Brian C. Martinez

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...

Soil engineering in vivo: harnessing natural biogeochemical systems for sustainable, multi-functional engineering solutions

Carbon sequestration, infrastructure rehabilitation, brownfields clean-up, hazardous waste disposal, water resources protection and global warming—these twenty-first century challenges can neither be solved by the high-energy consumptive practices that hallmark industry today, nor by minor tweaking or optimization of these processes. A more radical, holistic approach is required to develop the sustainable solutions society needs. Most of the above challenges occur within, are supported on, are enabled by or grown from soil. Soil, contrary to conventional civil engineering thought, is a living system host to multiple simultaneous processes. It is proposed herein that ‘soil engineering in vivo’, wherein the natural capacity of soil as a living ecosystem is used to provide multiple solutions simultaneously, may provide new, innovative, sustainable solutions to some of these great challenges of the twenty-first century. This requires a multi-disciplinary perspective that embraces the science of biology, chemistry and physics and applies this knowledge to provide multi-functional civil and environmental engineering designs for the soil environment. For example, can native soil bacterial species moderate the carbonate cycle in soils to simultaneously solidify liquefiable soil, immobilize reactive heavy metals and sequester carbon—effectively providing civil engineering functionality while clarifying the ground water and removing carbon from the atmosphere? Exploration of these ideas has begun in earnest in recent years. This paper explores the potential, challenges and opportunities of this new field, and highlights one biogeochemical function of soil that has shown promise and is developing rapidly as a new technology. The example is used to propose a generalized approach in which the potential of this new field can be fully realized.

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.

Seismic and Resistivity Measurements for Real-Time Monitoring of Microbially Induced Calcite Precipitation in Sand

A variety of biogeochemical processes, from inorganic mineral precipitation to bio-film formation to bio-gas generation, are being investigated as alternative methods to improve soil properties. Every process applied in a geotechnical application requires the ability to monitor the progression of treatment, preferably in real time. While monitoring of the biogeochemical processes is necessary to properly apply and manage the treatment process, ultimately verification that the treatment is improving the engineering soil properties as desired is necessary. Because direct measurements of soil properties (e.g., strength tests) during treatment are infeasible, the use of indirect non-destructive measurements during treatment is desirable. Development of these real-time, non-destructive measurements would increase the “certainty of execution” of bio-treatment methods. To this end, seismic velocity, and resistivity measurements are examined herein to assess their ability to monitor the extent and spatial distribution of microbially induced calcite precipitation (MICP) in sands. Shear wave velocity (S-wave) test results are used to develop a generalized correlation to the precipitated calcite mass; this in turn enables prediction of changes in void ratio (porosity), density, and shear modulus during treatment. Compression wave velocity (P-wave) measurements are determined under different saturation conditions and used in combination with S-wave measurements to observe how the Poisson’s ratio evolved during treatment. The applicability of resistivity measurements for monitoring the MICP treatment process is also examined. Finally, the seismic properties of MICP treated sand are compared with other conventional materials and the implications of these results for real-time monitoring during future field-scale applications discussed.

Microbial carbonate precipitation: Correlation of S-wave velocity with calcite precipitation

The use of microbial induced precipitation as a soil improvement technique has been growing in geotechnical domains where ureolytic bacteria that raise the pH of the system and induce calcium carbonate (CaCO3) precipitation are used. For many applications, it is useful to assess the degree of CaCO 3 precipitation by non-destructive testing. This study investigates the feasibility of S-wave velocity measurements to evaluate the amount of calcite precipitation by laboratory testing. Two sets of cemented specimen were tested. The first were samples terminated at different stages of cementation. The second were samples that went through different chemical treatments. These variations were made to find out if these factors would affect the S-wave velocity- cementation relationship. If chemical reaction efficiency was assumed to be constant throughout each test, the relationship between S-wave velocity (Vs) and the amount of CaCO3 precipitation was found to be approximately linear. This correlation between S-wave velocity and calcium carbonate precipitation validates its use as an indicator of the amount of calcite precipitation

Development of a Scaled Repeated Five-Spot Treatment Model for Examining Microbial Induced Calcite Precipitation Feasibility in Field Applications

Microbial induced calcite precipitation (MICP) has been heavily investigated in laboratory experiments with few forays into field-scale implementation. Conventionally, MICP refers to an alternative technology for improving the geotechnical properties of soils via microbially mediated urea hydrolysis inducing conditions for calcite precipitation at particle contacts. The study presented herein focuses on up-scaling the conventional treatment process to more realistic volumes through the development of a scaled repeated five-spot treatment model. Commonly used in oil recovery applications, the repeated five-spot well pattern provides for a flow symmetry condition allowing for improved laboratory model feasibility. A conventional MICP two-phase treatment technique resulted in improvement in the target treatment (0.5 m by 0.5 m by 0.15 m) zone with small spatial variation. Sensors, including bender elements and sampling wells, provided valuable insight into the evolution of biological, chemical, and mechanical changes spatially and temporally during treatment. Overall, the scaled repeated five-spot treatment model was successful at capturing a complex treatment scenario involving a bio-mediated soil improvement technology and demonstrated the potential to capture complex scenarios of soil improvement.