Enzyme induced carbonate precipitation (EICP) treatment is a stabilization method of dust mitigation that applies a spray-on treatment to form a soil crust and increase the wind erosion resistance of a disturbed soil surface. The purpose of this work was to evaluate the EICP treatment with multiple field and laboratory test methods for measuring the wind erosion resistance of EICP treated soil. The threshold friction velocity (TFV) is defined as the minimum wind speed required to initiate continuous particle movement and represents the wind erosion resistance of a soil surface. Tested soil type and textures included a clean fine sand to a loamy sandy soil that contained a significant amount of fines. Dry untreated soil and disturbed field soil surfaces were compared to a chloride salt solution treatment and an EICP treatment solution in both laboratory and field testing to evaluate the wind erosion resistance of the treatments.

Microbially- and enzyme-induced carbonate precipitation (EICP and MICP) offer potentially sustainable and cost-effective mitigation methods for fugitive dust by forming an erosion-resistant crust on the soil through precipitation of a natural calcium carbonate (CaCO3) cement. While there have been isolated studies on the efficacy of the carbonate precipitation process, there are few systematic studies of the influence of the properties of the soil being treated (e.g., gradation, salt content) on the precipitation and the resulting wind erosion resistance. Moreover, the influence of environmental conditions on the durability of the crust formed by the induced carbonate precipitation has not been systematically investigated. In this research program, the efficacy and durability of EICP and MICP for dust mitigation were investigated for a variety of soil types and in different environmental conditions. Soil samples from seven sites with fugitive dust problems were treated with MICP or EICP and subjected to lab or field testing. The results of these tests showed that the effectiveness of biocementation treatment varies depending on the grain size distribution of soil and mineralogical composition. Testing on iron ore tailings materials demonstrated that treating by application of EICP solutions at lower concentrations (i.e., 0.5M and 0.75M of urea and calcium chloride) yielded effective results for poorly graded fine sand-sized tailings but the same solutions were ineffective for the well graded sand-sized tailings that contained large gravel-sized particles. Additionally, the application of MICP and EICP on sediments adjacent to a shrinking lake (the Salton Sea) with different salt contents exhibited enhanced performance in soils with lower salt content. The effect of temperature during deployment and precipitation cycles are shown to be significant environmental factors by simulating wetting-drying and freeze-thaw cycles in the laboratory. A dust-resistance crust formed through biocementation remained mostly intact after undergoing multiple cycles of wetting-drying. However, the durability of a dust-resistance crust formed through biocementation to multiple cycles of freeze-thaw depended on treatment solution concentration and soil grain size. Additionally, high temperature during field deployment of MICP adversely effected crust formation due to rapid evaporation that inhibited the complete hydrolysis of urea and the precipitation of carbonate.

Trichloroethene (TCE) and hexavalent chromium (Cr (VI)) are ubiquitous subsurface contaminants affecting the water quality and threatening human health. Microorganisms capable of TCE and Cr (VI) reductions can be explored for bioremediation at contaminated sites. The goal of my dissertation research was to address challenges that decrease the efficiency of bioremediation in the subsurface. Specifically, I investigated strategies to (i) promote improve microbial reductive dechlorination extent through the addition of Fe0 and (ii) Cr (VI) bio-reduction through enrichment of specialized microbial consortia. Fe0 can enhance microbial TCE reduction by inducing anoxic conditions and generating H2 (electron donor). I first evaluated the effect of Fe0 on microbial reduction of TCE (with ClO4– as co-contaminant) using semi-batch soil microcosms. Results showed that high concentration of Fe0 expected during in situ remediation inhibited microbial TCE and ClO4– reduction when added together with Dehalococcoides mccartyi-containing cultures. A low concentration of aged Fe0 enhanced microbial TCE dechlorination to ethene and supported complete microbial ClO4– reduction. I then evaluated a decoupled Fe0 and biostimulation/bioaugmentation treatment approach using soil packed columns with continuous flow of groundwater. I demonstrated that microbial TCE reductive dechlorination to ethene can be benefitted by Fe0 abiotic reactions, when biostimulation and bioaugmentation are performed downstream of Fe0 addition. Furthermore, I showed that ethene production can be sustained in the presence of aerobic groundwater (after Fe0 exhaustion) by the addition of organic substrates. I hypothesized that some lessons learned from TCE Bioremediation can be applied also for other pollutants that can benefit from anaerobic reductions, like Cr (VI). Bioremediation of Cr (VI) has historically relied on biostimulation of native microbial communities, partially due to the lack of knowledge of the benefits of adding enriched consortia of specialized microorganisms (bioaugmentation). To determine the merits of a specialized consortium on bio-reduction of Cr (VI), I first enriched a culture on lactate and Cr (VI). The culture had high abundance of putative Morganella species and showed rapid and sustained Cr (VI) bio-reduction compared to a subculture grown with lactate only (without Morganella). Overall, this dissertation work documents possible strategies for synergistic abiotic and biotic chlorinated ethenes reduction, and highlights that specialized consortia may benefit Cr (VI) bio-reduction.

