AbstractThis study investigated the mechanical behavior of a low-plasticity silt treated by microbially induced carbonate precipitation (MICP). Also, the biogeochemical reactions of the fine-grained soil during MICP treatment were investigated, as soil minerals can react with MICP solutions due to increased pH (pH=9) and generation of carbonate ions. Two types of MICP treatments were investigated: (1) bacterial suspension in the urea medium (UB treatment); and (2) urea medium, bacteria cells, and cementation medium (UBC treatment). The two treatments were applied to silt samples that were then subjected to direct shear tests at different confining pressures (12, 25, and 35 kPa). The relationships of shear stress versus horizontal displacement, compression displacement versus horizontal displacement, and equivalent CaCO3 contents and distributions were measured. Also, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), and Raman spectroscopy were used to investigate the elemental and mineral compositions and microscale morphology of the silt samples. The peak and ultimate shear strengths of the silt samples were improved by the UB and UBC treatments. The improvements of the mechanical properties of UB-and UBC-treated samples are mainly attributed to the precipitations of calcium carbonate, iron carbonate, and iron in the soil.