Date of Thesis

7-16-2014

Thesis Type

Masters Thesis

Degree Type

Master of Science in Civil Engineering

First Advisor

Michael Malusis

Abstract

Geosynthetic Clay Liners (GCLs) are manufactured flat sheets composed of a layer of bentonite sandwiched between two geotextiles. GCLs are used as containment material in environmental and hydraulic applications (Koerner 1998). Previous studies have shown that hydraulic conductivity and diffusion coefficient of bentonite tend to increase when in contact with chemical solutions, making this material less efficient. The purpose of this study was to investigate the relationship between the membrane efficiency coefficient and effective salt-diffusion coefficient of a dense, prehydrated geosynthetic clay liner (DPH-GCL) in the presence of monovalent salt (KCl) solutions. It was also aimed to compare the DPH-GCLs to conventional GCLs based on the aforementioned properties. The relationship between membrane efficiency and effective salt-diffusion coefficient determined for DPH-GCLs was found to be accurately approximated by linear fit as in the case of conventional GCLs tested in previous studies. The membrane efficiency coefficients and effective salt-diffusion coefficients, Ds*, were determined for 5-mm-thick DPH-GCL specimens subjected to five different KCl solutions (source concentration, Cot = 8.7, 20, 47, 80 and 160 mM) in rigid-wall diffusion cells under no-flow conditions. The source KCl solutions and de-ionized water (DIW) were circulated across the top and bottom specimen boundaries, respectively, in a closed system and values of membrane efficiency coefficients were determined based on the differential pressure induced across the specimens due to prevention of chemico-osmotic liquid flux. The Ds* values were calculated using the measured concentrations and electrical conductivities of the solutions exiting top and bottom boundaries. The DPH-GCL specimens exhibited the same trends of decreasing membrane efficiency coefficient with increasing Ds* as conventional (non-prehydrated) granular GCL specimens tested in previous studies. However, the DPH-GCL specimens exhibited higher membrane efficiency and lower Ds* values per given source concentrations relative to conventional GCL specimens tested at the same source concentrations. These findings are consistent with the lower hydraulic conductivities, k, measured for the DPH-GCL specimens and are attributed primarily to the higher dry density of the DPH-GCL specimens (~1.2 Mg/m3) relative to the conventional GCL specimens (~0.4 Mg/m3), although differences in bentonite texture (i.e., powdered bentonite in the DPH-GCL versus granular bentonite in the conventional GCL) and polymer treatment of the bentonite in the DPH-GCL (via the prehydration solution) also may have contributed to higher membrane efficiency and lower Ds* for the DPH-GCL.

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