This research addresses a compelling opportunity in geoengineering to apply recent discoveries in the fundamental mechanisms controlling silica nucleation and growth in silicifying organisms. Biomimetic silicification (biosilicification) uses insights into biochemical pathways developed by these organisms to replicate, for a directed purpose, natural silicifying processes and products. This approach confers a number of advantages for ground treatment: 1) Potential for a new, sustainable means of creating cemented soil that can be tailored to specific geomechanical performance problems; 2) Uses readily?available, environmentally benign chemical materials; 3) Changes to soil and groundwater chemistry are not required and the process proceeds within the pH range of most natural soils; and, 4) Modifies the energy barriers to silica precipitation onto soil grain surfaces and may also improve the properties of the silica polymer that forms within the interstitial space. This approach has the distinct advantage of putting to work the biochemical machinery of organisms without the difficulties of culturing and maintaining active populations. Biosilicification has the potential for cost savings on ground improvement projects over traditional grouting methods because concentrations of silicic acid required could be much less. Cements formed by this process can potentially yield higher grouted soil strength and better long?term deformation behavior compared to traditional methods.
Basic laboratory research will build on insights from preliminary studies to create cemented sand specimens using biosilicification. Materials will consist of Ottawa 20/30 sand, commercially available silicate solutions and amine?based, polyelectrolyte macromolecules. Compositions, concentrations and delivery methods of silicate and macromolecule solutions will be varied. Nondestructive in-situ microstructure characterization and distribution of cement in specimens will be made using x-ray tomography. Unconfined compression and drained triaxial compression tests will be used to assess strength gain with time; stress-strain, stiffness and strength behavior; and, long-term, constant load strength. The potential for biomimetic healing of broken cement bonds will be evaluated. Silicification experiments will be conducted to determine cement strength and the strength of cement to grain bonds. Vickers hardness and elastic modulus of the cement will be determined by nano?indentation tests. Comparisons will be made with traditional silicate?grouted sand specimens.
Through the unique combination of the geoengineering and biogeochemistry expertise of the PIs, this project is an opportunity for innovative, and possibly transformative, advances with broad societal, economic and educational impacts. Societal benefits are derived from a ground improvement method that could lower the cost of infrastructure, using materials that are non?toxic to humans and ecosystems, and prevent worker exposure to chemical hazards. Considering the potential for commercial application of this new process, impacts to the geotechnical profession are possibly large. Interactions with practitioners during the project will be an especially fruitful avenue for transferring the discoveries to practice. Undergraduate students will be encouraged to work on this exciting interdisciplinary research project. Graduate students on the project will have the opportunity to develop expertise in the growing area of applied biogeochemistry. As such, they will have frontline experience in transforming rapidly advancing scientific discoveries from the bench top to the geoengineering scale. In conjunction with the Center for Enhancement of Engineering Diversity, academically qualified women, minorities and first generation university students will be recruited. Women students will especially benefit from mentoring by co-PI P.Dove and from the scholarship opportunities available through the NSF sponsored AdvanceVT program.
Over the last decade, the engineering and scientific communities have made advances in developing new materials that draw upon a rapidly expanding understanding of biological materials and processes. Of particular interest is the ability of organisms to direct the mineralization of tissues to produce highly functional materials. These structures are the result of natural selection that is, in itself, the result of "tuning" mechanical efficiency in response to environmental conditions. Thus, scientists, engineers, and businesses are highly motivated to mimic processes in nature to produce new industrial materials. The goal is to produce lighter, stiffer, stronger materials using smaller inputs of energy and component chemicals. This goal holds the promise of translating into new employment opportunities for trained individuals, and improved economic and environmental sustainability. This project used insights into biological silicification to produce a new cementing agent to strengthen soils for the construction industry. Improvement of soils supporting structures is often necessary before construction can take place or unfavorable performance can occur. Corrective action can be expensive. The current state of ground strengthening practice includes use of Portland cement based materials or chemical grouts. These materials can be problematic because: 1) Potential harmful effects on soil, groundwater and buried structures due to high concentrations of caustic and/or other materials in the subsurface; and, 2) Production of Portland cement and lime result in substantial release of carbon dioxide into the atmosphere. Through extensive laboratory testing, this project has developed a process that uses commercially available materials to strength sand soils. A provisional patent (Serial No. 61/017,273) was filed through Virginia Tech Intellectual Properties, Inc. Unconfined compressive strengths are found to be equal to or higher than soils treated at similar silicate concentrations using traditional and alternative formulations. In preliminary experiments, silicification was found to substantially improve the strength of compacted fine-grained soil. However, additional study that is beyond the scope of this project is required before the silicification process is applied to fine-grained soils. Six US-born students consisting of five males and one female participated in this project while students in the Department of Civil and Environmental Engineering at Virginia Tech. One male student and the female student worked while undergraduates, gaining valuable experience. All students earned graduate degrees in engineering and are employed in professional practice. The results were transferred to practice by personal interaction with construction company representatives, through seminars and in papers published in the engineering literature.
