What is the key mechanism by which Geosynthetic Clay Liners (GCLs) achieve low permeability?

The key mechanism by which Geosynthetic Clay Liners (GCLs) achieve low permeability is the hydration and swelling of the bentonite clay component when it comes into contact with water. A GCL consists of a thin layer of bentonite clay, a highly expansive clay mineral, sandwiched between two geotextile layers, which are then needle-punched or stitched together. When the bentonite is hydrated, it expands significantly, filling the void spaces within the GCL and forming a tight, impermeable barrier. The geotextile layers provide structural support and prevent the bentonite from being washed away. The degree of swelling and the resulting permeability depend on the type of bentonite used (typically sodium bentonite has higher swelling capacity than calcium bentonite) and the confining stress applied to the GCL. The confined swelling creates a dense, tortuous flow path for water, resulting in a very low hydraulic conductivity, often comparable to or even lower than that of compacted clay liners (CCLs).