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Author ORCID Identifier

https://orcid.org/0000-0001-9116-0218

AccessType

Campus-Only Access for One (1) Year

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Civil Engineering

Year Degree Awarded

2021

Month Degree Awarded

September

First Advisor

Guoping Zhang

Second Advisor

Don J. DeGroot

Third Advisor

H. Henning Winter

Subject Categories

Geotechnical Engineering

Abstract

Fine-grained clay particles of <2 >µm in size are ubiquitous in soils and sedimentary rocks. Due to their platy shape, high aspect ratio, and surface charges, clay particles play a dominant role in controlling the mechanical properties of those fine-grained geomaterials such as soft clays and shales. In physics and engineering, some materials show that the strength and stiffness increases with time even if the boundary conditions remain unchanged (e.g., no change in pressure, temperature, or composition), and such a phenomenon is called thixotropy. It is generally agreed that the principle of thixotropy of wet clays is complex. For a closed, isothermal physical system (such as a wet clay without changes in pressure, temperature, volume, or composition), any change in macroscopic mechanical properties with time (e.g., stiffness and strength) must stem from certain internal processes occurring within the system.

Both the macroscale mechanical and microscale structural mechanisms of thixotropic hardening of soft clays were uncovered through multiscale experimental investigations to better understand the underlying mechanisms of thixotropy and develop connections between quantitative time-dependent clay fabric evolution and macroscale thixotropic process. Macroscale laboratory experiments, including bender element and undrained shear strength testing, were applied to obtainmechanical properties (e.g., stiffness and strength) during thixotropy. Owing to the small sizes (e.g., <2 >µm) of the clay particles, it is challenging to quantify non-destructively the clay texture evolution with time. Fortunately, both conventional one-dimensional X-ray diffraction (1DXRD) and advancement in two-dimensional synchrotron X-ray diffraction (2DXRD) has provided a tangible and powerful approach to determine the clay particle orientation for such geomaterials as wet soils and shales. A better, systematic understanding of the thixotropic behavior of soft clays can hence be expectedly achieved through directly quantitative clays’ microfabric evolution (e.g., particle rearrangement, reorientation, and reaggregation) with time.

In the final phase of this study, to further understand the influences of physical and chemical factors on the thixotropic behavior of wet clays, clay specimens with different mineralogy cured at different temperatures (e.g., 4, 24, 44, 84 ºC), various salinities (e.g., 0, 0.017, 0.034, 0.068 g/g NaCl as pore fluid concentration), and different initial water contents relative to liquid limit (e.g., 0.8LL, 1.0LL, 1.2LL) were also studied following methods developed in the first phase. In general, all studied soft clays possess thixotropic hardening behavior, which is affected by the porewater salinity, temperature, and initial water content, and their interplay in controlling the thixotropy is rather complex. The temporal evolution of the microfabric of soft clays is complex, including reorientation from the high shearing-induced parallel orientation, hydrogen bond, or other interparticle force-induced aggregation to form face-to-face associated particle groups, further flocculation of aggregates, and continuous formation of thicker aggregates. Different clay minerals may exhibit different degrees of thixotropic hardening rate and magnitude, and the salinity and temperature at the maximum of the hardening rate and magnitude are also affected by the clay mineralogy. These findings here can have significant impacts on a series of engineering practices and problems, including offshore applications, such as floating offshore wind turbine and oil and gas platform, geothermal structure, and pile installation, which involve clays, especially soft clays.

DOI

https://doi.org/10.7275/24517400

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Available for download on Thursday, September 01, 2022

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