Mechanical Effects of Biogenic Nitrogen Gas Bubbles in Soils

The fluid bulk stiffness of a soil is very sensitive to the presence of gas, and a small volume of bubbles can significantly affect the pore pressure response to loading, including Skempton’s B parameter, P-wave velocity, and liquefaction resistance. Biologically mediated processes can lead to the production of gases in soils; nitrogen is particularly advantageous because it is not a greenhouse gas, it is not combustible, and it has low solubility in water. Sands, silts, and clayey sands inoculated with Paracoccus denitrificans were monitored to assess the effects of nutrient availability, fines content, and pressure-diffusion on the evolution of nitrogen gas generation and bulk stiffness. Results show clear evidence of biogas bubble formation, earlier gas generation and entrapment in specimens with higher fines content, and a strong correlation between biogas volume and P-wave velocity. The volume of gas is correlated with specific surface, suggesting that biogas bubble formation develops as heterogeneous nucleation and that it is directly linked to the availability of nucleation sites on mineral surfaces, which in turn also affect the degree of attainable supersaturation. Results support the viability of biogenic gas generation as a tool to increase the liquefaction resistance of soils subjected to cyclic loading.