Performance Variability of Geosynthetic Clay Liners (GCLs)

by Ben Lewis(GeoKonect) and John Scheirs(ExcelPlas)

Overview

Geosynthetic Clay Liners (GCLs) may appear similar on data sheets (e.g., same bentonite mass per area or index properties), yet their field performance can differ markedly. Key factors include the quality of the bentonite clay, the internal reinforcement method (affecting shear strength), the types of geotextiles used (influencing interface friction), and whether the GCL is a multi-component product (e.g., with a polymer coating such as Naue’s Bentofix® X series). This report examines these factors in depth, drawing on international research and industry experience. We also compare coated GCLs to traditional composite liners (a GCL overlain by an HDPE geomembrane) in terms of seam integrity, chemical compatibility, cation exchange resistance, and long-term durability. Case studies and independent test data are included to highlight real-world outcomes.

Bentonite Quality Variability and Hydraulic Conductivity

Not all bentonite is the same. GCL performance is highly sensitive to the bentonite’s mineralogy and chemistry even if two products list similar bentonite mass per square meter on their data sheets. Sodium bentonite with higher purity (greater montmorillonite content and higher plasticity) generally yields lower hydraulic conductivity in clean water, but it can be more vulnerable to chemical attack. For example, one study compared two GCLs: one with high-quality bentonite (≈86% sodium montmorillonite, PI ~548%) and one with lower-grade bentonite (≈77% montmorillonite, PI ~393%). The higher-purity bentonite GCL had about three times lower permeability than the lower-quality bentonite GCL when permeated with deionized water. This implies that under ideal conditions (e.g., fresh water, proper hydration), a more premium bentonite can form a tighter seal.

However, differences emerge when the clay is exposed to chemical stress (e.g., salty or multivalent solutions). In the same study, the high-quality bentonite GCL experienced a much larger increase in hydraulic conductivity upon permeation with calcium chloride solutions than the lower-quality bentonite GCL. With moderately saline solutions (5–20 mM CaCl₂), the “good” bentonite GCL became 2–3 times more permeable than the “lesser” bentonite GCL, and under high CaCl₂ concentrations (50–500 mM), its permeability was tens to hundreds of times higher. In other words, the GCL superior in clean water became more susceptible to chemical attack, likely because its sodium-rich clay was more prone to cation exchange (sodium being replaced by calcium). By contrast, the lower-grade bentonite (with initially more calcium/less sodium) showed smaller proportional changes. This finding underscores that a bentonite’s initial swell index or lab permeability with deionized water doesn’t tell the whole story one must consider the soil or leachate chemistry in the field.

Bentonite cation exchange leads to a loss of swell and a rise in permeability when GCLs encounter electrolytes. Divalent cations like Ca^2+ are especially harmful, as they readily replace Na^+ on the clay and collapse the clay’s structure. Even typical ground soils and leachates contain enough calcium/magnesium to raise a GCL’s permeability by an order of magnitude if the bentonite is unprotected. To mitigate this, some GCLs use polymer-modified bentonite or require prehydration with clean water. Polymer additives (e.g., polyacrylamides) can enhance a bentonite’s resistance to salt attack and reduce the increase in permeability, albeit only if the polymer remains in the clay and does not leach out over time. For instance, a polymer-treated bentonite GCL showed minimal permeability increase (less than one order of magnitude) when exposed to a harsh organic leachate in tests, whereas untreated bentonite would likely have fared worse. These modifications and prehydration practices illustrate how bentonite quality and conditioning are critical: two GCLs with “sodium bentonite” in their specs can behave very differently under real-site conditions if one uses a more robust bentonite (or protective measures) than the other.

Takeaway: Variability in bentonite quality (mineral composition, purity, and any additives) has a direct impact on a GCL’s hydraulic performance. A GCL that appears equivalent on paper may have much lower field permeability if its bentonite is high-purity and kept in a sodium form or conversely, it may suffer higher leakage if the bentonite is more prone to cation exchange. Designers should review detailed bentonite properties (swell index, CEC, montmorillonite content) and request site-specific permeability testing with project leachates to truly understand how one GCL will perform versus another.

Table 1: Example of bentonite qualitydifferences and theirimpact on GCL performance (data from Shackelford etal. 2005).

Internal Shear Strength of Different GCL Products

GCLs must maintain their integrity on slopes and under loads, which depends on their internal shear strength (the shear needed to slide or rupture the GCL internally). Even if two GCLs have similar tensile strength on a data sheet, their internal shear behavior can differ due to how the geotextiles and bentonite are bonded:

