Biogas desaturation and bio-gelation methods for mitigation of sand liquefaction

• Main Author: Wang, Kangda
• Other Authors: Chu, Jian
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Civil engineering::Geotechnical
• Online Access: https://hdl.handle.net/10356/148811
• Institution: Nanyang Technological University

Biogas desaturation is an emerging technique for mitigating the liquefaction hazard of sand. Compared with conventional soil treatment methods, the biogas desaturation method is cheaper and more environmentally friendly. In this thesis, a further study on the biogas desaturation method and its possible enhancement using the bio-gelation method is presented. The biogas desaturation method is based on a biological process called microbial denitrification in which the nitrate is converted to nitrogen via enzymes produced by bacteria. The liquid batch experiments on the denitrification process reveal that acetate is more suitable than glucose as carbon source for the denitrification process and a pH value around 7.5 is optimum. Furthermore, microbially induced carbonate precipitation (MICP) can also be triggered by introducing 40 mmol/L calcium ions in the denitrification solution to produce a limited amount of calcium carbonate in sand to enhance the stability of the gas bubbles. This study has also identified that the 40 mmol/L calcium ions is an optimized value. The MICP process will become inactive when the amount of calcium ions used is higher than this value. The undrained monotonic and cyclic triaxial tests carried out on the biogas desaturated sand have indicated that 1) the undrained shear strength of loose sand can be doubled when the degree of saturation is reduced from 100% to 86.4% by biogas desaturation; 2) under one-side compressional cyclic load, the number of cycles to failure can be increased by 100 times by slightly reducing the degree of saturation from 100% to 95%; 3) under symmetrical cyclic load, desaturating the loose sand to a degree of saturation of around 90% can increase the liquefaction resistance of sand by 75% which has the similar effect as to densify a fully saturated loose sand to a medium dense state; and 4) the failure state in the undrained cyclic triaxial tests can be identified using the instability line determined from the monotonic triaxial tests at the same degree of saturation. The seismic response of biogas desaturated sand was studied in a fully instrumented laminar box installed on a shake table. Test results show that the excess pore water pressure, the volumetric strain, and the structure settlement are all reduced by nearly a half when sand is desaturated to a degree of saturation of around 90%. Furthermore, shake table tests on sand on a sloping ground have also indicated that biogas desaturation is also effective in reducing the lateral strain or the lateral spreading of loose sand. Geophysical measurements such as wave velocity and electrical resistivity can be used to measure the degree of saturation and the relative density of soil. Shear wave velocity measured in the triaxial tests increases with the reduction of void ratio of sand under the same effective confining stress. But the shear wave velocity does not change with the change in the degree of saturation, which reflects the expectation that the matric suction caused by the biogas desaturation is negligible for clean sand. A correlation between the apparent electrical resistivity and the degree of saturation has been established which may be potentially used for in-situ applications. The long-term gas stability in sand under seepage is investigated by a series of sand column tests. For a clean sand with an initial degree of saturation of around 90%, the degree of saturation can be increased to 100% after 10-day’s upward seepage under a hydraulic gradient of 0.1. However, when the biogas desaturated sand is treated using bio-gelation method, little gas is lost under the same test condition. Therefore, the bio-gelation method is effective in enhancing the stability of gas bubbles. Results from the quicksand model tests show that a single round of bio-gelation treatment with 1 g/L alginate concentration can increase the critical hydraulic gradient of the medium loose sand by 20% and reduce its permeability by 3 orders of magnitude. Triaxial tests on the bio-gelled sand demonstrate that the slope of failure line of the sand can be increased from 1.08 to 1.27 after bio-gelation treatment. Under undrained cyclic loading conditions, the number of cycles at failure can be increased from 183 for clean sand to 435 for bio-gelled sand using 5 g/L alginate concentration.