Enzyme-induced carbonate precipitation (EICP) is a biogeotechnical soil improvement method that involves the precipitation of calcium carbonate via hydrolysis of urea (ureolysis) catalyzed by free urease enzyme in a calcium chloride solution. When this reaction takes place in the pore space of a sand, the precipitated calcium carbonate may bind soil grains together, thereby improving strength. Three studies on EICP are presented in this dissertation. In the first study, chemical equilibrium modeling via PHREEQC is used to develop a method for evaluating urease activity from electrical conductivity (EC) measurements in a closed reactor containing urea and urease. It is shown that a commonly used correlation to estimate urease activity from EC measurements overestimates the initial urea hydrolysis rate (thereby overpredicting the urease activity as well). In the second study, the crystal structure and mechanical properties of calcium carbonate minerals formed by EICP are studied. It is shown that a “modified” precipitate synthesized by the inclusion of nonfat dry milk in the EICP solution is more ductile than a “baseline” precipitate synthesized from an EICP solution without nonfat milk. Additionally, in sands biocemented using the modified EICP solution, precipitation occurs preferentially at the grain contacts. This may contribute to relatively high unconfined compressive strengths at low carbonate contents in some EICP-treated sands. The third study discusses the role of some sand characteristics on the strength following modified EICP treatment. Three batches of Ottawa 20-30 sand from different sources were treated identically using the modified EICP solution. Subsequent testing showed large differences in their unconfined compressive strengths. It is shown that this variation in unconfined compressive strength is due to differences in the surface microtexture and surface mineralogy of the sands.The fundamental studies presented in this dissertation provide a deeper understanding of some aspects of the EICP process.

In this project, the potential of ferrous iron precipitation as an alternative for ground improvement applications is investigated. This study analyzes the potential of naturally occurring iron oxidation, which uses Fe2+ as an electron donor to produce Fe3+ precipitate. The goal of this study was to stimulate or accelerate the naturally occurring iron oxidation and precipitation process, to form a ferruginous crust in the subsurface, that would reduce hydraulic conductivity or increase soil strength. Iron precipitation can occur through aerobic or anaerobic iron oxidizers. Initial experimental test results in falcon tubes and a literature review showed that to obtain significant oxidation of ferrous iron and consequent precipitation of iron minerals required a buffer to prevent acidification. Experimental studies in which aerobic and anaerobic iron precipitation is stimulated in sand columns under various boundary conditions also leads to an optimization of conditions for mineralization. Mineralized zones are evaluated via permeability loss tests, extent of iron oxidized and characterization tests which show that the crust has the most concentration of precipitated iron, which can be used in targeting pollution mitigation, erosion control, etc. The results show a significant loss of permeability- by a factor of two, in high concentration of iron with a balanced buffer control. In this study, the knowledge on ground stabilization by studying the naturally occurring mechanism of iron precipitation, leading to possible industrially relevant geotechnical applications are successfully investigated.

