Mechanisms Controlling Hydraulic Conductivity and Service Life of Bentonite-Polymer Composite Geosynthetic Clay Liners Permeated with Aggressive Solutions

Bentonite-polymer composite geosynthetic clay liners (BPC-GCLs) are used to line containment systems such as landfills, leach pads, and impoundments with aggressive leachates that adversely affect conventional sodium bentonite GCLs. BPC-GCLs were permeated with aggressive leachates to understand the mechanisms controlling hydraulic conductivity in BPC-GCLs under various conditions. Empirical results were compared to computational models to develop methods for predicting hydraulic conductivity and service life.
Polymer elution in BPC-GCLs resulted in preferential flow paths and dramatic increases in hydraulic conductivity for several BPC-GCLs. Low hydraulic conductivity in BPC-GCLs is maintained as long as narrow, tortuous pore paths result from the swelling of bentonite granules and/or the retention of hydrated polymer gels between bentonite granules. The product of the swell index of the bentonite component and the flow stress of the hydrated polymer component, herein referred to as flow-swell index, represents both of these mechanisms and shows promise as an index of hydraulic conductivity for BPC-GCLs to aggressive solutions. BPC GCLs permeated or batch aged at 60 ⁰C maintained comparable or lower hydraulic conductivity to those permeated at 20 ⁰C, regardless of changes to swell index and flow stress, provided sufficient polymer is retained in the pore spaces. Hydraulic models developed using COMSOL are consistent with the mechanisms identified through experimental observations, whereby flow is directed at lower velocities through narrow pores when larger pores are filled with polymer gel. Computational models demonstrate that decreases in viscosity of polymer gels resulted in increased elution rates.