New, exciting opportunities for utilizing biological processes to modify the engineering properties of the subsurface (e.g. strength, stiffness) have recently emerged. Enabled by interdisciplinary research at the confluence of microbiology, geochemistry, and civil engineering, this new field has the potential to meet society's ever-expanding needs for innovative treatment processes that improve soil supporting new and existing infrastructure. An overview of bio-mediated improvement systems is presented, identifying the primary components and interplay between different disciplines. Geometric compatibility between soil and microbes that restricts the utility of different systems is identified. Focus is then narrowed to a specific system, namely bio-mediated calcite precipitation of sands. Various microscopy techniques are used to assess how the pore space volume is altered by calcite precipitation, the calcite precipitation is distributed spatially within the pore space, and the precipitated calcite degrades during loading. Non-destructive geophysical process monitoring techniques are described and their utility explored. Next, the extent to which various soil engineering properties is identified through experimental examples.;The remainder of the dissertation is focused on utilizing urea hydrolysis to induce calcite precipitation. Work was done to gain an understanding of the environmental factors that affect the growth of the bacterium Sporosarcina pasteurii, the metabolism of the bacterium, and the calcite precipitation induced by this bacterium to optimally implement the biological treatment process in situ. Soil column and batch tests were used to assess the effect of likely subsurface environmental factors on the Micorbial induced calcite precipitation (MICP) treatment process. The results presented herein indicate that the biological treatment process is equally robust over a wide range of soil types, concentrations of ammonium chloride and salinities ranging from distilled water to full seawater; on the time scale of an hour, it is not diminished by the absence of oxygen or lysis of cells containing the urease enzyme. The results advance the biological treatment process MICP towards field implementation by addressing key environmental hurdles faced with during the upscaling process.;As previously discussed, non-destructive geophysical process monitoring techniques are utilized during the treatment process. Bender elements are commonly used to monitor the shear wave velocity of soils in laboratory tests. When used in aggressive soil environments, such as MICP treated soil, electromagnetic crosstalk can distort the received bender element signal, preventing accurate shear wave velocity measurements. Under these conditions, the electrical source is transmitted from source to receiver bender, dominating any received shear wave signal propagating through the soil. Bender elements can be constructed to greatly reduce the electromagnetic crosstalk, and simple tests can be performed to help ensure the bender element system is not susceptible to crosstalk. Practical guidelines are presented regarding fabrication, operation, and health monitoring of bender elements that will help ensure clear shear wave velocity measurements in aggressive soil environments.;MICP increases the strength and stiffness of loose sand deposits, increasing resistance to seismically-induced soil liquefaction. Experimental tests, including triaxial and centrifuge, were used to assess the increase in shear strength and the resulting improvement in liquefaction susceptibility.;Triaxial tests are used to evaluate the evolution of stiffness and strength in specimens subjected to various stress paths. The cementation process is monitored using non-destructive geophysical measurements (i.e. bender elements). Shear response of the treated and untreated samples are evaluated for drained and undrained triaxial specimens subjected to compression, extension, and constant p loading paths. A marked increase in peak stress ratio of up to about 1.5 times that of the untreated samples is observed in moderately cemented samples and up to 1.9 times that of untreated samples for heavily cemented samples. Degradation of the cementation as reflected by changes in stiffness is observed using geophysical monitoring. The results are used to plan centrifuge tests that will evaluate the increased resistance to liquefaction triggering from microbial induced calcite precipitation treatment.;Microbial induced calcite precipitation (MICP) binds sand particles together through calcite crystal formation at particle-particle contacts. This results in an increase in the small-strain stiffness and strength of treated loose sand. Geotechnical centrifuge tests were used to evaluate the increased resistance of MICP treated sand relative to untreated loose sand when subjected to seismic shaking. Results of one model with a structure founded on sand treated to a moderate level of cementation and another model with the structure founded on loose untreated sand are compared. The centrifuge models were subjected to ground motions consisting of sine waves with increasing amplitudes. The accelerations, pore pressures, and shear wave velocities measured in the soil during shaking are presented. The resistance to liquefaction and deformation in the MICP treated model showed significant increases, as evidenced by substantial decreases in excess pore pressure ratios and vertical strains beneath the structure.
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