Controlling Colloidal Silica Grouts Using Microbial Fermentation Activity

Abstract

Colloidal silica grouts have long been recognized as an environmentally-conscious ground improvement solution capable of stabilizing weak, problematic soils and achieving large reductions in soil hydraulic conductivities for geotechnical applications including liquefaction mitigation and seepage reduction. In the conventional approach, various chemical accelerants are first added to colloidal silica suspensions, solutions are injected into the intended treatment zone, and gelation occurs over time resulting from both colloidal negative surface charge neutralization and van der Waals attraction between colloids. Although the method holds significant promise for practical ground improvement applications, effectively controlling the gelation time for these grouts and ensuring their stability when subjected to various field-representative subsurface conditions has remained a significant challenge. When high ionic strength brackish/marine subsurface conditions are encountered, silica grouts can set too quickly resulting in a limited extent of improvement with associated hydraulic conductivity reductions that prevent subsequent treatment attempts. In other cases, unexpected differences in soil mineral and solution chemistries can result in grouts that fail to set long after injections resulting in minimal engineering improvements. In an effort to overcome these limitations, this research investigates the potential of bio-mediated processes to control the rate of colloidal silica gelation thereby forgoing the need for chemical accelerants and improving the probability of treatment success for various subsurface conditions. In this study, the use of microbial fermentation activity was examined which was expected to control gelation through both solution pH reductions and ionic strength changes. A series of batch and soil column experiments were performed to investigate this process using both augmented mixed-acid fermenting bacteria and stimulated microbial communities under varying chemical conditions. Results suggest that both augmented and stimulated bacteria can be used to successively control colloidal silica gelation and that the rate of gelation is dependent on both the magnitude and rate of fermentation-induced pH reductions. Furthermore, soil column experiments, which up-scaled treatment techniques developed in batch, demonstrated the ability of both aqueous solution and geophysical measurements to effectively monitor gelation progression. Although additional characterization of the mechanical behavior of these biologically-improved soils is needed, treated poorly-graded sands in this study exhibited modest tensile strengths and hydraulic conductivity reductions near two orders of magnitude.