MICROBIAL BIOFILM IN POROUS SEDIMENTS: EFFECTS ON SOIL BEHAVIOR
Ta, Hien Xuan
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This dissertation describes the results of a study on biofilm formation and its effects on soil behavior. Column experiments stimulating the production of biofilm by various bacteria were performed along with monitoring of changes in permeability, responses of P- and S-waves transmission, and electrical resistivity. Resonant column tests were conducted to study the effects of biofilm on dynamic properties of saturated sand. Flow through the porous medium was simulated in COMSOL and used to compute bulk permeability reduction with biofilm formation. The column experiment results showed that biopolymer Dextran produced by Leuconostoc mesenteroides resulted in the reduction of bulk permeability by more than one order of magnitude. It did not contribute to changes in stiffness of the soil but significantly increased the seismic wave attenuation. The electrical resistivity doubled as the result of Dextran occupying the pore spaces and the corresponding decrease in porosity. The study on biofilms produced by Shewanella oneidensis and Pseudomonas putida showed similar reduction in permeability. Compressional stiffness of the sediment was found unchanged even in the presence of significant biofilm formation. High frequency P-wave technique was unable detect any changes in P-wave attenuation. However, there was a slight increase in S-wave velocity along with a significant increase in shear wave attenuation. The resonant column test results showed a slight decrease in shear and flexural wave velocity. On the other hand, the damping ratios in both torsional and flexural modes showed significant increase due to the accumulation of highly viscous, ductile biofilm material in the specimen. Prediction of permeability reduction by existing analytical models was examined in light of observed data. The predictions by pore-filling models were closer to observed values whereas those by grain coating models differed very much. However, the inability of analytical models to capture the microstructure morphology of the biofilm formation in soils will limit their usefulness in practice. The numerical solution of the fluid flow using COMSOL in a simulated porous structure with biofilm based on observed SEM images is shown to give an accurate prediction of the permeability reduction as a function of the amount of biofilm formation in the sediment.