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Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2017)

Part I of this dissertation describes a simple experimental technique that can be used to obtain the slopes of the plastic potential and yield functions during shear based on the deformation theory of plasticity. The method imposes small perturbations in the direction of the stress increment by closing the drainage valve, thereby abruptly switching from drained to undrained loading conditions during plastic loading. Elastoplastic moduli are obtained immediately before and after the perturbation from the measured deviatoric stress, mean effective stress, deviatoric strains, and volumetric strains for the stress paths immediately before and after closing the drain valve. During drained shear, samples were sheared while the mean effective stress was maintained constant. Combining tests performed at several confining stresses, the proposed method can map yield and plastic potential surfaces and predict their evolution for a wide range of stresses.

Part II of the dissertation focuses on providing insights into the effects of changes in clay mineralogy and pore-fluid chemistry on cyclic behavior of low-plasticity fine-grained soils. A series of cyclic and monotonic direct simple shear experiments was conducted on three low-plasticity fine-grained mixtures of non-plastic silt with either bentonite or kaolinite clay minerals blended with fresh deionized water or saline water. The clay fractions were adjusted to achieve a plasticity index of PI = 9 for all three mixtures to study differences or similarities in their behavior and to examine the effectiveness of index properties in identifying the observed differences in lab results.

Even though all three blends have the same plasticity index, significant differences in their cyclic response were observed. Results indicate that plasticity index is an insufficient indicator of the cyclic behavior of low-plasticity fine-grained soils, and corrections for pore fluid chemistry and clay minerology may be needed for future liquefaction susceptibility and cyclic softening assessment procedures. Site-specific cyclic and monotonic testing for important projects remains to be the recommended approach for properly characterizing the seismic behavior of such soils with the current level of understating of their complex behavior.

The collected experimental data, visualization and post-processing tools, as well as documented experimental procedures of Part I are curated and published for public access on http:www.DesignSafe-ci.org. The data from Part II is being curated and will be published online in the near future.

    Cover page: Experimental Mapping of Elastoplastic Surfaces for Sand and Cyclic Failure of Low-Plasticity Fine-Grained Soils
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2018)
    Two Jupyter notebooks are prepared for visualizing, analyzing, and post-processing data from constant-height direct simple shear tests performed on three fine-grained soil mixtures. The experimental data and Jupyter notebooks are available and implemented in the DesignSafe cyberinfrastructure (www.designsafe-ci.org). To categorize the dataset, the experiments are grouped into three events based on their designations for the mixtures tested in the lab. Each experiment contains two stages; consolidation and shear, including time series of load, displacement, stress, strain, equivalent excess pore-water pressure etc. The data has been categorized in this manner to facilitate the use of the visualization notebook and to locate specific tests in the dataset. A spreadsheet similar to the AGS4 format was created to provide metadata about the soil specimens that were tested. The data plotter notebook visualizes the experimental data. A separate analysis notebook creates an environment for the user to combine outcomes of multiple tests, synthesize the data, and enables regression analyses for finding critical state lines, shear strength normalization parameters, and generates CSR vs. number of uniform cycles for cyclic tests for arbitrary strain levels.
      Cover page: Processing, Visualization, and Analysis of Direct Simple Shear Test Data Using Jupyter Notebooks in the DesignSafe Cyberinfrastructure
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2018)
      A series of cyclic and monotonic direct simple shear experiments was conducted on mixtures of non-plastic silt and bentonite, prepared with two different pore fluids: fresh deionized water, and saline-water with a 35 g/L concentration of NaCl (consistent with sea water). The clay fractions were adjusted to achieve a plasticity index of PI = 9, with the fresh-water blend requiring 5% bentonite and the saline water blend requiring 10% bentonite. Though both blends have the same plasticity index, differences in cyclic behavior were observed. The relationship of cyclic stress ratio versus number of cycles and the ratio of cyclic stress to monotonic undrained strength was higher for the saline water blend than for the fresh water blend. Results indicate that plasticity index alone is an insufficient indicator of cyclic strength. Corrections for pore fluid chemistry (and by extension, depositional environment) may be necessary, in addition to possibly other factors, for procedures to assess the potential for liquefaction susceptibility and cyclic softening.
        Cover page: Cyclic Behavior of Low-Plasticity Fine-Grained Soils with Varying Pore-Fluid Salinity
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2018)
          Cover page: Yield Surface Mapping and Triaxial Compression Test Data Curation
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2018)
          Elastoplastic models are commonly used in modern geotechnical practice to numerically predict displacements, stresses, and pore pressures in large construction projects. These elastoplastic models use presumed functional forms for yield and plastic potential functions that are rarely obtained from experimental measurements. This research describes a simple experimental technique that can be used to obtain the slopes of the plastic potential and yield functions during shear based on the deformation theory of plasticity. The method imposes small perturbations in the direction of the stress increment by closing the drainage valve, thereby abruptly switching from drained to undrained loading conditions during plastic loading. Elastoplastic moduli are obtained immediately before and after the perturbation from the measured deviatoric stress, mean effective stress, deviatoric strains, and volumetric strains for the stress paths immediately before and immediately after closing the drain valve. During drained shear, samples were sheared while the mean effective stress was maintained constant. Combining tests performed at several confining stresses, the proposed method can map conventional isotropic yield and plastic potential surfaces and predict their evolution for a wide range of stresses. The proposed technique can also be used for kinematic yield surface and may be used to develop new and more accurate elastoplastic constitutive models.
            Cover page: Experimental mapping of elastoplastic surfaces for sand using undrained perturbations
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2023)
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