Undrained Responses of Microbially Desaturated Sand under Monotonic Loading

Desaturation is a method for the mitigation of liquefaction of sand. This method has gained increasing interest in recent years as it may become a more cost-effective solution than many conventional methods. In this paper, a microbial denitrification process was adopted to generate nitrogen gas for desaturation of sand. This approach offers several advantages. First, nitrogen gas does not dissolve in water easily. Second, nitrogen gas is chemically inert. Third, as the gas is generated in situ, its distribution is also more uniform. Fourth, the gas bubbles generated by the microbial process are tiny, and thus the potential of escaping from the ground is less. In this paper, undrained triaxial compression and extension tests were adopted to evaluate the liquefaction behavior of sand desaturated using nitrogen under static monotonic loading conditions. Test results show that, under an undrained axisymmetric condition, when the degree of saturation of loose sand is reduced from 100% to a range of 95–88%, the undrained shear strength can increase by more than two times. The reduction in the degree of saturation also leads to a transition from strain softening to strain hardening in the stress-strain behavior in compression tests on sand with relative density $D_r\approx 10\%$ and extension tests on sand with $D_r\approx 30\%$. Moreover, the slope of the instability line that specifies the condition for instability in sand also increases as the degree of saturation reduces. This study proves that the use of biogenic nitrogen gas is effective in enhancing the liquefaction resistance of sand under static, monotonic loading conditions.