCL acceptance limit. It does, however, challenge the idea that an acceptable bond must always deliver the highest available peel result.

If a proposed membrane-to-GCL bond returns 300 or 600 N/m, neither number is an automatic pass or fail. A rejection should identify the relevant failure mechanism, construction demand, durability concern or required design margin. Being lower than another supplier's result is not, on its own, that explanation.

We also need to compare the correct interfaces. Internal GCL peel measures the connection between the GCL's top and bottom geotextile layers. It is different from the attachment of a polymer membrane to the GCL. ASTM D6496 addresses the former. A published 6.1 N/cm GCL peel value—610 N/m—cannot simply be relabelled as membrane adhesion. ASTM D6496 scope, published GCL peel data.

ASTM D7005 also cautions against direct comparisons of geocomposite peel results across different materials or thicknesses. The test includes effects from bending the separated layers, and a general relationship with interface shear resistance has not been established. ASTM D7005.

For slope stability, test the actual product and adjoining materials under representative conditions. For procurement, establish a bond requirement that supports that design and checks production consistency.

A higher peel result does not verify polymer durability, bentonite compatibility or a field seal.

Taped overlaps and welded membrane joints deserve separate assessment.

A strong factory bond between the polymer and GCL says little about the seal between adjacent panels. These are different connections performing different jobs.

Some coated GCL systems specify proprietary tape at their overlaps. Published product information identifies this arrangement; it should be assessed as a taped joint, with evidence for the actual coating, tape and installation detail. Published coated-GCL overlap arrangement.

A taped overlap relies on the tape and its bonded interfaces to maintain the seal. I would want evidence covering surface preparation, application conditions, water and chemical exposure, movement and ageing, together with practical site checks. Taped geomembrane joints have their own test methods, including ASTM D7272 where applicable. They should not be assumed to have the same behaviour or verification requirements as a fusion-welded joint. Testing geomembrane seams made with premanufactured tape.

A purpose-designed laminate can provide exposed, weldable PE edges or flaps, kept free of bentonite, geotextile and adhesive in the weld area. That allows the membrane joint to be designed around a qualified polymer sheet and a validated welding procedure. The adhesive attaching the membrane to the GCL has a separate role from the weld joining neighbouring membrane panels.

This offers a useful construction advantage. Suitable dual-track welds provide an air channel for pressure testing under ASTM D5820. Destructive peel and shear tests under ASTM D6392 assess appropriate nonreinforced geomembrane weld specimens. These checks provide evidence of seam quality during installation; long-term joint durability still needs separate assessment. Dual-seam air-channel testing, thermofusion seam testing.

Weldability must be designed into the laminate. A thin film, fully bonded edge or unsuitable formulation does not become a qualified field-welded system simply because it is called laminated. Equally, coating OIT and factory peel results cannot establish that a coating is suitable for field welding.

Where water can enter an inadequately sealed overlap and follow a connected interface, the system may have a preferential leakage path. The issue is the continuity and flow capacity of that path, the pressure driving the water and the ability of the joint to seal.

The useful evidence is hydraulic testing of the actual overlap under the expected head, confinement and chemistry. The GCL overlap must still perform its own sealing role beneath the polymer connection.

Side joints, end joints, intersections, penetrations and repairs all need workable details. A thicker polymer layer and a higher peel result do not resolve them automatically.

Where a continuous, weldable polymer barrier is required, a laminate designed with qualified membrane joints deserves clear credit. Its factory peel number should not obscure that benefit.

Granular bentonite deserves the same fair assessment.

The argument for wider product choice also applies to the bentonite inside the GCL.

Powder can hydrate more quickly under some conditions. Particle size can affect both how quickly and how fully a GCL hydrates from the available subsoil moisture. Research on GCLs over lateritic soils illustrates why the subgrade and GCL construction matter. Research on particle size and GCL hydration.

With suitable chemistry, hydration and confinement, both powdered and granular sodium bentonite can form a swollen, low-permeability barrier. The useful comparison is their hydraulic performance under the conditions they will face.

Particle form should be assessed alongside mineral quality, bentonite mass and distribution, chemistry, hydration conditions and the way the GCL is reinforced. A favourable result in clean water does not establish long-term performance in a saline or chemically aggressive liquid.

Granular products can offer lower-dust handling and better bentonite retention, depending on the product construction. Published manufacturer literature identifies both benefits. They deserve consideration in manufacturing, transport and installation, supported by evidence for the actual product. Published granular-GCL characteristics.

Granular GCLs can also be manufactured with high internal shear strength. Published product literature attributes higher peak strength to dense needle-punch reinforcement. Any claimed advantage must be established on the finished, hydrated product; dry particle size alone does not determine its shear strength. Published reinforcement characteristics.

EPA Victoria's published landfill guidance recognises both powdered and granular bentonite and addresses bentonite properties, GCL construction and performance. Product selection must meet the applicable approval and project specification. EPA Victoria landfill guidance, Appendix E.

Where a specification excludes granular bentonite, or imposes a particular limit on fines, the technical basis should be clear. Particle-size limits can serve a purpose. They should be linked to demonstrated performance, manufacturing consistency or an applicable requirement.

A properly qualified granular option should receive credit for its handling, retention and mechanical performance. It should not be dismissed simply because powder is more familiar.

Dust belongs in the product assessment.

A product that releases less dust offers a practical benefit to the people manufacturing and installing it. That deserves attention alongside hydraulic and mechanical data.

Bentonite may contain crystalline silica, depending on its source. Published GCL safety data sheets identify the potential for respirable dust and crystalline silica exposure. Suppliers should provide the current safety data sheet for the supplied product and clear handling instructions. Granular material can also generate fines; lower dust does not mean no exposure risk. Example GCL safety data sheet.

Dust-generating work needs planned controls. Reduce dust release, use suitable containment or extraction where practical, and assess worker exposure. Where respiratory protection is required, it must be suitable for the exposure and used correctly; tight-fitting respirators need fit testing. A generic instruction to “wear a mask” is insufficient. Safe Work Australia silica code.

Suppliers should make these requirements easy to find. Designers and buyers should consider lower-dust alternatives where they meet the required engineering performance.

A better specification would reward the complete product.

I would assess proposed systems against the following requirements:

This gives innovation a clear path: demonstrate an improvement in the functions that matter.

My preference is for adhesive-laminated GCLs that combine a separately verified geomembrane with a well-engineered bentonite component and a durable, adequate bond. That combination deserves serious consideration even when its peel result is lower than a coated alternative.

At Kontain, we work in geosynthetic design and supply, and the systems we propose should meet the same scrutiny.

Competition improves when specifications explain their requirements and allow alternatives to demonstrate performance. It narrows when one index property becomes a substitute for the whole assessment. Where two systems meet the design requirements, a price premium based only on additional peel resistance needs a clear justification.

As the polymer takes on a full containment role, every part of its qualification matters: formulation, thickness, cracking and deformation, oxidative durability, texture and joints. None of those questions disappears because the layer is called a coating.

Engineers and asset owners should set the required performance first, then require manufacturers to develop and demonstrate a product that meets it. The limits of an existing production line should not set the limits of the specification.

Give manufacturers a clear brief and a fair route to qualify better products. Require evidence for the complete barrier. That is how we turn competition into useful innovation and better value for asset owners.