Date of Award

Fall 2022

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Inthuorn Sasanakul

Abstract

Cyclic response and liquefaction behavior are soil mechanisms under earthquake conditions. The study of dynamic soil response is important to evaluate soil behaviors under dynamic loading for a wide range of strains. Soil liquefaction phenomena involving a significant reduction in the strength and stiffness of saturated cohesionless soils has caused catastrophic ground failure in numerous earthquakes. Liquefaction mostly occurs in sand, but liquefaction of gravelly soil has been observed. To date, only a few studies have focused on the liquefaction of gravelly soils; hence, research data is limited. This research aims to improve the understanding of cyclic response and liquefaction behavior of gravelly soils through experimental methods including cyclic triaxial tests and dynamic centrifuge modeling tests.

In undrained cyclic triaxial (CTX) tests, the cyclic response of mine waste rock; which consists of gravel, sand, and fines; was evaluated. A total of forty-two CTX tests were performed on undisturbed and reconstituted waste rock samples that have comparable grain size distribution and normalized shear wave velocity (Vs,1) ranging from 260 to 400 m/s. Shear modulus and damping curves were also evaluated to characterize dynamic soil properties. Results from the CTX tests were analyzed for the number of cycles required to cause liquefaction (NL) at a given cyclic stress ratio (CSR) and for the behavior of pore water pressure development leading to soil liquefaction. It was found that as CSR increases, the NL decreases. The relationships between the CSR and NL were found to be relatively close to sand, and the effect of the effective confining stress on the CSR was also observed in the gravelly soil samples. For these gravelly soils, the ultimate friction angle, the phase transform friction angle, and the post-cyclic friction angle were found to be 51.5, 41.6, and 32.5 degrees, respectively. During preliminary evaluation of the shear wave velocity (Vs) of these gravelly soils, it was also found that the Vs-based liquefaction assessment developed in previous studies for sand does not apply to the materials tested in this study because the Vs of these gravelly soils are significantly higher than the Vs of sand. One of the factors contributing to the liquefaction behavior is the type of matrix soil structure that consists of gravel, sand, and fines. Different types of matrix soil structures, including Interlocked Gravel Matrix, Gravel-Sand Matrix, and Gravel-Fines Matrix, can be assessed based on the basic understanding of phase relationships. It was found that the highest potential of liquefaction is when the soil has Gravel-Sand Matrix, as gravel particles are not in contact and interlocked with each other.

The liquefaction behavior of four loose mining waste rock mixtures with variable amounts of gravel, sand, and fines has been investigated in eight geotechnical centrifuge modeling tests. Mixtures of these soils are 40-48-12, 50-38-12, 50-50-00, and 70-30-00 for gravel-sand-fines content. The Vs,1 of these materials is approximately 127, 128, 125, and 117 m/s, respectively. The relative densities, void ratios, and shear wave velocities of these samples are also comparable, but the samples containing fines had an order of magnitude lower permeability. During the shaking events, acceleration time histories and pore water pressure ratios were obtained and used to establish shear stress-strain behaviors. To evaluate the effect of soil composition, a 0.27-g sinusoidal base shaking amplitude was applied to four centrifuge models. It was observed that soil liquefaction occurred in all models except for a model comprised of 70% gravel and 30% sand. The acceleration time histories of all models exhibited dilative behavior, which was dependent on the development of excess pore pressure and effective stress. In models without fines, pore pressure dissipation was observed during shaking. After the tests, models with fines exhibited more settlement and volumetric strain. According to the findings, an increase in the amount of sand and fines increases the liquefaction potential of mine waste rock. The impacts of fines content on the liquefaction behavior and dynamic response of gravelly soils were also evaluated. Six models of two loose saturated gravelly soil samples were investigated in the centrifuge. The first gravelly soil mixture consisted of 50% gravel, 38% sand, and 12% fines, whereas the second soil mixture consisted of 50% gravel and 50% sand. Each soil mixture was subjected to uniform sinusoidal shaking with amplitudes of 0.19 g, 0.27 g, and 0.40 g. Based on the results, the fines content had a relatively low impact on cyclic shear strain, the rate of pore pressure development during shaking, the cyclic resistance ratio, and the reduction of shear modulus. However, the fines content has a noticeable impact on soil dilatancy, the rate of pore pressure dissipation during and after shaking, damping, volumetric strains, and soil fabric after shaking.

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© 2022, Pitak Ruttithivaphanich

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