Date of Award
Master of Science
Civil, Construction, and Environmental Engineering
When a fully saturated loose sand is subjected to a seismic load the sand liquefies which results in little to no shear strength. It has been shown that by reducing the degree of saturation through the microbial denitrification process that the shear strength of sands under isotropic conditions can be greatly improved. However, previous studies have only studied the effects of biogas on isotropically consolidated samples. Soils in the ground are considered K0 consolidated, therefore a study on the effects of K0 consolidation of the biogas desaturated sand was performed. Triaxial undrained compression tests were performed on isotropically consolidated and K0 consolidated loose sand specimens. The isotropically consolidated test results showed a significant increase in shear strength when Skempton’s pore water pressure coefficient B-value decreased from 0.99 to 0.39. With this reduction in B-value, the loose sand went from a strain softening behavior to a strain hardening behavior by reducing the maximum excess pore pressure generated during undrained shearing. This behavior is well reported in the literature. However, the results of the K0 consolidated triaxial samples showed much less improvement compared to the isotropically consolidated samples. It was found that by decreasing the degree of saturation the development of pore water pressure was increasingly delayed. This delay may prevent a catastrophic failure caused by brittle behavior under in-situ conditions.
It has been found that the permeability of soils can be decreased by reducing the degree of saturation. For this reason, a study on the use of the microbial denitrification method to reduce the permeability of a clean sand was performed. A series of sand column permeability tests were performed to observe the change in the coefficient of permeability with decreasing the degree of saturation using the microbial denitrification method. Using the denitrifying bacteria, a fully saturated sand can be partially desaturated to a degree of saturation of around 70%. By reducing the degree of saturation to around 70%, the coefficient of permeability of a fully saturated sand at a void ratio of 0.65 decreased from 8.3 (10-4) m/s to 2.7 (10-4) under a hydraulic gradient of 0.1. Although a reduction in the permeability of 67% was found, the microbial denitrification method was found to be unsuitable for reducing the permeability of clean sands for use as an impermeable layer.
Many geotechnical structures such as slopes, embankments, and retaining walls can be approximated to plane strain conditions. To understand plane strain conditions it is better to perform actual plane strain tests rather than conventional triaxial tests. Therefore, a plane strain apparatus modeled after the Nayang Technological University plane strain device was constructed as a part of this thesis. An overview of the design and the control program has been discussed. A series of undrained and drained plane strain compression tests were performed on loose sand samples. The plane strain device was found to provide accurate and reliable results for K0 consolidation, drained shear, and undrained shear. Therefore, this device can be used for future research projects concerned with plane strain conditions.
Hansen, Seth, "Engineering properties of saturated and biogas desaturated sand" (2015). Graduate Theses and Dissertations. 14562.