• Unreinforced (adhesive-bonded) GCLs: These were an early type of GCL where the clay was glued between geotextiles without needle-punched fibers. They have the lowest internal shear strength. When hydrated, bentonite behaves like a weak gel with a friction angle often around 5° in a fully sheared state. Tests on an unreinforced (adhesive) GCL showed a peak internal friction angle of only ~10° and a residual (post-peak) angle of ~4–5°. This means once sliding initiates, the liner offers almost no resistance beyond the clay’s slick friction. A documented slope failure in a landfill liner was attributed to using an unreinforced GCL it simply couldn’t hold the slope once the clay weakened by hydration. Today, adhesive-only GCLs are seldom used in critical applications.
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• Stitch-bonded GCLs: These use stitching yarn to bind the layers, providing some reinforcement. They have better strength than adhesives but are still relatively modest. Their peak shear strength increases somewhat with normal load, but not as much as needle-punched GCLs. In one comparison, the peak strength envelope of a stitch-bonded GCL was lower than that of a needle-punched GCL, especially at higher stresses. Stitch-bonded products were more common in the 1990s but have largely been supplanted by needle-punched ones for critical uses.
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• Needle-punched (NP) GCLs: These dominate the current market. They are reinforced by thousands of fibers punched through the clay, anchoring the two geotextile faces together. NP GCLs have substantially higher peak shear strength because the fibers can carry tensile load across the weak clay layer. Under low normal stress, the fibers mobilize friction between the GCL’s faces, often giving an apparent cohesion intercept and a higher secant friction angle. For example, a typical needle-punched GCL (nonwoven to nonwoven, ~3.7 kg/m² bentonite) had a peak shear strength described by an effective friction angle of ~23–24° at moderate stress (with a cohesion of ~80 kPa), though at very high normal stress, the fiber contribution leveled off to ~10° friction with a larger cohesion. NP GCLs can sustain peak shear stresses of 100–300+ kPa depending on loading, far exceeding what unreinforced GCLs can hold. As normal stress increases, needle-punched GCLs gain strength more rapidly than stitch-bonded ones. High-quality NP GCLs often undergo a Thermal Lock process (heat-treating the fibers after punching) to lock them in place. This boosts internal shear stability by preventing fiber pullout and reducing clay extrusion, preserving strength even at low confining stress.

Importantly, all GCLs regardless of reinforcement tend to converge to a similar residual shear strength after large displacements because the reinforcing fibers either break or pull out, leaving a layer of slick bentonite. Residual shear strength envelopes for different GCL types are essentially the same (~5° friction, near zero cohesion) once the GCL is fully sheared. Thus, it’s the peak shear strength that differentiates products. Needle-punched GCLs can sustain much higher shear before failure (often 2–4× higher peak stress than an unreinforced GCL). This is why all major GCLs for slopes and bottom liners today are needle-punched. Indeed, there have been no known internal failures of reinforced needle-punched GCLs reported in the literature they are robust when properly designed. In contrast, the low internal strength of early unreinforced GCLs contributed to slope instabilities (hence their virtual disappearance in critical uses).

One index property related to internal shear is the GCL peel strength (ASTM D6496), which measures the bonding between geotextiles (via fibers) in a dry state. Higher peel strength generally indicates more fibers and better reinforcement. Some studies found a correlation between peel strength and peak internal shear strength e.g., a needle-punched GCL with higher peel had higher peak shear capacity. However, other studies (e.g., Zornberg et al. 2005) did not find a clear correlation for certain products, suggesting that factors like fiber orientation and bentonite consistency also play a role. Regardless, manufacturers use peel strength as a quality control metric, and specifying a minimum peel strength can help ensure consistent reinforcement across batches.

Practical implication: When two GCLs “look the same” on a spec sheet, check how they are constructed. A needle-punched, thermally locked GCL will have a dramatically higher slope stability capacity than an older adhesive or simple stitch-bonded GCL. If one product advertises, say, a peak internal friction angle of ~20–25° (with some cohesion) and another is silent on this, it’s likely the latter is weaker internally. Always request internal direct shear test results under relevant normal stresses for the GCL being considered especially for applications on slopes or vertical walls. Never assume all GCLs have the same shear strength. Using an insufficiently reinforced GCL can lead to failure when the liner is under load or on an incline.

Interface Friction: Geotextile Types and Surface Characteristics

Beyond internal strength, interface friction between the GCL and adjacent materials is critical for slope stability and overall performance. GCLs come with different surface geotextiles (woven or nonwoven, sometimes scrim-reinforced), affecting how the GCL interacts with soils and geomembranes:

