Biocementation of a Well-Graded Gravelly Soil and Macromechanical Characterization

Abstract

Soil biocementation through microbially induced carbonate precipitation (MICP) is a promising technique for improving soil behavior in a nondisruptive manner, particularly for rehabilitation and retrofitting applications. Previous studies characterizing the shear behavior of biocemented soils have concentrated on poorly graded sands, whereas research on well-graded gravelly soils, which are extensively used in shallow geotechnical structures, has been lacking. Mohr–Coulomb strength parameters have been predominately employed to interpret the macromechanical effects of biocementation, but the previously reported findings show significant contradictions. In this study, a well-graded aggregate, representative of commonly used well-graded gravelly soils, was biocemented and subjected to monotonic drained triaxial compression. The test results show remarkable improvements in shear behavior, with the observed changes in stress–strain responses, strength and stiffness development, and stress dilatancy agreeing with those reported for biocemented sands as well as conventional cemented soils. Relatively low cementation levels can effectively rectify the mechanical performance caused by poor compaction to that seen at optimal levels, demonstrating the feasibility and potential of biocementation for improving soils of this type. Detailed analysis of the results reveals the decisive role of cementing bonds and their degradation in causing behavioral changes at different shearing stages. The theories of bonded structure and force-chain evolution are used to explain the preyielding observations, while an analytical approach capable of quantifying the evolution of different strength components is presented for postyielding macromechanical characterization. Conversely to the inference drawn from the strength parameters, the largest improvement is found in the frictional rather than the dilative and cohesive components of strength. Further analysis reveals the commonality of the macromechanical effects of biocementation, density, and confinement, and a unique relationship between macromechanical composition and peak stress ratio emerges.