Distribution and fixation characteristics of microorganism in MICP treated silt column

SHAO Guanghui;HOU Min;LIU Peng;College of Civil Engineering,Nanjing Forestry University;Jiangsu Province Key Laboratory of Soil and Water Conservation and Ecological Restoration,Nanjing Forestry University;

microbial induce calcite precipitation(MICP);;silt;;microorganism distribution;;homogeneity

Cementing silt by microbial induced calcite precipitation(MICP) is a newenvironmentally-friendly technology. The distribution of microorganisms is an important factor that affects the uniformity of silt cemented by grouting.By measuring the amount of bacteria,the concentration of NH+4 and the pH in the pore fluid,as well as the content of CaCO3 in the cemented silt,the impact that the concentrations of bacteria and cement solution exerted on the distribution of bacteria and reaction products in the process of grouting columnar silt sample were investigated in this study.The results showed that the amount of microbial biomass in columnar silt sample decayed significantly along the direction of seepage when the original bacterium suspension of 1.223×108 CFU/mL was injected. In addition,the bacteria were mainly distributed within 25 cm of the grouting end. The increasing concentration of cement solution mainly influenced the amount of calcium carbonate within 5 cm of the grouting end. There was a good agreement between the CaCO3 content in cemented silt columns and the amount of free bacteria in pore fluid along the direction of seepage.For comparison,the supernatant was diluted to 8.20×106 CFU/mL and bacterium suspension was diluted to 1.80×106 CFU/mL with nutrient solution,respectively,which were used instead of the original bacterium. When the diluted supernatant or diluted bacterium suspension was injected into the silt column,the amount of bacteria in the pore fluid was much larger than that in the original suspension after 2 h of static adsorption because of the rapid reproduction of bacteria,and the distribution of microorganism was relatively uniform. The diluted supernatant or diluted bacterium suspension demonstrated an excellent infiltration capacity. The cemented silt columns presented more uniform than the columns injected original bacterium suspension. The decrease in CaCO3 content was less than microbial biomass in columnar silt sample along the direction of seepage. Fixed bacteria were composed of adsorbed bacteria and detained free bacteria which both affected the content of calcium carbonate produced in the process of grouting columnar silt samples. When a supernatant or bacterium suspension diluted by a nutrient solution was injected,the change in cement solution concentration had little effect on the content of calcium carbonate produced. The results of this study revealedthat the heterogeneity of silt reinforced by microbial grouting is mainly determined by the heterogeneity of fixed bacteria. Injecting diluted bacterium supernatant or suspension can improve the uniformity of biocemented silty soil.