SGER: Biological Improvement of the Mechanical Properties of Soils

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Abstract:This SGER project will demonstrate the feasibility of three different mechanisms for microbiologically based improvement of the physical properties of soil through a series of bench-scale experiments. While the potential for employing microbiological process to remediate soil and groundwater contamination has been extensively explored in recent years, the potential for employing microbiological processes to improve the physical properties of soils for geotechnical engineering applications is largely unexplored, despite its occurrence in many natural geologic processes. Potential microbiologically-induced improvements in the engineering process include increased shear strength, decreased compressibility, decreased hydraulic conductivity, and reduced liquefaction potential. The three microbiological processes that will be addressed by the project are mineral precipitation, mineral transformation, and biofilm and biopolymer growth. These processes are known to improve the engineering properties of soil on a geological time scale and are also known to induce potentially beneficial changes in engineering properties of soils in shorter time frames, but in situations where the context renders these effects undesirable (e.g., clogging of landfill drains and water treatment plant filters). The engineering challenges in developing beneficial applications of these microbiological processes involve inducing the desired process over a timeframe of engineering interest in the location of interest and controlling the process to avoid unwanted side effects. This research program consists of three series of bench scale experiments to demonstrate the feasibility of these microbiological mechanisms for improving the physical properties of soil over time frames of engineering interest. One series of experiments will attempt to induce carbonate precipitation in granular soil to increase shear strength and reduce liquefaction potential using three different mechanisms: sulfate reduction, denitrification, and ureolysis. The second series of experiments will evaluate the potential for microbial transformation of smectite, the clay mineral found in most expansive soils, to illite, a significantly less expansive clay mineral and thereby mitigate soil expansion (swell potential). The third series of bench scale experiments will evaluate the impact of biopolymer plugging on the mechanical properties of granular soil. Together, these experiments will evaluate three of the primary candidate mechanisms for microbiological improvement of the physical properties of soils.Success with microbial improvement of the physical (geotechnical) properties of soil would open up a new era in geotechnical engineering - the era of bio-improvement. Mineral precipitation would provide significant advantages in remediation of liquefaction potential of granular soils, especially near or beneath existing structures where traditional soil improvement techniques are limited because of associated ground deformations and/or high cost, stabilization of slopes, control of soil erosion and scour, reduction of settlement and increase in bearing capacity for shallow foundations, stabilization of excavations, and mitigation of flowing sands in tunnels. In addition, microbial mineral precipitation could be used to seal the cracks within a fractured rock formation. Mineral transformation could be used to mitigate problems associated with residential foundations and roadways founded upon expansive soils, a problem that is estimated to cost the US economy over $5 billion per year. Potential applications of biopolymer and biofilms include temporary and permanent groundwater control, mitigation of liquefaction potential, and possibly corrosion protection of steel and concrete structures. Success with microbial improvement of soil will also spur the creation of a new inter-disciplinary field that marries microbiology and geochemistry with geotechnical engineering. The greatest advancements in science and engineering today come at the interfaces of heretofore unrelated fields. Bringing microbiology and geochemistry to bear on geotechnical engineering is an exceptional opportunity for creating such an interface, one with profound scientific and practical benefits.