• Nonwoven (NW) Geotextile Surface: Many GCLs have at least one nonwoven polypropylene geotextile face. Nonwovens are fuzzy and can “grip” better against soils and textured geomembranes. A nonwoven-to-soil interface typically exhibits relatively high friction, as the nonwoven can interlock with soil particles. For example, in one study with a residual clayey soil, a GCL with a nonwoven surface achieved an interface friction angle of about 33° against the soil. Nonwovens also deform to the texture of geomembranes, providing a good interfacial bond if the geomembrane is textured.
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• Woven Geotextile Surface: GCLs often include a woven (typically slit-film) as a carrier layer for strength. Woven surfaces are flatter and can be more slippery against other materials. Interestingly, the study mentioned above found a GCL with a woven surface had a slightly higher friction angle (~37°) against that particular residual soil. However, the nonwoven side provided some adhesion (~12 kPa apparent cohesion) while the woven had essentially zero adhesion. This suggests the nonwoven fabric engaged the soil enough to generate a small cohesion intercept, whereas the woven relied purely on friction. In general, differences of a few degrees can occur based on surface texture. It’s worth noting that with different soils or liners, the trend could reverse coarse or rough surfaces usually favor nonwovens.
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• Composite (Scrim-Reinforced) Geotextiles: Some GCL products use a dual-textured carrier, e.g., a nonwoven bonded to a woven (scrim) as one layer. The idea is to combine strength with friction. For instance, Naue’s Bentofix “B” series GCLs use a nonwoven/woven composite carrier. The manufacturer notes this provides “additional interface friction” and better internal shear and puncture resistance. Essentially, the nonwoven component of the composite adds surface roughness for friction, while the woven provides tensile strength and dimensional stability. These composite carriers are often chosen for landfill side slopes where the GCL lies beneath a geomembrane the higher friction can reduce the risk of the geomembrane and GCL sliding on each other or on the subgrade.
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• Coated Surfaces: (Discussed more in the next section on coated GCLs.) A smooth polymer coating on a GCL is usually low-friction against other smooth surfaces, but some coatings are textured or embossed to improve grip. For example, Bentofix X GCLs have an “embossed” polyethylene coating to increase surface roughness. When placed against a textured geomembrane or rough subsoil, this can yield decent friction, but it may be less than a fibrous nonwoven interface. Manufacturers of coated GCLs often provide interface test data to show they can meet slope stability requirements (e.g., one project used a PE-coated Bentofix X on a 3H:1V slope with a textured geomembrane and achieved high interface shear strength per Geosynthetica reports).

Key Point: The outer surfaces of a GCL (which can be nonwoven/nonwoven, woven/nonwoven, or even coated) will determine its interface friction with other layers. If two GCLs use different geotextiles, their friction angles against soils or geomembranes can differ by several degrees enough to impact a slope stability safety factor. It’s not safe to assume friction angle values; instead, use product-specific interface test data. For example, one test showed a woven-faced GCL vs. soil at 37° and a nonwoven-faced GCL vs. the same soil at 33°, while another scenario might find the nonwoven side superior against a textured liner. Always identify which side of the GCL will contact which material (subgrade, geomembrane, drainage layer, etc.) and verify the interface strengths. Modern GCL products address this by design: many have a nonwoven on the side intended to face a geomembrane (since textured geomembrane to nonwoven GCL is a common high-friction interface), and a composite or woven side on the opposite for reinforcement and to avoid friction loss against subgrade. If GCLs look “the same” in general description but one has a woven carrier vs. another’s double nonwoven, this seemingly minor difference can result in different interface friction performance on-site.

Finally, interface friction can also be affected by bentonite extrusion or hydration timing. If a GCL hydrates and the clay extrudes, it can grease the interface. This is why designers try to keep GCLs unhydrated until after cover placement (especially on slopes). Coarser geotextiles and scrims help limit bentonite squeeze-out, maintaining friction. In sum, the GCL’s geotextile type and construction influence both internal strength and interface strength both must be considered to avoid slope failures or excessive movement.

Coated GCLs (Multi-Component GCLs) – Bentofix® X and Others

In recent years, manufacturers have introduced multi-component GCLs that incorporate a thin polymer layer (coating or film) in addition to the typical geotextiles and bentonite. These are often called “coated GCLs” or “polymer-enhanced GCLs.” Naue’s Bentofix® X series is a prominent example: it is a needle-punched GCL with a polyethylene (PE) coating extruded onto one side. Global Synthetics distributes Bentofix X in Australia, promoting it as a GCL that provides an immediate barrier to liquids and gases. Essentially, a coated GCL aims to combine the functions of a geomembrane (the polymer layer) with the self-sealing attribute of bentonite in one product.

Features and Claimed Advantages of Coated GCLs:

