Evaluation of Prototype Geosynthetic Clay Liners in Landfill Cover Applications

Geosynthetic clay liners (GCLs) are factory manufactured clay barriers, typically consisting of 3.6 kg/m2 sodium bentonite clay between two geotextiles. They are used as hydraulic and environmental barriers for a variety of civil and environmental applications. Because of their low hydraulic conductivity, they are frequently used as a replacement for regulatory prescribed compacted clay liners. Where quality clay is not available nearby, GCLs are frequently used as a substitute. With their many benefits, GCLs do have their limits. Over the last decade, several field studies have assessed the in-service performance of GCLs in landfill covers. In some of these cases, the GCLs exhibited high hydraulic conductivities compared to laboratory tested GCLs. Past research has demonstrated that GCLs which undergo both wetting and drying cycles combined with cation exchange (sodium for calcium or magnesium) can experience increases in hydraulic conductivity over time. To prevent this from occurring, the current industry standard is to either test the adjacent soil for chemistry to evaluate compatibility (i.e. concentrations of calcium, magnesium, sodium, and other major cations), or to install either a separate overlying geomembrane over the GCL or a geomembrane-laminated GCL to prevent clay desiccation. This study exposed prototype polymer-modified bentonite GCLs to several wetting and drying cycles in a calcium-rich solution. After each hydration cycle, samples were allowed to air dry. GCL samples were then tested for hydraulic performance using index test methods and hydraulic conductivity, and compared to traditional GCLs that underwent the same conditions. Four separate polymer-modified bentonite (PMB) GCLs were evaluated in this study. PMB-1 demonstrated poor performance after repeated cycles. PMB-2, PMB-3, and PMB-4 demonstrated improved performance, and have the potential to work well as stand-alone barriers in landfill cover applications.