Bacteria in Sediments: Soil Bio-engineering

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BACTERIA IN SEDIMENTS: BIO-ENGINEERED SOILSMicrobial activity has influenced the evolution of the earth's surface, converted the atmosphere from reducing to oxidizing, changed the mineral nature of the earth (today most minerals contain either oxygen or hydroxide), accelerated the rate of rock weathering and altered soil formation, influenced the composition of groundwater, and participated in the formation of gas and petroleum hydrocarbons. In this research, we put forward that the tremendous transforming power of bioactivity can be harnessed to bio-engineer near surface soils in short, engineering-time scales. We explicitly recognize limiting size effects: while bacteria are ~1 micron in size, soils and soil pores can range from nanometers to meters. Therefore, we have special interest in defining the geometrical and mechanical limits for bio-activity in soils (while recognizing other potential limiting factors such as carbon, water and nutrients).The specific goals of this research are (1) to develop controlled bio-mineralization methods to increase the small strain shear stiffness and dilatancy of the granular skeleton, (2) to identify the potential effects of bio-generated gas associated to various mineralization pathways, and (3) to determine the conditions required for metabolism and 'bio-survivability' in sediments as a function of sediment grain size and effective stress. The research methodology is based on targeted experimentation, using carefully selected bacterial species and sediments to magnify the phenomena under study, or to falsify the underlying hypotheses. Detailed experimental protocols are designed for each task. Extensive instrumentation and geophysical measurements will permit monitoring the evolution of processes in real time, including the potential long-term reversibility. Experimentation will be complemented with the development of analytical models that capture the governing physical and biological processes, and discrete element simulations that extend the experimental range.Results will affect both engineering (e.g., enhanced seismic performance, healing of microfractures, and new foundation, excavation and tunneling practices) as well as science (e.g., bounds for bio-geological processes, reassessment of bioactivity in the deep biosphere, or authigenic carbonate formation in marine sediments).This research will advance soil behavior and geotechnical engineering in the new dimension of biological processes, beyond today's physical-mechanical-chemical framework, and it will explore potentially revolutionary new alternatives for engineering practice. Educational modules will be prepared and distributed to the community.