Geosynthetic clay liners get specified the way most geosynthetics do: by name and by thickness. Both are close to meaningless. A GCL's performance comes from the bentonite inside it and the conditions it meets in the ground, and two products of identical thickness can behave completely differently.
Four things decide it.
1. Bentonite mass, measured dry
This is the property that most directly governs sealing performance, and it's the one most often left out of specifications.
Mass per unit area must be reported at dry moisture content under ASTM D5993, so that values from different products are comparable - a figure quoted at delivered moisture content tells you nothing. Reported values in the literature indicate that a natural sodium bentonite can seal at around 2,700 g/m², while a sodium-activated bentonite needs roughly 3,700 g/m² to reach the same hydraulic conductivity.
That distinction matters commercially. Natural sodium bentonite and sodium-activated calcium bentonite are not interchangeable, and a mass figure without a bentonite type is incomplete.
The performance target itself is well established: a maximum hydraulic conductivity of 5 × 10⁻¹¹ m/s under ASTM D5887, or the EN 16416 flux index in European specifications.
2. Chemical compatibility with what it will actually contact
This is where GCLs fail in mining, and the mechanism is specific.
Calcium, magnesium and other divalent cations in the permeating liquid or the adjacent soil displace the sodium in the bentonite. The clay loses swelling capacity, and permeability can rise by two to three orders of magnitude. Exhumed samples and laboratory work confirm increases of one to two orders of magnitude under cation exchange, desiccation or inadequate confinement.
A GCL that meets 5 × 10⁻¹¹ m/s against deionised water in a test cell may be several orders worse against your process water. Tailings liquors, acid rock drainage, hard groundwater and calcareous subgrade all supply the divalent cations that drive the exchange.
The consequence for specification is simple: conductivity testing against project-representative fluid, not against distilled water. An index value tested to ASTM D5887 tells you the product was manufactured correctly. It does not tell you it will work on your site.
3. Hydration sequence
A GCL has to hydrate from the subgrade before it meets the contained fluid. Hydrated bentonite that then encounters aggressive chemistry performs far better than dry bentonite that hydrates directly in it.
That makes subgrade moisture and installation timing engineering decisions, not site logistics. Placing a GCL onto dry subgrade in arid conditions and covering it late, or exposing it to process fluid before it has taken up water, undermines the barrier before it has started work.
There's a related exposure problem. Where a GCL sits beneath a black geomembrane left uncovered, solar heating drives moisture out of the bentonite during the day and it condenses on the underside of the membrane at night, running down slopes and eroding bentonite as it goes. Field studies have documented significant bentonite loss on slopes from exactly this mechanism. Prompt covering isn't housekeeping - it's the difference between a functioning barrier and a depleted one.
4. Shear strength, on any slope
The bentonite that makes a GCL work is also, when hydrated, a low-friction plane. Internal shear strength and interface shear strength against the adjacent layers are measured under ASTM D6243, and on side slopes they govern the design.
Reinforced GCLs - needlepunched or stitch-bonded - exist because unreinforced ones cannot carry shear. Specifying a GCL for a slope without specifying its shear parameters, at the normal stress and hydration state the design assumes, is specifying a stability problem.
Where a GCL is the right answer
In a composite liner beneath a geomembrane, a GCL is excellent. It seals against defects and wrinkles in the membrane above, it's a fraction of the construction effort of a compacted clay layer, and it removes the problem of achieving consistent moisture and density in the field - which at a remote site, or in arid conditions, is a real and expensive constraint.
Where the argument gets weaker is as a standalone barrier. A GCL is very thin, and diffusive contaminant transport depends on thickness. Against dissolved species, a metre of clay does something a 10mm mat cannot, whatever their relative hydraulic conductivities. This is why the strongest configurations in the literature combine a geomembrane, a GCL and a clay layer, rather than substituting one for another.
As Bouazza put it in a review of case histories, GCLs shouldn't be seen as a panacea for all containment problems, and their use warrants site-specific evaluation.
What to put in the specification
- Bentonite type - natural sodium or sodium-activated
- Bentonite mass per unit area, dry, to ASTM D5993, as a minimum
- Hydraulic conductivity to ASTM D5887 or flux index to EN 16416, plus compatibility testing against project fluid
- Reinforcement type and shear parameters to ASTM D6243 at design normal stress
- Overlap width and any supplementary bentonite at seams
- Covering window after deployment
Six lines. They're the difference between a product that meets a name and one that meets a design.
We supply and install GCL systems across Africa, with full conformance documentation. Talk to our technical team about compatibility testing for your site chemistry.