Microbial-Induced Desaturation in Stratified Soil Conditions

A feasibility study was performed to assess the potential of microbially induced desaturation and precipitation (MIDP) through denitrification to reduce the risk of earthquake-induced liquefaction and improve the resilience of embankments along the lower Fraser River in Richmond, British Columbia, Canada. The denitrification process produces nitrogen gas, which gradually desaturates the soil, and dissolved inorganic carbon, which in presence of dissolved calcium results in precipitation of calcium carbonate minerals, cementing the soil particles. The trapped gas bubbles dampens pore pressure build-up during cyclic loading. Consequently, microbially induced desaturation (MID) the first phase of the MIDP process, has great potential as soil improvement technique, especially for liquefaction mitigation, independently from the precipitation phase of the process. Numerous studies have demonstrated the effect of biogenic gas formation on the mechanical response for treated sand samples at lab scale. However, there is still limited knowledge on the impact of partial desaturation on the hydraulic properties of the treated sediments and the durability of the entrapped gas phase, particularly in naturally stratified and heterogenous formations. This is important because changes in porosity and hydraulic conductivity may affect the injection flow patterns and distribution of substrates and products in the subsurface. The objective of this study was to develop a test procedure to evaluate the applicability and performance of the MID technique in stratified subsurface conditions. The study involved a two-dimensional model (2D) tank set-up, in which substrates were laterally injected and extracted. The results demonstrated the effectiveness of treatment and showed how gas formation, migration and entrapment and resulting degree of desaturation and hydraulic conductivity are affected by stratifications in a natural soil.