• Immediate Low Permeability: Standard GCLs require hydration by water to swell the bentonite and achieve their low permeability (which can take days/weeks and may not happen uniformly if the environment is dry). In contrast, a coated GCL has a plastic film that is impermeable from the moment of installation. This creates a near-immediate hydraulic barrier (as long as the overlaps/seams are sealed) rather than having to wait for bentonite hydration. The bentonite still acts as a backup seal, but the coating is the primary barrier initially. For instance, Bentofix X is marketed as “ensuring an immediate and enhanced barrier” to liquids, gases, and even radon, while also protecting the clay from desiccation.
• Gas Barrier: The PE coating functions as a geomembrane layer, blocking vapor and gas transmission, which dry bentonite alone would not. (Dry bentonite is quite permeable to gases until it hydrates and swells.) With a coated GCL, even if the bentonite is not yet hydrated, gases like methane or radon cannot readily pass because of the poly layer. This is beneficial in applications like landfill covers or radon barriers for building foundations.
• Desiccation and Hydration Control: The polymer film can limit the evaporation of moisture from the bentonite, reducing desiccation cracking of the clay and controlling hydration. If the coating faces upward (toward the liquid to be contained), it forces the GCL to hydrate from below (subsoil) or from the edges, preventing immediate inundation of the bentonite with potentially aggressive liquids. This can slow down cation exchange by keeping the bentonite initially isolated from a chemical solution. Conversely, if the coating is face-down, it might be used to isolate the bentonite from high-sulphate subgrade water and only allow hydration from the contained fluid above. In either case, the coating acts as a protective barrier for the bentonite, maintaining its moisture and chemistry until needed.
• Enhanced Slope Stability (No Bentonite Erosion): One impetus for coated GCLs came from the phenomenon of downslope bentonite erosion observed when GCLs are left exposed under geomembranes (such as in exposed landfill liners). Temperature-induced geomembrane wrinkles can cause water to condense and drain, carrying bentonite fines with it. Laboratory and field studies (the “QUELTS” experiments in Canada) showed that standard GCLs could lose bentonite from under an exposed geomembrane within a few wet-dry cycles. However, a “multicomponent GCL with coating side up” experienced no bentonite erosion in those tests. The coating essentially seals the bentonite in, so there is no mechanism for water to pick up clay and transport it. This is a significant advantage for cases where a composite liner might be exposed for some time. By preventing bentonite erosion, the coated GCL maintains its long-term sealing ability better than an uncoated one in the same scenario.
• High Internal Shear and Interface Friction: Modern coated GCLs like Bentofix X still retain needle-punch reinforcement through the coating (they punch the fibers and then apply the coating on one side). Bentofix X, for instance, is described as a shear-resistant needle-punched GCL (with a scrim-reinforced carrier) that just happens to have a PE layer on the woven side. The coating is “embossed” to create a textured surface, which improves friction against adjacent layers. One project example (from Geosynthetica) noted that a Bentofix X with a textured coating provided very good interface shear on steep slopes, comparable to traditional textured geomembrane-to-GCL friction. Additionally, Bentofix X incorporates extra powdered bentonite embedded into the cover geotextile which, upon overlapping panels, swells to self-seal the overlaps. While one might worry the plastic film would prevent bentonite at the overlaps from connecting, they address this by impregnating bentonite into the geotextile and using an overlap zone demarcation.
• Simplified Installation (in Theory): Having a single product to install (instead of separate GCL and geomembrane layers) can reduce installation steps. There is no separate geomembrane to deploy or weld in this scenario the GCL roll is simply laid like normal and overlaps are addressed in the field. This can save time if done correctly, although, as discussed later, the overlap sealing of coated GCLs requires careful attention (often using a special tape or other methods).

Example – Bentofix® X Structure: Bentofix X is constructed with a layer of high-swelling sodium bentonite (powdered) sandwiched between a nonwoven polypropylene top geotextile and a woven slit-film polypropylene bottom geotextile, which are needle-punched together. After needle-punching (and thermal locking of fibers), an embossed PE coating is applied to the entire outer surface of the woven bottom layer. The coating thickness is a fraction of a millimeter (exact thickness depends on type; Bentofix X ranges have coatings from ~0.2 mm up to ~0.5 mm or more, and in some cases, double coatings). The coating is "structured" (textured) for improved friction and is intact except at the edges of the roll (to allow overlapping bentonite-to-bentonite contact). A printed guideline on the roll shows the required overlap width (typically 0.3 m). Overlaps between adjacent panels can be sealed by simply overlapping so that the exposed bentonite on one panel contacts the bentonite of the next (which hydrates and self-seals), or by using a self-adhesive tape specifically developed for Bentofix X. Naue's guidance states, "unless otherwise required, overlaps and connections must be made with the one-sided, self-adhesive Bentofix® X Tape." This tape is essentially a strip of polymer that bonds the overlapping coatings, creating a continuous plastic layer across the seam.

Other manufacturers have similar multi-component GCLs: e.g., CETCO’s Bentomat CL (which has a thin geomembrane laminated to one side) or Agru’s SuperGCL. Each has its nuances, but the general idea is the same – a thin geomembrane-like layer integrated with the GCL.

On datasheets, two GCLs might both list "reinforced GCL with <0.5 x 10^-9 m/s permeability," but one could be a coated GCL. The coated one will perform differently on-site: initially essentially impermeable (until any damage or over time diffusion) and with different handling needs. It’s crucial to recognize if a GCL is coated or not, and not to assume their behavior is identical.

