Reactive Transport Model to Evaluate Process Performance of Bio-Mediated Liquefaction Mitigation underneath Existing Structures

A reactive transport model is presented, which can be used to predict the distribution of substrates and products for microbial induced desaturation and precipitation (MIDP). MIDP is an emerging bio-mediated ground improvement method, which can be used to improve the cyclic resistance of the soil underneath existing structures. The process involves the injection of a substrate solution containing calcium acetate and calcium nitrate into a targeted soil layer. Indigenous nitrate-reducing bacteria produce biogenic gas (nitrogen and carbon dioxide), calcium carbonate minerals, and biomass. The gas gets trapped in the pore space, desaturates the soil, and can reduce pore pressure build-up during cyclic loading, which increases the undrained shear strength of loosely packed granular soils. Precipitation of calcium carbonate minerals can cement the soil particles and improve shear strength. In this study, a numerical model is developed using COMSOL Multiphysics, which couples the biochemical reactions to a reactive transport model and can be used to relate changes in substrate and product concentration to changes in porosity, degree of saturation, and hydraulic conductivity. The model is used to evaluate how the treatment performance is affected by different treatment variables, such as flow rate, treatment time, and injected substrate concentrations, and environmental conditions such as treatment depth and grain size distributions.