The use of a Geosynthetic Clay Liner (GCL) as the low permeability layer in landfill lining systems and restoration caps can provide a cost effective and readily Construction Quality Controlled alternative to natural clay. In fact this material may be the only option in regions where the local geology is such that no clay of suitable quality is available.
A GCL uses Sodium Bentonite, a dehydrated clay that has long been recognised as an ideal impermeable barrier material, and which will expand once in the soil to six or more times its initial volume.
Claymat is an example of this type of product which comprises a sandwich of bentonite between two layers of geotextile. The result is a thin, flexible (it arrives in rolls, clean, easily transported and installed) lining system. Finesse Claymat is as manufactured by ABG of Meltham, and there are a number of other manufacturers.
Layers of sand or fine gravel (150 to 300 mm) are often placed on top and/or below the GCL as specified by the landfill capping system designer to protect it from damage during installation or thereafter.
The GCL membranes on the market have been tested to provide both an excellent self-healing capability, and chemical resistance, and are accepted by most environmental regulators. A paper available from Thomas Telford Journals shows that permeability can be compromised if suitable treatment is not applied at overlaps, so clearly the CQA Engineer will need to take care about ensuring good site procedures on this matter. (See “Forensic analysis of excessive leakage from lagoons lined with a composite GCL”, At Thomas Telford Journals.)
Assessments of the environmental protection afforded by a landfill liner require that all underlying soil and geosynthetics components are considered in landfill contaminant migration assessments. The results described in a 2004 paper provides published data for laboratory GCL diffusion and sorption coefficients, required to perform contaminant migration assessments for five VOC contaminants commonly found in municipal solid waste leachate. Assessment of diffusion coefficients and clay-leachate compatibility assessment is also deemed necessary to ensure acceptable long-term performance. In fact GCL membranes were shown to give permeabilities generally significantly lower than those reported in the literature for compacted clay liner materials.
Pozidrain ground water drainage membrane is also often used in conjunction with a Geosynthetic Clay Liner as when laid above the GCL it reduces the hydraulic head and stress on the geomembrane and it also provides additional physical protection against puncture.
A significant number of landfill sites have already utilised the benefits of Geosynthetic Clay Liners and many use Pozidrain and equivalent products within these systems.