Bridge scour at piers is a major problem for design and for maintaining old infrastructure. The current methods require their own upkeep and there may be better ways to mitigate scour. I looked to the mangrove forests of coastal environments for inspiration and have developed a 2D model to test the efficacy of placing a mangrove-root inspired system to mitigate scour. My model tests the hydrodynamics of the root systems, but there are additional benefits that can be used as bioinspiration in the future (altering the surrounding chemistry and mechanical properties of the soil).Adding a mangrove inspired minipile system to bridge piers changes scour parameters within my 2D COMSOL models. For the volume of material added, the minipiles compare favorably to larger sacrificial piles as they reduce A_wcz and 〖τ'〗_max by similar (or even better) amounts. These two parameters are indicators of scour in the field. Within the minipile experiments, it is more beneficial to place them upstream of the main bridge pier as their own ‘mangrove forest.’ The value of A_wcz and 〖τ'〗_max for complex 2D models of scour is unclear and physical experiments need to be performed. The model geometry is based on the dimensions of the experimental flume to be used in future studies and the model results have not yet been verified through experiments and field trials. Scale effects may be present which cannot be accounted for in the 2D models. Therefore future work should be conducted to test ‘mangrove forest’ minipile systems in 3D space, in flume experiments, and in field trials.

Enzyme-induced carbonate precipitation (EICP) is an emerging technology for ground improvement that cements soil with calcium carbonate to increase strength and stiffness. EICP-improved soil can be used to support new facilities or it can be injected under existing facilities to prevent excessive deformation. The limitations for commercial adoption of EICP are the cost and the lack of implementation at field-scale. This research demonstrated two ways to reduce the cost of EICP treatment at field-scale. The first was a modification to the EICP solution such that lower amounts of chemicals are needed to achieve target strengths. The second was to use a simple and inexpensive enzyme extraction method to produce the enzyme at a large-scale. This research also involved a two-stage scale-up process to create EICP biocemented soil columns using a permeation grouting technique. The first stage was at mid-scale where 0.6 m x 0.3 m-diameter EICP biocemented soil columns were created in boxes. This work confirmed that conventional permeation grouting equipment and methods are feasible for EICP soil treatment because the columns were found to have a uniform shape, the injection method was able to deliver the EICP solution to the edges of the treatment zone, and downhole geophysics was effectively used to measure the shear wave velocity of the biocemented soil mass. The field-scale stage was performed in the Test Pit facility at the Center for Bio-mediated and Bio-inspired Geotechnics' Soils Field Laboratory. Seven biocemented soil columns were created with diameters ranging from 0.3-1 m and heights ranging from 1-2.4 m. Effective implementation at this scale was confirmed through monitoring the injection process with embedded moisture sensors, evaluating the in situ strength improvement with downhole geophysics and load testing, and testing of the excavated columns to measure shear wave velocity, dimensions, carbonate content, and strength. Lastly, a hotspot life cycle assessment was performed which identified ways to reduce the environmental impacts of EICP by using alternative sourcing of inputs and extraction of byproducts. Overall, this research project demonstrates that EICP is a viable ground improvement technique by way of successfully producing field-scale biocemented soil columns.

Expansive soils pose considerable geotechnical and structural challenges all over the world. Many cities, towns, transport systems, and structures are built on expansive soils. This study evaluates stabilization of expansive soils using silicate solution extracted from rice husk taking advantage of an agricultural material waste. Rice husk ash production was optimized considering several factors including rinsing solution, rinsing temperature, burning time, and burning temperature. Results indicated that washing the rice husk with HCl (1M) produced an ash with surface area of 320 m2/g and 97% of silicon oxide. Two local soils were treated with sodium silicate solution, silica gel at pH 1.5, and silica gel at pH 4 to evaluate its mechanical properties at curing times of 1 day, 7 days, and 14 days. Results indicated that sodium silicate solution reduced the one-dimensional swell by 48% for Soil A, however, swell for soil B remained about the same. Silica gel at pH 1.5 reduced the one-dimensional swell by 67% for soil A and by 35% for soil B. Silica gel at pH 4 did also reduce the free swell by 40% for soil A and by 35% for soil B. Results also indicated that the swell pressures for all treated soils increased significantly compared to untreated soils. Soils treated with sodium silicate solution showed irregular compaction curves. Silica gel-treated soils showed a reduction in the maximum dry unit weight for both soils but optimum water content decreased for soil A and increased for soil B. Atterberg limits were also reduced for sodium silicate and silica gels-treated soils. Swelling index for bentonite showed a reduction by 53% for all treated bentonites. Soil-water characteristics curves (SWCC) for sodium silicate-treated soils remined almost the same as untreated soils. However, silica gels-treated soils retain more water. Surface area (SSA) decreased for sodium silicate-treated soil but increased for all silica gels-treated soils. It was concluded that curing times did not show additional improvement in most of the experiments, but the results remained about the same as 1-day treatment. The study demonstrated that silicate solution is promising and sustainable technique for stabilization of expansive soils.

