Exploratory Investigation of Bio-inspired Flexible Calcite Precipitation for Soil Improvement

点击翻译按钮获取中文摘要

Civil infrastructures (e.g., highways, railroads, bridges, and buildings) are commonly constructed on weak and/or loose soils that require improvement to resist applied loads including those generated by natural hazards (e.g., earthquake loading). Often such soils are improved using energy-intensive materials and techniques that are not environment-friendly. These issues will be exacerbated over the next 25 years, as civil infrastructure is expected to expand greatly to accommodate an anticipated 30% growth in the world's population. To address these issues, sustainable and resilient materials and construction techniques must be developed. One such approach for soil improvement relies on bio-mediated processes that use indigenous bacteria (microbes) in the soil, with recent research focusing on the microbial induced carbonate precipitation (MICP) process. When comparing the responses of untreated and MICP-treated soils, the data available in the literature show that soil shear strength can increase by up to 500%. However, applications of the MICP technique have encountered challenges and difficulties including: (1) breakage of the brittle cementation bonds at small strains; (2) the generation of ammonium (a toxic waste product); and (3) limited penetration of microbes through the pores of soils smaller than fine sands. These challenges and difficulties (a) limit the MICP application to specific soil particle sizes and (b) reduce the shear strength and modulus of soils, which limits the benefits of the MICP technique in geotechnical applications such as liquefaction mitigation and foundation support. This grant promotes the progress of science by exploring the development of a new resilient and sustainable method to improve soil properties using a bio-inspired process that naturally occurs in sea sponges. This approach provides strong and flexible bonding of soil particles and can be used in finer soils. The proposed concept is transformative, and it accelerates the development of biogeotechnical soil improvement techniques, motivates further innovative developments in the field of bio-inspired geotechnical engineering, and enhances cross-disciplinary collaboration. The project also enables the training and education of undergraduate and graduate students, and in particular, will develop new researchers who are knowledgeable and competent in the techniques and procedures employed in biogeotechnical engineering that are not usually employed in traditional geotechnical engineering education; and educate the general public, K-12 teachers and students, university students, and practicing engineers. The goal of this project is to explore the concept of bio-inspired flexible calcite (BiFC) precipitation to improve the bonding (cementation), ductility, stiffness, and strength of soils. This research focuses on the use of the silicatein-alpha enzyme, without microbes, to induce flexible calcite precipitation in soils. Preliminary experiments demonstrated that the silicatein-á enzyme can precipitate bio-inspired flexible calcite (BiFC) in the laboratory and confirmed that BiFC with 10% to 16% silicatein á enzyme content has exceptional flexibility with no sign of breakage when subjected to strains greater than 20% with shear resistance that is 9 times greater than that of naturally-formed calcite. Therefore, it is hypothesized that using silicatein-alpha enzyme, without microbes, to produce BiFC precipitate in soils will eliminate the breakage of cementation bonds at small strains and enhance the mechanical properties of soils upon shearing (ductility, stiffness and strength), avoiding the need for additional healing treatments after earthquake loading. The proposed process also avoids the generation of ammonium that occurs with MICP; does not require subsurface stimulation or augmentation of microbes potentially reducing the cost of the process; and may extend the range of soil particle sizes that could be treated. To achieve the goal of the project, this exploratory research will focus on: (1) optimizing the production of silicatein-alpha enzyme and BiFC using bench-scale reactors and fermenters and characterizing BiFC using high-definition optical microscopy and scanning electron microscopy (SEM); (2) characterizing the micro scale mechanical properties of precipitated BiFC using Atomic Force Microscopy (AFM) cantilever beam tests and particle-scale tests; and (3) investigating the mechanical properties of BiFC treated sand and silt using triaxial testing.