Hydraulic and Geochemical Characteristics of a Geosynthetic Clay Liner Exhumed from an Exposed Composite Liner

A geosynthetic clay liner (GCL) was exhumed from a composite liner (geomembrane over GCL containing granular sodium bentonite) in the base of a landfill cell after 12 years of atmospheric exposure. The GCL was altered appreciably during exposure, despite being overlain by a geomembrane. Water content, hydration, and bentonite erosion varied considerably along the slope. GCL samples from the top of the slope were dry, with visible bentonite granules comparable to those in a virgin GCL as if the bentonite had not hydrated. No substantial change in exchange complex or swell index (SI) occurred in samples from the top of the slope, and hydraulic conductivity of these samples to water was low (approximate to 10-11 m/s). GCL samples from the toe of the slope varied considerably, from moist and soft to dry and cracked, but all had undergone hydration, and had low SI, a small mole fraction of monovalent cations in the exchange complex, and high hydraulic conductivity (10-8 m/s or higher). Bentonite in GCL samples from midslope had a cracked structure commonly observed in GCLs that have undergone wet-dry cycling. Midslope GCLs typically had hydraulic conductivity greater than 10-7 m/s. A sample from the top of the slope from a location near a GCL panel separation was comparable to other samples from the top of the slope. GCL samples from the anchor trench differed considerably, being very permeable or having low hydraulic conductivity. Hydraulic conductivity was strongly correlated with SI; GCL samples with SI<15 mL/2 g were highly permeable, and those with SI>15 mL/2 g were comparable to a virgin GCL. The variation in SI was related directly to replacement of sodium by calcium in the exchange complex of the bentonite. Hydraulic conductivity also was affected by the thinning of the GCL.