The Atlantic razor clam burrows underground with effectiveness and efficiency by coordinating shape changings of its shell and foot. Inspired by the burrowing strategy of razor clams, this research is dedicated to developing a self-burrowing technology for active underground explorations by investigating the burrowing mechanism of razor clams from the perspective of soil mechanics. In this study, the razor clam was observed to burrow out of sands simply by extending and contracting its foot periodically. This upward burrowing gait is much simpler than its downward burrowing gait, which also involves opening/closing of the shell and dilation of the foot. The upward burrowing gait inspired the design of a self-burrowing-out soft robot, which drives itself out of sands naturally by extension and contraction through pneumatic inflation and deflation. A simplified analytical model was then proposed and explained the upward burrowing behavior of the robot and razor clams as the asymmetric nature of soil resistances applied on both ends due to the intrinsic stress gradient of sand deposits. To burrow downward, additional symmetry-breaking features are needed for the robot to increase the resistance in the upward burrowing direction and to decrease the resistance in the downward burrowing direction. A potential approach is by incorporating friction anisotropy, which was then experimentally demonstrated to affect the upward burrowing of the soft robot. The downward burrowing gait of razor clams provides another inspiration. By exploring the analogies between the downward burrowing gait and in-situ soil characterization methods, a clam-inspired shape-changing penetrator was designed and penetrated dry granular materials both numerically and experimentally. Results demonstrated that the shell opening not only contributes to forming a penetration anchor by compressing the surrounding particles, but also reduces the foot penetration resistance temporally by creating a stress arch above the foot; the shell closing facilitates the downward burrowing by reducing the friction resistance to the subsequent shell retraction. Findings from this research shed lights on the future design of a clam-inspired self-burrowing robot.

This thesis is part of a larger research project, conducted by Elizabeth Stallings Young, which aims to improve understanding about the factors controlling the process of MIDP and the interaction between the biochemical reactions and the hydrological properties of soils treated with MIDP. Microbially Induced Desaturation and Precipitation (MIDP) is a bio-geotechnical process by which biogenic gas production and calcite mineral bio-cementation are induced in the pore space between the soil particles, which can mitigate earthquake induced liquefaction (Kavazanjian et al. 2015). In this process substrates are injected which stimulate indigenous nitrate reducing bacteria to produce nitrogen and carbon dioxide gas, while precipitating calcium carbonate minerals. The biogenic gas production has been shown to dampen pore pressure build up under dynamic loading conditions and significantly increase liquefaction resistance (Okamura and Soga 2006), while the precipitation of calcium carbonate minerals cements adjacent granular particles together. The objective of this thesis was to analyze the recorded pore pressure development as a result of biogenic gas formation and migration, over the entire two-dimensional flow field, by generating dynamic pressure contour plots, using MATLAB and ImageJ software. The experiment was run in a mesoscale tank that was approximately 114 cm tall, 114 cm wide and 5.25 cm thick. Substrate was flushed through the soil body and the denitrifying reaction occurred, producing gas and correspondingly, pressure. The pressure across the tank was recorded with pore pressure sensors and was loaded into a datalogger. This time sensitive data file was loaded into a MATLAB script, MIDPCountourGen.m, to create pressure contours for the tank. The results from this thesis include the creation of MIDPContourGen.m and a corresponding How-To Guide and pore pressure contours for the F60 tank. This thesis concluded that the MIDP reaction takes a relatively short amount of time and that the residual pressure in the tank after the water flush on day 17 offers a proof of effect of the MIDP reaction.