Examining the Liquefaction Resistance of Lightly Cemented Sands Using Microbially Induced Calcite Precipitation (MICP)

摘要

Microbially induced calcite precipitation (MICP), or bio-cementation, is a bio-mediated, environmentally-conscious soil improvement technology that can improve the engineering properties of granular soils through the precipitation of calcium carbonate (CaCO_3) on soil particle surfaces and contacts. While past studies have shown the potential of bio-cementation to improve the resistance of granular soils to earthquake-induced soil liquefaction, the spectrum of behaviors during the transition from the uncemented to lightly cemented conditions has remained poorly characterized. In this study, a series of direct simple shear tests were performed to improve our understanding of the effect of CaCO_3 bio-cementation on the liquefaction triggering and post-triggering strain accumulation behavior of loose sands. A poorly-graded Ottawa F-65 sand material was treated to varying degrees of very light bio-cementation and subjected to undrained cyclic shearing events. All specimens were cemented and sheared under a vertical effective stress of 100 kPa and subjected to cyclic stress ratios (CSR) of 0.1 and 0.2. During these tests, shear wave velocity (V_s) measurements were completed to non-destructively monitor cementation progression and degradation during shearing. Results suggest that even very light levels of bio-cementation (△V_S ≈ 0 to 100 m/s) can significantly increase the number of cycles needed to trigger soil liquefaction (3% single amplitude shear strain), e.g., a 10-fold increase in the number of cycles needed to trigger liquefaction was observed when V_s values were increased by only 25 m/s. Despite this significant improvement in small-strain pre-triggering behavior, little improvement was observed with respect to strain accumulation before and after initial triggering. While additional testing is needed, results from this study can improve our understanding of the behavior of lightly bio-cemented and naturally-cemented soils subjected to earthquake-induced undrained cyclic loading.