This award supports fundamental research necessary to quantify the improvements made to the engineering properties of sand by microbially induced cementation, so that this method can be used to mitigate against earthquake-induced liquefaction damage. Liquefaction is the rapid transformation of a saturated soil deposit from a solid to a viscous fluid during seismic shaking. Some of the most dramatic and costly examples of infrastructure damage in earthquakes such as the 1964 Prince William Sound, Alaska earthquake, the 1971 San Fernando earthquake, and the 2010-2012 Christchurch earthquakes were caused by liquefaction. Bio-mediation, the use of naturally occurring microbes to modify the engineering properties of soils, may be a promising technology for liquefaction prevention. Bacterial processes can be used to generate natural cementation within the grains of saturated sand deposits, improving the soil's resistance to liquefaction. Bio-mediation may additionally be a more environmentally friendly, sustainable option than traditional chemical soil improvement techniques. Results from this research will make US infrastructure and housing more resilient to earthquake damage, resulting in lower loss of property and life in future earthquakes. This research involves several disciplines, including civil engineering, environmental engineering, biochemistry, and sustainability sciences. The multi-disciplinary approach will help broaden participation of underrepresented groups in research, and positively impact engineering and science education.
The objective of this project is to test the hypothesis that the efficacy of microbially induced calcium carbonate precipitation in dynamically loaded saturated sands is governed by microstructural changes in interparticulate cementation dependent upon cyclic strain loading magnitude, number of large strain load cycles, and cementation density. The effectiveness of varying bio-cementation densities will be quantified in terms of volumetric cyclic threshold strains and shear moduli, damping ratios, and excess pore water pressures as functions of cyclic strain magnitudes and number of large strain load cycles. Saturated bio-mediated laboratory specimens with varying MICCP densities will be prepared alongside untreated specimens. Bender element, resonant column, and strain-controlled dynamic cyclic triaxial tests will be used to measure the effects of bio-cementation densities and dynamic loading conditions under undrained, unconsolidated conditions. Scanning electron microscopy images of microbial reinforcement will be used to identify microstructural sources of macroscopic behaviors. The intellectual merit of this work is in: (1) identification of microstructural changes in microbially induced cementation at varying densities due to dynamic loading conditions and (2) quantification of their effects on the macroscopic shear moduli and damping ratios of saturated sand, which are necessary dynamic mechanical properties for predictive models of dynamically loaded soil systems. The microstructural mechanisms responsible for the changes in the dynamic mechanical properties and behaviors under variable loading conditions will be identified, advancing a mechanistic understanding of how bio-cementation physically evolves and reinforces saturated sands under dynamic loads.