Coated GCLs vs. Traditional GCL + Geomembrane Systems

Multi-component coated GCLs invite the question: Can they replace the conventional composite liner of a geomembrane over a GCL? What are the pros and cons of each approach? We compare several aspects:

1. Seaming and Overlap Integrity:A traditional composite liner uses a separate HDPE or LLDPE geomembrane that is field-welded. Geomembrane seams are made by fusion welding two overlaps, typically producing a double-track weld with an air channel that can be pressure-tested for leaks. This yields a high-confidence seam (welds are tested by vacuum box or air pressure and destructive peel tests). In contrast, a coated GCL’s polymer layer is much thinner and attached to geotextile, so it cannot be welded in the same manner as a thick geomembrane. Instead, overlaps of coated GCL are usually sealed by bentonite plus an adhesive strip or tape. For Bentofix X, Naue provides a proprietary self-adhesive tape for the PE coating seam. This tape is applied over the 30 cm overlap area, bonding the coating of one panel to the next. While this can effectively seal the overlap, it is more akin to a single-seam tape seaming a thin geomembrane. Quality assurance of these seams is a concern unlike a double-weld, there’s no built-in test channel. Installers must rely on visual inspection and perhaps peel tests on sample overlaps. If a project requires, an extrusion weld could potentially be done on the overlapping coatings, but the thinness (~0.5 mm) makes it tricky and risks damaging the geotextiles. Thus, weld quality is generally superior for a separate 1.5 mm geomembrane, whereas taped overlaps of a coated GCL introduce some uncertainty. Proper installation guidance and inspection are essential to ensure no gaps or wrinkles in the tape. A poorly executed tape seam could be a weak link where liquids can penetrate if the bentonite doesn’t swell perfectly. In summary, composite liners have field-proven welding techniques, while coated GCL seams are a newer practice without decades of track record.
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2. Interface Friction and Slope Stability:In a traditional composite liner, you have a geomembrane in contact with a GCL. The critical interface is often between the geomembrane and the GCL (or the geomembrane and another layer like a drainage geocomposite). Designers usually specify a textured geomembrane to be against the GCL’s nonwoven to achieve high friction. With a coated GCL, the “composite” is already bonded, so internal sliding within the GCL is unlikely (the needle-punch fibers hold it together). The potential interface to check is between the coating and whatever is above it (e.g., if a geomembrane or cover soil is placed on top of the coated GCL) or below it (coating vs. subgrade). If the coated GCL is the primary liner, typically cover soil or a protection layer will go above it so friction between the coating and soil is relevant. An embossed/textured coating provides reasonably good friction, but it may be lower than a traditional textured geomembrane interface. However, since the coated GCL does not require a geomembrane on top, one less interface exists. Overall, slope stability can be adequate with coated GCLs if designed properly; for instance, the Bentofix X10F product achieved high interface friction on 2.5:1 slope without a separate geomembrane. Each case needs testing one shouldn’t assume friction angles; instead, use direct shear data for “coating vs. soil” or “coating vs. geomembrane” as applicable.
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3. Chemical Compatibility and Cation Exchange Resistance:A major advantage of composite liners (geomembrane + GCL) is that the geomembrane physically isolates the bentonite from the leachate, preventing cation exchange and chemical attack on the clay for as long as the geomembrane is intact. In a coated GCL used as a single liner, the thin coating serves the same isolation role. Coated GCLs have shown impressive performance in aggressive fluids when the coating faces the fluid. For example, a study noted that a polymer-coated GCL in contact with a high-strength organic leachate only had a modest increase in permeability, far less than an uncoated GCL would. The coating acted as a barrier, so the permeating liquid reaching the bentonite was greatly reduced and possibly altered (the coating can slow diffusion of multivalent ions). Additionally, if a coated GCL is placed coating-up, the bentonite underneath can be prehydrated with clean water from below or before installation, ensuring it swells with mainly sodium ions. This can preserve long-term low permeability. Thus, coated GCL vs. separate geomembrane in terms of chemical resistance: both effectively shield the clay from harsh chemicals initially. The coated GCL’s bentonite may actually stay in better condition if the coating is fully intact, since even a pinhole in a separate geomembrane could expose a small spot of bentonite to attack, whereas in a coated GCL the polymer is bonded across the whole area (harder for liquid to find a path to the clay except via diffusion or a damaged spot). That said, the polyethylene coating is much thinner than a standard geomembrane, so over years, permeation of chemicals through it (by diffusion) will be faster. A 0.5 mm film has 3× the permeation rate of a 1.5 mm membrane of the same material. If a site has high concentrations of organic solvents, for instance, a thin coating might not hold up as long as a thick geomembrane before breakthrough. This must be considered in design sometimes coated GCLs are used in conjunction with a thin geomembrane or secondary liner for extra safety. Additionally, one must ensure the coating polymer itself is chemically compatible (usually it’s HDPE or LLDPE, which are broadly chemical-resistant, but perhaps slightly more prone to stress crack if not as thick)
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4. Durability, UV, and Long-Term Degradation:One of the sharpest critiques of coated GCLs is the uncertainty in the long-term durability of the thin polymer layer. Geomembranes (especially HDPE) have well-established formulations with antioxidants, carbon black for UV protection, and standardized testing (like ASTM GM13, which sets criteria for tensile strength, stress crack resistance, oxidative induction time, etc.). They are often 1.5–2.5 mm thick and can survive decades, even exposed, if properly formulated. In contrast, the polyethylene coating on a GCL is typically 0.1 to 1.0 mm thick, and its exact composition may not be as tightly controlled by industry-wide standards. A geosynthetics industry bulletin noted that if a GCL coating is only, say, 0.5 mm, it is “far below the range demanded of geomembranes for longevity (1.5 mm+).” A thinner layer will age faster from processes like oxidation there’s simply less cross-section to lose before it becomes compromised. Moreover, the entire surface area of the polymer in a coated GCL is bonded to geotextile and exposed to any percolating substance on one side, so stresses like thermal expansion, chemical exposure, and mechanical strain act on a very thin film. If not adequately stabilized, such a coating could embrittle or crack over time. The same bulletin cautioned that these coatings have “unknown density, stabilizer packages and unknown stress behaviour” compared to well-characterized geomembranes. In other words, there is often no publicly available specification for the polymer coating’s properties (it’s part of a proprietary GCL product), which makes it harder for engineers to assess its lifespan. This lack of a clear standard leads to concerns that coated GCLs might be suitable only for short or mid-term use until further longevity data is available. Indeed, the Geosynthetic Research Institute’s GCL5 guidance includes a note echoing that multicomponent GCLs should be treated cautiously for long-term design. UV exposure is another aspect: a thin polyethylene layer can degrade very quickly under
   
