Available at: https://digitalcommons.calpoly.edu/theses/3071
Date of Award
6-2025
Degree Name
MS in Civil and Environmental Engineering
Department/Program
Civil and Environmental Engineering
College
College of Engineering
Advisor
Robb Moss
Advisor Department
Civil and Environmental Engineering
Advisor College
College of Engineering
Abstract
Geosynthetic clay liners (GCLs) are used in municipal solid waste landfill (MSW) liner and cover systems as an alternative to thick compacted clay layers. Previous field studies have revealed potential for bentonite erosion in GCLs when subjected to wet-dry cycles that occur when GCLs are placed on slopes, covered with a geomembrane, and left exposed without additional soil cover. A laboratory test program was conducted to quantify the effects of bentonite erosion on hydraulic conductivity of sodium-bentonite geosynthetic clay liners and samples subjected to cation exchange treatment in addition to bentonite erosion. This test program determined what level of bentonite erosion in non-woven needle-punched reinforced GCL samples resulted in increased hydraulic conductivity and how erosion coupled with cation exchange changes this relationship.
Erosion levels of 10, 20, 40, 60, and 80 percent of original bentonite mass lost were studied, corresponding to erosion levels observed in previous field and laboratory studies. A procedure was developed to remove bentonite from the test samples in a uniform manner. For samples subjected to cation exchange, representative low and high levels of cation exchange were reached by pre-hydrating the test samples in 0.05M and 0.2M solutions of calcium chloride, representative of ranges seen in MSW leachate and corresponding to solution strengths used in prior studies. Samples treated for cation exchange were pre-hydrated in the cation-bearing solutions after bentonite erosion was completed. After bentonite erosion and cation exchange treatment, hydraulic conductivity tests were performed on the samples using a flexible wall permeameter. Average water, a solution of deionized water, calcium chloride, and sodium chloride, was selected as the permeant fluid for testing to mimic in- situ conditions for GCLs in a MSW landfill.
Hydraulic conductivity of the GCL samples not subjected to cation exchange was within the same order of magnitude as the non-eroded GCL samples up to the 40 percent erosion level. Between erosion levels of 0 and 40 percent, samples subjected to cation exchange had hydraulic conductivities between 2x10-6 and 2x10-5 cm/s, compared to a range of 3x10-9 to 7x10-9 cm/s for samples that were not treated for cation exchange. A bentonite erosion level of 60 percent and higher in the erosion-only samples resulted in a hydraulic conductivity approximately four orders of magnitude higher than the untreated GCL. At bentonite erosion levels of 60 percent and above, samples subjected to cation exchange had hydraulic conductivities within the same order of magnitude as the samples subjected only to bentonite erosion. The results of this study indicate that GCLs subjected to bentonite erosion lose effectiveness as landfill liners at an erosion level between 40 and 60 percent and that, at lower bentonite erosion levels, cation exchange impacts the effectiveness of the GCL more than bentonite erosion. The results of this study can be used as a baseline to inform future research to refine the relationship between bentonite erosion, cation exchange, and hydraulic conductivity.