Soil liquefaction is a geologic hazard that must be addressed in most seismically active regions of the United States and throughout the world to reduce the potential for loss of life and economic impact. To reduce the potential of liquefaction, various soil improvement methods have been used such as blasting, vibrocompaction, deep dynamic compaction, drains, and grout injection. These methods, with the exception of injecting grouts, are generally limited to undeveloped sites, and the use of grouts is only economical for treating small volumes of soil. Due to limitations of existing methods, new methods need to be developed to mitigate liquefaction for developed sites. Methods currently being studied to accomplish this include passive site stabilization using colloidal silica (Gallagher and Koch 2003, Gallagher and Yuanzhi 2005) and microbial remediation using Flavobacterium johnsonaie (Roth et al. 2005) or Bacillus subtilis (Martinez et al. 2003). The objective of this study is to demonstrate that introduction of appropriate microbial cells into a soil matrix will promote biomineralization within soil pore spaces such that the likelihood of liquefaction is significantly decreased.
To reduce the potential for liquefaction, two bacteria are being introduced separately into a volume of loose, saturated sand. These bacteria are Leptothrix discophora and Sporosarcina pasteurii. These bacteria are considered a better alternative for microbial remediation compared to biofilm-forming bacteria because they precipitate iron and manganese or calcite external to their cells. The anticipated mechanisms for reducing liquefaction are increased density by filling voids and cementation of soil particles during precipitation of the insoluble iron and manganese oxides or calcite. After precipitating the insoluble oxides, the bacteria can die without significantly degrading the improved shear strength of the soil. The mechanisms causing an increase in shear strength from introducing Flavobacterium johnsonaie are poorly understood, but the proposers hypothesize that if the Flavobacterium johnsonaie bacteria die, the shear strength may decrease toward pre-treatment levels.
Soil grain size distribution will be quantified and compared with the grain size distribution of non-treated soil to qualitatively assess production of insoluble oxides. Stiffness, strength and liquefaction resistance are being assessed using shear wave velocity testing with bender elements, miniature cone penetration testing, vane shear testing, and triaxial shear testing.