   UV if left uncovered (matter of weeks to months), especially if it’s not heavily stabilized. Most applications cover the GCL promptly, but if a coated GCL were ever used in an exposed cover (e.g., a solar evaporation pond liner), one would need to ensure the coating has UV additives or plan for replacement. Standard geomembranes have 2–3% carbon black and can handle sunlight; a GCL coating might not. Thus, the coated GCL should be covered as soon as possible, just as one would do for a non-UV resistant geomembrane.
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5. ‍Leakage and Failure Modes:In a composite liner (GM + GCL), the geomembrane takes nearly all the hydraulic head; any leakage through a hole is then attenuated by the GCL. A coated GCL in solo acts similarly the coating takes the head, and if there’s a flaw, the bentonite is right there to seal. But consider a possible failure scenario: If the coating has a hole or a tear (from installation damage or later settlement), will the bentonite successfully seal it? Possibly yes, if the bentonite is hydrated and adjacent to the hole it can swell into the defect and self-heal as bentonite is known to do. However, if the coating prevented the bentonite from hydrating (say the liner has not seen water yet or was under a dry cover), the bentonite at that spot might be dry and won’t swell instantly to plug the hole. Traditional composite liners don’t have this issue because if the geomembrane has a hole, the GCL below is likely hydrated by subsoil moisture and can respond. With a coated GCL, especially coating-up configuration, the bentonite could remain relatively dry if the subgrade was dry. One solution is to purposely prehydrate the GCL (spray water on subgrade or rolls) before placing in critical applications but this must be balanced against slope stability issues. So, a concern is that coated GCLs rely on the integrity of the thin coating as the primary barrier, and any breach could be more consequential if the bentonite isn’t in an ideal state to take over. In contrast, a composite liner relies on the geomembrane but typically also has a leak detection layer or at least the GCL as a full secondary layer.
   
   Another subtle point brought up by designers is that if you have a coated GCL under a geomembrane (i.e., using both which sometimes is done, coating as a secondary barrier), you’ve essentially created a double liner with a gap between the GM and the coating. Should that gap be treated like a standard double-liner leak detection layer? If not vented or drained, any leak that gets between the geomembrane and the coated GCL could travel laterally (potentially causing “whales” geomembrane blisters because the coating is relatively impermeable and can trap fluid). The Geofabrics technical note suggests that if a coated GCL is used beneath a geomembrane, it starts to mimic a dual-liner system, and therefore a drainage layer between them would be wise. If one is going that route, however, the note argues one might as well install a full second geomembrane with a proper geonet, which is a known design, rather than depending on an “unknown” thin coating for similar cost.
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6. Cost and Regulatory Acceptance: While not aperformance trait, it’s worth noting that many regulations (e.g. for landfillsin the US, EU, Australia) explicitly requirea geomembrane of certain thicknessover a clay liner. A coatedGCL may or may not be directlyaccepted as meeting a “composite liner” requirement. There might need to beequivalency demonstrations. In terms of cost, coated GCLs save the cost of aseparate geomembrane installation, but the product itself is more expensivethan a standard GCL. For large projects, separate systems might still becheaper or comparable. The Geofabrics piece pointed out that a thicker coatedGCL (with ~0.5 mm coating for betterdurability) could cost about the same as a dual-liner system with leakdetection when all is said and done. Thus, the economicsaren’t overwhelmingly in favourof coated GCLs except in niche cases or where the logistics of installing ageomembrane are challenging.
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7. Summary comparison: A coated GCL can provide equal short-termhydraulic performance as a composite liner(both effectively impervious) and can simplify the liner system, while alsoprotecting the bentonite from early chemical exposure and erosion. However,its thin polymerlayer raises questions for long- termdurability, seam quality assurance, and code acceptance. TraditionalGCL+geomembrane systems have a long track record, clear standards (1.5 mm HDPE per many regs), and well-proven installation/testing methods, but involvemore material and installation steps. Some engineers view coated GCLs as aninnovation for specific cases (temporary liners, liners in remote areas wheredeploying two materials is cumbersome, additional barrier for covers, etc.),rather than a wholesale replacement for standard composite liners in critical hazardous waste applications. When evaluating two GCL solutions –one being a coated product and one being a separate geomembrane + GCL – it’s important to weigh these factors.Don’t just comparepublished permeability values (whichwill look great for both); also consider seam/sealing method, known servicelife of the barrier, and whether additional drainage or monitoring layers are required.

