Brina M. Montoya

Experimental Optimization of Microbial-Induced Carbonate Precipitation for Soil Improvement

AbstractImplementation of laboratory-tested biomediated soil improvement techniques in the field depends on upscaling the primary processes and controlling their rates. Microbial-induced carbonate precipitation (MICP) holds the potential for increasing the shear stiffness and reducing the hydraulic conductivity by harnessing a natural microbiological process that precipitates calcium carbonate. The study presented herein focuses on controlling MICP treatment of one-dimensional flow, half-meter-scale column experiments. Treatment was optimized by varying procedural parameters in five pairs of experiments including flow rates, flow direction, and formulations of biological and chemical amendments. Monitoring of column experiments included spatial and temporal measurements of the physical, chemical, and biological properties essential to the performance of MICP, including shear wave velocity, permeability, calcium carbonate content, aqueous calcium, aqueous ammonium, aqueous urea, and bacterial density. Rela...

Stress-Strain Behavior of Sands Cemented by Microbially Induced Calcite Precipitation

AbstractMicrobial induced calcite precipitation (MICP) is a novel biomediated ground improvement method that can be used to increase the shear strength and stiffness of soil. The evolution of the shear strength and stiffness of sand subjected to undrained and drained shearing is evaluated using triaxial tests. MICP treated sands with cementation levels ranging from young, uncemented sand to a highly cemented sandstonelike condition are subjected to undrained shear. A transition from strain hardening to strain softening behavior and a corresponding transition of global to localized failure as cementation is increased is observed. Moderately cemented specimens are subjected to various stress paths, which result in a change to the shear strength and volumetric behavior. Shear wave velocity is used to nondestructively monitor the change in small-strain stiffness during shearing, which provides an indication of cementation degradation as a function of strain level. Because shear wave velocity is influenced by ...

Fabrication, Operation, and Health Monitoring of Bender Elements for Aggressive Environments

Bender elements are commonly used to monitor the shear wave velocity of soils in various tests, including triaxial, consolidation, and centrifuge tests. When used in aggressive soil environments, electromagnetic crosstalk can distort the received bender element signal, preventing accurate shear wave velocity measurements. Aggressive soil environments are defined herein as conductive soils with high relative permittivity. Under these conditions, the electrical source is transmitted from source to receiver bender, dominating any received shear wave signal propagating through the soil. Careful attention must be paid to reducing the transmission of the electromagnetic signal, particularly in aggressive soil environments. When the waterproof coating of a bender element degrades and the inner and outer electrodes become electrically connected in a saturated environment, the bender element will no longer operate. However, when the waterproofing material is degraded so that only a single electrode on the source element is exposed, electric current can enter the pore fluid and affect the received signal. Further, even if the waterproofing coating is intact, electromagnetic crosstalk from the induced electrical field generated by the transmitting bender element can still affect the received signal when the conductivity of the pore fluid is high. Bender elements can be constructed so as to greatly reduce the electromagnetic crosstalk, and simple tests can be performed to help ensure that the bender element system is not susceptible to crosstalk. The objective here is to present details and practical guidelines regarding the fabrication, operation, and health monitoring of bender elements that will help ensure clear shear wave velocity measurements in aggressive soil environments. The fabrication steps presented improve on previous recommendations. Bender element operation (including signal form, frequency, and amplitude) also affects signal quality and the accuracy of the measured travel time. Finally, recommendations for monitoring the health of the bender elements throughout the transducer life are outlined.

Liquefaction Mitigation Using Microbial Induced Calcite Precipitation

The potential of a novel, bio-mediated soil improvement to increase resistance to liquefaction triggering and to reduce the consequences of liquefaction if it occurs was evaluated. 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.