Stimulation of native microorganisms for biocementation in samples recovered from field scale treatment depths

Microbially induced calcite precipitation (MICP) is a biomediated cementation process that uses natural microbial enzymatic activity to improve the geotechnical properties of granular soils. In this study, two sets of experiments are completed using soil samples obtained from different depths to evaluate the feasibility of stimulating native ureolytic microorganisms for MICP at depths relevant to geotechnical applications. Batch and column experiments completed using five different stimulation solutions demonstrate that stimulation of native microbial ureolysis is improved with an enhanced stimulation solution, which differs from a standard stimulation solution used in previous studies through initial solution pH adjustment to 9.0 and higher concentrations of ammonium chloride and yeast extract of 100 mM and 0.2 g/L, respectively. A sterile sampling and column testing program is completed using soil materials obtained at shallow (2 m), middle (5.9 m), and deep (12 m) depths from a geotechnical boring and treated with both standard and enhanced stimulation solutions. Despite significant differences in stimulated urea degradation between soil depths and stimulation solution types, all tested columns achieved ureolysis rates sufficient to induce MICP, although at different times. Following 14 cementation treatments, soil columns achieved final V_s values as high as 1,020 m/s and unconfined compressive strengths as high as 1.9 MPa. The results of this study suggest that native ureolytic microorganisms may be successfully stimulated in natural soil deposits to induce calcite precipitation at treatment depths up to 12 m for geotechnical ground improvement.