Effect of Degree of Saturation on the Liquefaction Resistance of Non-Plastic and Low-Plastic Soils

Liquefaction and cyclic softening of fine-grained soils pose a significant hazard during earthquakes, yet the cyclic behavior of transitional soils such as silts remains poorly understood. Existing empirical liquefaction evaluation frameworks and ground improvement techniques are largely developed for sands and are not directly applicable to silts, particularly beneath existing infrastructure where conventional mitigation methods are often impractical or cost prohibitive. This knowledge gap is critical in regions such as the Pacific Northwest, where young, loosely deposited silts are prevalent and susceptible to seismic loading. Microbially induced desaturation (MID) has emerged in recent years as a promising, minimally invasive ground improvement technique for liquefaction mitigation. By generating biologically produced gas within the soil matrix, MID reduces the degree of saturation and increases pore fluid compressibility, which has been shown to improve liquefaction resistance in sands. However, applicability and effectiveness of MID as a liquefaction mitigation method in fine-grained soils has not been systematically evaluated in prior studies. This research experimentally investigates the hypothesis that increased pore fluid compressibility enhances liquefaction resistance under cyclic loading. A cyclic direct simple shear (CDSS) device was modified with a custom baseplate to enable truly undrained testing at prescribed degrees of saturation. The testing program was first validated using Ottawa 20–30 sand, with results showing agreement with published literature. The same apparatus and methodologies were then applied to two reconstituted silt slurries: a non-plastic silt (PI 0) and a low-plasticity silt (PI 16), tested over a range of saturation ratios. Results demonstrate that pore fluid compressibility significantly influences cyclic response, but its effect varies with soil plasticity. While reduced saturation markedly increased liquefaction resistance in sand and non-plastic silt, little change in cyclic resistance was observed in the low-plasticity silt. Distinct differences in cyclically induced pore pressure generation, shear strain accumulation during cyclic shear loading, and post-cyclic volumetric strain were observed between the two silts, indicating different cyclic responses and failure mechanisms and transitional behavior between liquefaction and cyclic softening. This dissertation provides new experimental insight into the cyclic behavior of silts, highlights the limitations of sand-based liquefaction criteria when applied to fine-grained soils, and establishes a framework for evaluating MID-treated silts under cyclic loading. The findings contribute toward the development of improved liquefaction assessment and mitigation strategies for fine-grained soils beneath existing infrastructure.