Effect of Mix Water Quality on the Design and Hydraulic Conductivity of Soil-Bentonite Cutoff Wall Backfill
Date of Thesis
This study examined the effect of mix water quality on the behavior of soil-bentonite backfill materials for use in contaminant containment applications. Previous studies have observed the behavior of soil-bentonite backfills when permeated with electrolyte solutions. To date, no studies have sought to study backfills prepared with slurries hydrated with electrolyte solutions. In this study, slurries of multiple bentonite contents were prepared with natural sodium bentonite (NSB) and a range of sodium chloride (NaCl) and calcium chloride (CaCl2) solutions. These solution concentrations were chosen to encompass a range of hardness and salinity values that could be encountered in groundwater in the US. These slurries were tested for mud density, Marsh viscosity, filtrate loss, and bleed. Several of these slurries were selected and used to mix soil-bentonite backfills for hydraulic conductivity testing. Each backfill mixture was mixed with dry bentonite, slurry, and sand (raised to 10 % moisture with the same electrolyte solution used to mix the slurry for that backfill mixture). The results of this study indicate that the performance of bentonite slurries is highly dependent on the concentration of CaCl2 in the mix water (CM). The Marsh viscosity, filtrate loss, and bleed volume worsen with respect to a specified set of performance criteria with increasing CaCl2 concentration. Increasing the bentonite content in the slurries caused an increase in Marsh viscosity and a decrease in filtrate loss and bleed. At CM = 10 mM, 7 % NSB slurries met the criteria, while 5 and 6 % NSB slurries did not. At CM = 25 mM (highest mix water CaCl2 concentration in this study), the performance criteria were met only when the NSB content was raised to 12 %. The results of the backfill testing in this study show that hydraulic conductivity tends to increase with the CaCl2 concentrations of mix water and permeant water. The mix water CaCl2 concentration has a larger impact on the hydraulic conductivity of a backfill than the permeant water CaCl2 concentration. At a mix water CaCl2 concentration of 10 mM or higher, the hydraulic conductivity appears to behave independently of permeant water CaCl2 concentration. All backfills tested in this study were created with slurries that met a set of performance criteria. The results indicate that a backfill mixed with a high level of CaCl2 may perform poorly despite being mixed with a slurry that meets the performance criteria specified in this study. Two other clays, namely multiswellable bentonite (MSB) and a proprietary contaminant resistant bentonite (SW-101), were tested in this study as possible alternatives to natural sodium bentonite. Further investigations are recommended to fully understand the behavior of these clays in contaminant containment applications.
backfill, bentonite, mix water, chemical compatibility, cutoff wall, hydraulic conductivity, vertical barrier
Masters Thesis (Bucknell Access Only)
Master of Science in Civil Engineering
Civil and Environmental Engineering
Mary Beth Gray
Replogle, Melissa Ann, "Effect of Mix Water Quality on the Design and Hydraulic Conductivity of Soil-Bentonite Cutoff Wall Backfill" (2016). Master’s Theses. 178.