The research team will investigate the use of oxidation of organic compounds by denitrifying bacteria (?denitrification?) to induce carbonate cementation in sand. Microbially induced carbonate precipitation (MICP) offers the promise of a sustainable, non-disruptive and energy efficient engineering solution to a variety of important infrastructure development and geologic hazard mitigation problems, including remediation of liquefaction potential, improving foundation bearing capacity, and reducing (or even eliminating) excavation and tunnel support requirements. MICP through ureolysis is being investigated by several research groups in the U.S. and abroad. However, denitrification is potentially superior to ureolysis as a MICP process in that denitrifying bacteria are ubiquitous in the subsurface and the process works under anaerobic conditions, e.g. below the water table, while ureolysis requires oxygenation. Denitrifying bacteria and nutrients will be introduced into a laboratory soil column with pore water containing calcium carbonate in solution. Carbonate precipitation will be monitored by mass balance measurement of calcium concentrations in the pore fluid and the development of cementation will be monitored by shear wave velocity measurements in the soil column. These column experiments will be used to refine a biogeochemical model for denitrification, establish an optimal nutrient mix for denitrification, and establish precipitation and cementation rates for MICP by denitrification. Triaxial and simple shear tests on specimens from the column experiments will be used to evaluate the relationship between the amount of MICP and changes in the strength, stiffness, and liquefaction potential of the specimens. The proposed research has the potential to transform geotechnical practice, not only by development of a new, sustainable approach to ground improvement but also by advancing the emerging field of biogeotechnical engineering. This work is a continuation of work initiated under a National Science Foundation Small Grant for Exploratory Research (SGER).
Results of the research conducted for this project suggest that microbial denitrification, or dissimilatory reduction of nitrate, offers the potential of a sustainable and cost-effective method for mitigation of earthquake-induced liquefaction. The 2011 earthquake in Christchurch, New Zealand, dramatically illustrated the devastating effect earthquake-induced liquefaction can have on modern infrastructure, damaging over 10,000 homes beyond repair, disrupting water, sewer, and transportation systems for weeks, and rendering the central business district uninhabitable. While laboratory testing showed that carbonate precipitation via microbial denitrification is a slow process and may take years to precipitate enough carbonate to provide sufficient mitigation against liquefaction, the large amounts of nitrogen gas generated immediately upon the onset of denitrification is in itself a liquefaction mitigation mechanism. Therefore, liquefaction mitigation via microbial denitrification may be viewed as a two-phase process: an initial phase wherein desaturation via gas generation provides mitigation and a second phase in which sufficient carbonate precipitation has occurred to provide mitigation via inter-particle cementation and/or an increase in dilatancy (the tendency of a soil to increase in volume during shear). A particular advantage of microbial denitrification as a liquefaction mitigation technique is that is suitable for remediation under and around existing facilities because it is non-disruptive. Advantages of microbial denitrification compared to other techniques for microbially induced carbonate precipitation include that its primary by-products are benign (non-toxic) and that denitrifying bacteria are ubiquitous in the subsurface. Field testing is required to demonstrate the ability to induce microbial denitrification in liquefiable soils in situ.
