Collaborative Research: Laboratory-to-Field Scaling and Geophysical Monitoring for Soil Bio-Improvement

The growth of large urban populations worldwide and the associated need to build on sites with problematic soils has increased the need for low-noise/low-energy ground improvement techniques to hinder soil settlements, increase soil strength, and prevent subsurface soil instabilities such as liquefaction. Microbial-induced carbonate precipitation (MICP) has emerged as a promising technique to improve soil sites because microorganisms capable of causing calcium carbonate precipitation are ubiquitous in earth systems, and naturally-occurring biogeochemical reactions can be augmented and engineered. However, MICP faces inherent soil type limitations and spatial variability issues, requires adequate quality control/adaptation, must satisfy required 'permanency,' and may pose unwanted environmental consequences. There is limited experience with upscaling successful laboratory results to field conditions, and there are high early costs inherently related to new technology. The goal of this research is to combine emerging geophysical technologies with established soil strengthening techniques to first investigate the feasibility of MICP in the laboratory and then upscale the results in a field study case.

MICP has emerged as a promising bio-mediated soil improvement technique. Carbonate precipitation can be engineered by biostimulation, whereby selected nutrients are injected into the soil and indigenous soil microorganisms are used to catalyze carbonate precipitation throughout the sediment porous network as a direct consequence of byproducts formed during cell growth or active metabolism. The effect of cementation on soil behavior depends on the amount and type of cementing agent, grain size distribution of the soil, density, and degree of confinement at the time of cementation, i.e., the stress-cementation history. Carbonate precipitation reduces porosity, stiffens and strengthens the soil mass, alters the response of the internal fabric to stress changes, and increases the dilative tendency upon shear. Quality control is a critical component of any soil improvement effort. The process can be quantitatively examined in real time to assess the evolution and spatial extent of the bio-treatment and to adapt/optimize it to increase its efficiency. This research will explore complementary, real-time monitoring concepts. The non-invasive, geophysical tools selected for this study will be developed and optimized in the laboratory, and then will be scaled up to field conditions. This research will include the following activities: (1) explore environmentally-safe, optimal deployment strategies, (2) identify conditions to minimize spatial variability and develop control techniques, (3) test complementary process-monitoring techniques (elastic shear wave velocity and electrical-spectral induced polarization using both local and tomographic test conditions and penetration CPTu in the field), and (4) scale up laboratory studies to the field.