Case Studies and Independent Evaluations

To ground the above discussions, here are a few illustrative findings from practiceand third-party research:

• Bentonite quality effect (chemical attack example): A municipal waste landfill in Germanyreported issues with a GCL that had high-quality Wyoming bentonite when it wasleachate-exposed without a good prehydration. Lab tests like those byShackelford et al. (2005) explain why: the high-swell bentonite lost more ofits low permeability upon leachate exposure than a lesser bentonite might have.This led the operator to specify prehydration of GCLs with tap water prior towaste placement, and in later cells they even switched to a polymer-amendedbentonite GCL to improve chemicalresistance. The lesson was that paperspecifications (e.g. “permeability ≤ 5×10^-11 m/s in water”) didn’t guaranteeperformance once calcium-rich leachate came into play. Site- specifictesting and selecting bentonite suited for the leachate prevented recurrence ofthe problem.
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• Unreinforced GCL slope failure: Inthe 1990s, a landfill side slope slid soon after construction. Investigations(documented in a case history) found that an unreinforced, adhesive-type GCLhad been used beneath a textured geomembrane. When the GCL hydrated, its internal strengthwas so low that it failed at the interface or internally, causing thegeomembrane and cover soil to slide. Afterthis, the landfill had to be rebuilt with a needle-punched GCL and gentlerslope. This case underscores that using the wrong GCL (one with inadequateinternal shear strength) can lead to failure, even though the GCL might have met all “hydraulic” specs. Many landfilldesigners now mandateonly needle- punched GCLs witha minimum peel strength for slopes, precisely to avoid such failures.
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• Interface friction differences: A tailings dam project in South America found that one GCL product gave persistent low interface frictionagainst a textured geomembrane in large-scale tests, while another GCL(different geotextile) gave much higher friction. Both were needle- punched,but one had a slick woven carrier on the interface whereas the other had agritty nonwoven. The difference was on the order of 8° in friction angle andgreatly affected the required anchorage of the liner on the slope. Thisreal-world testing aligns with the lab findings that geotextile type matters. As a result, the project chose the GCL with the higher interface friction toensure stability, even though the alternative had a slightly lower cost. Thisis a case where the datasheets didn’tmention interface friction at all – only through independent testing didthe performance difference emerge.
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• CoatedGCL vs composite performance (QUELTSstudies): AtQueen’s University in Canada, a series of field plots (QUELTS) examined GCLs under exposed conditions. Itwas noted that standard GCLs under a black geomembrane exhibited significantbentonite erosion after repeated wet-dry cycles (leading to some panel sectionshaving increased permeability). However, a coated GCL (coating facing up underthe geomembrane) showed no detectable erosion even after over a year. Thisvalidated one benefit of coated GCLs. On the other hand, when researchersconsidered the long term, they raised the question of how the thin coatingwould hold up. It was suggested that for long-term design, either the coatingneeded to be much thicker, or one should use a conventional design. The manufacturerof the coated GCL used in QUELTS responded by highlighting that their producthad UV stabilisers and was notintended to be left exposed indefinitely, and that in covers (once buried) itslongevity would be more than sufficient. The back-and-forth between researchers and manufacturers here is healthy– it’s pushing the industry to gather more long-term data onmulticomponent GCLs.
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• Laboratory compatibility tests (Rowe et al., 2011): Extensivepermeability tests have been done comparing different GCLs in aggressivesolutions. A notable result – a polymer-coated GCL in a calcium-rich acid mine drainagemaintained a low permeability (<1×10^- 10 m/s) whereas a standardGCL increased to ~1×10^-8 m/s under the same conditions. This is attributed tothe coating reducing ion exchange and preventing clay erosion (the coated GCL’seffluent had no clay particles, whereas the uncoated GCL leachate had colloidalclay, indicating internal erosion). Such third-party tests (often published in Geotextiles & Geomembranes journal)give credibility to some of the performance claims of coated GCLs, while alsohighlighting that not all GCLs are equal in tough environments.
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•  Independent technical critiques: The Geofabrics Australia technicalnote (2020) from which we cited severalobservations reflects an industryviewpoint that is a bit sceptical of coated GCLs as a wholesale replacement fora geomembrane. It raised concerns about the lack of standardised specs for thecoating and suggested that a properly designed dual-liner system might often bepreferable for critical applications. It also noted that all GCLs in Australia contain some additives (e.g. a pH buffer orpolymer) to meet rigorous local EPA criteria,and urged transparency from suppliers on what is in their bentonite. Thisreminds end-users that not only the presence of a coating, but also any polymermixed into the bentonite, can affectperformance – positively or negatively. For instance, polymer additives canimprove chemical resistance but might reduce shear strength or be prone toleaching out; thus, knowing the exact formulation is important for long-term assessment.

