Hydraulic and mechanical behavior of GCLs in contact with leachate as part of a composite liner

When a geosynthetic clay liner (GCL) containing sodium bentonite is brought into contact with fluids and soils containing other cations, the latter may exchange with the sodium present between clay layers. This modification of clay surface chemistry may change the clay microstructure and its mechanical behavior. The influence of cation exchange on the potential for squeezing of bentonite was studied by submitting 10-cm-diameter GCL samples to squeezing tests. Those samples were part of 1-m-diameter GCL specimens that had intentionally been put in contact with leachate during a one year period through the discontinuity in a damaged geomembrane as part of a composite bottom liner, and an underlying soil containing calcium, thus generating cation concentration gradients through the GCL specimens. The impact of the hydration mode of the GCL on flow rate through the composite liner and cation exchange was studied. Results obtained tend to show that if pre-hydration of GCL on the whole surface under no load as compared to pre-hydration through the discontinuity in the geomembrane slightly prevented from cation exchange, it resulted in a larger flow rate in the composite liner. Consequently, based on the experiments presented in this paper, pre-hydration of GCLs prior to confining is discouraged. Furthermore, an increase in the calcium concentration combined with a decrease in potassium and ammonium concentrations at the surface of clay particles lead to a softening of bentonite resulting in a greater susceptibility to bentonite squeezing in the GCL samples.