Date of Award

Fall 12-2-2020

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Advisor

Aaron Gallant

Second Committee Member

Bill Davids

Third Committee Member

Shaleen Jain

Additional Committee Members

Ben Mason

Carlos Vega-Posada

Abstract

Entrapped gas bubbles in quasi-saturated porous media are of practical importance in the field of science and engineering. Gas bubbles, which may occur naturally or be introduced artificially, significantly influence the mechanical behavior of soil. In this study, the durability of gas is examined from two geotechnical engineering perspectives. In the field of geotechnics, the artificial introduction of gas is being considered, as it has been widely recognized that entrapped gas, even in nearly-saturated sediments, has an appreciable influence on soil’s mechanical behavior. Entrapped gas bubbles in quasi-saturated sediments significantly increase the pore fluid compressibility and suppress the generation of positive excess pore water pressure, thus increasing liquefaction resistance. The first topic considers the influence of gas durability as it relates to a novel ground improvement method, induced-partial-saturation (IPS). Recognizing that harnessing the mechanical benefits of gas may offer an economical liquefaction mitigation method with a low carbon footprint—that is relatively non-intrusive and applicable to new and existing civil infrastructure systems—previous studies have successfully demonstrated IPS increases the liquefaction resistance of the soil. However, the efficacy of IPS is linked to the long-term persistence of entrapped gas after emplacement. Motivated by the potential benefits of IPS, this research aims to address the salient consideration of gas durability and its longevity after emplacement—an effort that so far has been secondary to demonstrating IPS can mitigate liquefaction, and has not yet been meaningfully addressed. Assessment of gas longevity is challenging, particularly through physical demonstrations, which cannot be performed practically on time-scales of interest (i.e. decades). Modeling of the physical and chemical mechanisms that influence the durability and persistence of entrapped bubbles was undertaken, as it is a practical avenue to overcome these limitations and provide novel insight. The governing aqueous-phase advection-diffusion processes and inter-phase gas kinetics associated with bubble dissolution are simulated in a finite-difference framework. A greater understanding behind material-dependent tortuosity linked to effective diffusion coefficients used in the modeling effort is derived from experiments performed at the Advanced Geotechnics Lab in the civil and environmental engineering department at the University of Maine, using equipment readily available in many geotechnical engineering laboratories. The modeling framework is also validated with elemental and bench-scale experiments performed by others and then extended to address soil resaturation rates under a variety of subsurface conditions. The numerical modeling framework, which simulated the aqueous-phase gas mobility contributing to the dissolution of gas, was capable of predicting experimental observations of gas dissolution from several independent studies—which had varied spatial scales, pore fluid constituents, gas solubilities, and boundary conditions—under both hydrostatic and groundwater flow conditions. Under hydrostatic conditions, the thickness of the gassy layer decays due to a diffusion-induced resaturation front that advances from the boundary of quasi-saturated sediment. Under seepage conditions, a saturation front progressively advances downgradient due to imbibing groundwater that acts as a sink. The numerical study demonstrates that emplaced gas is durable to the extent where diffusion- and groundwater seepage-induced dissolution should not discourage the advancement of IPS, but will not remain indefinitely. Potential solutions to mitigate the decay of a gassy soil layer are discussed. The second topic considers the sediment response to tsunamis loading. Tsunamis are an extreme coastal hazard that causes catastrophic damage and disruption in the nearshore environment. In addition to inundation and flooding, tsunamis are attributed to the formation of deep-seated scour features and erosion in the soil bed, which can be exacerbated by the generation of excess pore water pressures that instigate hydraulic gradients and momentary liquefaction. The pore water pressure response in the soil bed is influenced by i.) seepage arising from the changing boundary pore water pressure at the soil bed surface and ii.) a partially undrained mechanical response that pressurizes the pore fluid as the soil skeleton deforms under the changing weight of the wave. Shallow nearshore coastal sediments will contain entrapped gas due to tidal fluctuations. Differential pressurization of pore water that instigates groundwater flow is intimately linked to entrapped gas and the associated pore fluid compressibility. Tsunami waves can be in excess of 10 m, generating fluid pressures in the sediment that will compress, and possibly dissolve, air bubbles as the wave height increases. Therefore, from a gas durability perspective, tsunami loading conditions are extreme and relevant to the assessment of the sediment under loading imposed by this hazard. Consideration of gas kinetics and the durability of gas as it relates to the pore fluid compressibility and pore water pressure response has not been addressed. To address the role of gas durability on the pore water pressure response and liquefaction during tsunami loading, the modeling effort was extended, and gas kinetics were incorporated in a poroelastic seepage-deformation model to examine the durability of gas and pore fluid hardening under shorter-duration, but extreme, loading scenarios where large excess pore water pressures are generated and high hydraulic gradients develop. A tsunami loading event was chosen to demonstrate the influence of gas durability because a.) duration of the event is on the order of tens of minutes (where earthquakes only last seconds to minutes); b.) tsunamis impose large changes in total stress on the sediment that is linked to the mechanical generation of large excess pore water pressure; which is further exacerbated by c.) a dynamic boundary pore water pressure imposed at the seabed surface during runup and drawdown of the wave. Results were compared with simpler pore fluid compressibility assumptions (i.e. constant compressibility) to highlight the influence of gas kinetics on the pore water pressure response in the sediment. Numerical studies indicate that the temporal evolution of excess pore water pressure generated in the sand bed was appreciably influenced by the consideration of gas kinetics and pore fluid hardening. When the inter-phase gas exchange is considered to simulate the pore fluid compressibility, stabilizing hydraulic gradients (i.e. infiltration) during runup of a tsunami wave is appreciably less than when a constant pore fluid compressibility (i.e. no compression or dissolution) of the gas is considered. The duration of sustained liquefaction after the wave has receded is significantly less than when a constant pore fluid compressibility was assumed. The maximum depth of liquefaction increases with the thickness of a layer where gas is entrapped, but only to an extent. Additionally, the assumed tsunami wave-height time series plays a role in the maximum depth of liquefaction. Notably, when the rate of drawdown is greater, the maximum depth of liquefaction increases.

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