In summary, real-world cases and tests confirm that seemingly similar GCLs can perform very differently. One GCL might fail on a slope where another succeeds; one might remain impermeable in a chemical environment where another leaks; one might endure weather exposure while another suffers bentonite loss. These outcomes depend on the construction and composition details we’ve discussed: bentonite quality, reinforcement method, surface texture, presence of coatings, etc. The best approach for designers and specifiers is to require project-specific performance tests (shear tests at expected loads, permeability tests with site fluids, etc.) for any GCL considered and to scrutinize beyond the basic datasheet. Likewise, owners should ask for third-party certifications or case histories for new innovations like coated GCLs before relying on them in critical containment.

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Conclusion

GCLs are not commodity products where one “equivalent” can simply replace another without evaluation. Variability in bentonite quality can lead to order-of-magnitude differences in hydraulic conductivity, especially under chemical stress. Differences in internal reinforcement (needle-punched vs. unreinforced) radically alter slope stability and strength, explaining why historical failures occurred with unreinforced GCLs. Geotextile types and surface treatments change the frictional behavior at interfaces, affecting the design of cover systems and liner stability. The advent of coated GCLs adds another layer of complexity providing enhanced performance in some areas (immediate barrier, chemical protection, erosion resistance) while introducing new considerations for seams, longevity, and standards compliance.

When two GCL products have identical index properties on paper, one must “look under the hood.” Examine the bentonite’s source and modifiers, the reinforcing method and tested shear strength, the type of carrier and cover geotextiles (and now, whether there’s a polymer coating). International research and field data make it clear that these factors can make one GCL far outperform another in a given scenario. Likewise, a coated GCL should not be assumed equivalent to a thick geomembrane without evidence; issues of seam QA and long-term durability must be addressed.

In practice, successful projects have emerged from carefully matching the GCL choice to the project demands for example, using salt-resistant polymer-treated GCLs for mining leach pads, or using a coated GCL in a cover system that will see thermal cycling, or insisting on high peel strength needle-punched GCLs for steep landfill slopes. The failures and problems, on the other hand, often trace back to assumptions that “a GCL is a GCL.” By learning from the wealth of laboratory studies and field experiences worldwide, engineers can avoid those pitfalls. In specification, it is wise to go beyond the datasheet: include requirements for minimum montmorillonite content or swell index, minimum reinforcement peel strength, interface friction testing, and if considering coated GCLs, demand information on the coating thickness, polymer properties, and any available long-term testing or case studies.

In conclusion, GCLs are high-performance geosynthetics but only as good as their constituent materials and design details. Two products that look alike in marketing brochures may, in fact, be “apples and oranges” in the field. Through an understanding of bentonite variability, shear strength mechanisms, interface behavior, and the nuances of multi-component GCLs, one can make informed choices to ensure the GCL system performs as intended for the long haul.

Always consult reputable technical publications, independent test data, and experienced geosynthetics experts when evaluating GCL options the stakes in containment performance are too high for surprises. The references and cases cited here, from Australia to North America and Europe, collectively emphasize due diligence: treat GCL selection and design as an engineered process, not a commodity purchase.

Sources:

• Shackelford, C. et al. (2005). Impact of Bentonite Qualityon Hydraulic Conductivity ofGCLs. Journal of Geotechnical & Geoenvironmental Engineering, 131(1), 64-75.
• Fox, P. et al. (1998 & 2013). Studieson GCL shear strength in Journal ofGeotechnical & Geoenvironmental Engineering and Geosynthetics Magazine.
• Research on GCL interface friction(e.g. Vukelic et al. 2008)and manufacturer data from Global Synthetics (Bentofix product info).
• Technical notes on multi-componentGCLs: Geofabrics Australasia (2020), Designing for Bentonite Erosionon Slopes andGRI White Paper (GRI-GCL5).
• Naue GmbH Bentofix® X specifications and Global Synthetics Bentofix brochures.
• Rowe, R. & Hosney, M. (2019). Geosynthetic clay liners: Perceptions and misconceptions, Geotextiles & Geomembranes (citedin summary).
• QUELTS field study summaries(Rowe et al. 2016) on bentonite erosion and polymer additives.
• U.S. EPA Archive (ClintonLandfill case) – Bentofix technical letter on cationexchange and hydration practices.
• Additional references from GeosyntheticsMagazine.com and Geosynthetica.com for case historiesand product reviews.

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Authors

Ben Lewis Geosynthetics Technical Consultant and Founder of GeoKonect

Specialising in HDPE, LLDPE, GCLs, and geotextile tube solutions for waste, water, and mining containment. (For inquiries or further discussion, please reach out via GeoKonector email ben.lewis@kontainsolutions.com.)

Dr. John Scheirs (ExcelPlas) Geosynthetics Polymer Consultant and Founder/Editor of GNA john@excelplas.com.au

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