On evolving size and shape of gas bubble in marine clay under multi-stage loadings: microcomputed tomography (μCT) characterization and cavity contraction analysis

Abstract

Discrete bio-gas bubbles commonly form in fine-grained marine sediments and have modified many aspects of the behavior of these sediments, including strength, stiffness, and permeability. Although the level of such modifications is known to govern by bubble shape and size, limited studies have been undertaken, mainly due to difficulty in nondestructively characterizing bubbles within a soil under in situ stresses. In this study, a mini-loading device was developed to perform one-dimensional loading tests on gassy marine clay and gassy silt in a microcomputed tomography (μCT). The evolving bubble shape, size, and pressure during loading were quantified, and the resulting stress fields around the bubble cavities were evaluated via elliptical cavity contraction analysis considering stress anisotropy. As the vertical load increased, bubble cavities were found to compress predominantly along the vertical loading direction, with little horizontal compression, because localized soil failure (LSF) and thus cavity collapse occurred mainly near the roof of the at-rest lateral earth pressure coefficient (K0)-stressed elliptical bubble cavities. The evolution of bubble shape and size under loading is significantly affected by stress anisotropy, which governs the extent and location of the LSF. A Gaussian mixture model is adopted to quantify the evolving distributions of bubble structure parameters, which are essential for developing more physically rigorous gassy soil models.

A state-dependent constitutive model for coarse-grained gassy soil and its application in slope instability modelling

“…A unique feature of the gassy soil, as revealed experimentally, is that the gas bubbles can either weaken or strengthen the shear strength of both fine-grained and coarse-gained soil, depending on the initial states (Wheeler, 1986;Grozic et al, 1999;Vega-Posada et al, 2014;Hong et al, 2019b;Yang et al, 2019). Increasing offshore construction activities on gas-bearing seabed have generated interests in developing theoretical models of gassy soils.…”

Constitutive modelling of fine‐grained gassy soil: A composite approach

“…A smaller x (lower rate of bubble flooding) is thus used, as compared with that for gassy Malaysian silt and gassy Combwich mud (see Table ). This could be due that the gas bubbles formed in soil with finer particles tend to be larger than those in soil with coarser particles, given the stress level of the two are identical . Since Speciwhite kaolin clay is finer than the other two soils reported herein, it contains bubbles of larger size, which is less susceptible to bubble flooding (being beneficial) but could have caused more yielding around the large cavities .…”

“…Influence of undissolved gas on the mechanical behaviour of soil is dependent on the grain size. For fine‐grained soils, the gas bubbles are typically much bigger that the soil particles, and therefore, they fit in the soil matrix rather than the pore water, as characterized microscopically by Hong et al (see Figure ). This indicates that fine‐grained gassy soil is not a soil with compressible pore fluid.…”

“…Microstructure of the fine‐grained gassy soil: A, a typical X‐ray microtomography (μCT) image; B, a typical scanning electron microscopy (SEM) image [Colour figure can be viewed at wileyonlinelibrary.com]…”

3D Elastoplastic Model for Fine-Grained Gassy Soil Considering the Gas-Dependent Yield Surface Shape and Stress-Dilatancy

“…Micro-structure of a typical fine-grained gassy soil (Hong et al, 2019a): (a) micro-computed tomography (μCT) image; (b) scanning electron microscopy (SEM) image.…”