A geosynthetic clay liner, usually shortened to GCL and often called a bentonite liner, is an engineered barrier that combines natural sodium bentonite clay with geotextiles. Once hydrated and covered, the bentonite swells to form a very low-permeability layer that holds water in, or keeps contaminants out.
Stratec supplies bentonite geosynthetic clay liners across the UK for SuDS basins, ponds, lakes, lagoons, reed beds and landfill (where a design-specified GCL is required), and offers consulting, assessment and supply so the liner suits the site rather than the other way round.
What is a geosynthetic clay liner?
A GCL is a composite product. A layer of natural sodium bentonite clay is sandwiched between a woven and a non-woven geotextile. The geotextiles give the liner its mechanical strength and hold the clay in place during handling and installation; the bentonite provides the seal.
Because the key ingredient is natural clay, a GCL is often chosen where environmental credentials matter. Because the clay is held in an engineered carrier, it is still classed as a geosynthetic, combining the benefits of natural clay with the consistency of a manufactured product.
How a bentonite liner works
Sodium bentonite has a very high swelling capacity. When the liner is hydrated and confined under the required minimum of 300 mm of cover material, the clay particles expand and form a very low-permeability layer.
That same swelling gives a GCL its best-known property: it is self-sealing. If the liner is punctured after installation, for example by a sharp stone, the bentonite swells around the hole and seals it. Polymer liners, by comparison, need to be found, cleaned and welded or taped to repair a puncture.
Why choose a GCL over a polymer liner?
Contractors usually pick a bentonite liner for one or more of these reasons:
- No welding or taping. Panels are overlapped rather than welded, so there is no need for specialist welding equipment, a power source or a specialist installation team.
- Installation in poor weather. GCLs can be laid in conditions where polymer or rubber liners are difficult or impossible to fit, such as wet or windy weather, which helps programmes on exposed and remote sites.
- Self-healing. Once hydrated and confined, the liner seals around punctures, adding long-term resilience.
- Slope stability. Bentonite liners offer good slope stability and can follow undulating ground, helping keep the soil layers above in place.
- Chemical and climate resistance. They offer chemical and climate resistance and tolerate differential settlement; suitability for a given site chemistry should be checked against the product data sheet.
- Environmental credentials. The natural clay core suits projects, particularly in the public sector, where sustainability is part of procurement.
There are trade-offs to plan for. A GCL is heavy, which affects delivery and usually means placing rolls with plant, and the up-front material cost can be higher than some polymer liners. On many projects the savings in labour, equipment and weather downtime bring the overall installed cost close to, or level with, a standard liner.
Where a site has hydrocarbon or VOC contamination, or needs a gas or radon barrier, an HDPE liner is often the better choice. Our team can advise on which suits your project.
Where geosynthetic clay liners are used
Typical UK applications include:
- SuDS detention basins, ponds and swales (see SUDS Geosynthetics UK)
- Ponds, lakes and lagoons
- Wet systems and reed beds
- Landfill caps and lining (design-specified GCL only)
Project example: a remote Highland substation basin
For a substation detention basin in a very remote Highland location, reached by a five-mile access road, the main contractor needed a sustainable basin that would blend into its surroundings. Stratec recommended a geosynthetic clay liner because it could be installed without a specialist crew or welding equipment, and supplied it in small rolls so it could be laid even in strong wind and poor weather. The result was a shorter installation, with no taping or on-site welding, and a durable, self-sealing basin. Read the Melgarve case study.
Installation essentials
Good performance depends on preparation. The subgrade should be smooth, compacted and free of sharp stones, roots and debris. Rolls are laid with overlaps according to the manufacturer’s guidance and the design, and the liner must be covered with the required minimum of 300 mm of cover material so the bentonite is confined when it hydrates. Because the rolls are heavy, plan placement around the plant already on site; because the liner can be laid in most weather, that is often easier to schedule than for a welded liner.
Talk to Stratec about geosynthetic clay liners
Tell us about the application, the ground, the area to be lined and your programme, and our specialist team will help you choose the right liner and supply it anywhere in the UK.
- Phone: 020 7887 2999
- Email: enquiries@stratecgroup.co.uk
- Or use the contact form
Frequently asked questions
What is a geosynthetic clay liner? An engineered liner made of natural sodium bentonite clay held between a woven and a non-woven geotextile. When hydrated and covered, the clay swells to form a very low-permeability barrier.
Is a bentonite liner the same as a GCL? Yes. "Bentonite liner" and "geosynthetic clay liner" describe the same type of product, because bentonite is the clay used.
Does a GCL need welding? No. GCL panels are overlapped rather than welded or taped, so no welding equipment, power source or specialist welding crew is needed.
How much cover does a bentonite liner need? The liner must be confined under the required minimum of 300 mm of cover material for the bentonite to seal properly and self-heal.
What does self-healing mean? If the hydrated, confined liner is punctured, the bentonite swells around the puncture and seals it.