Abstract:Three Gorges Reservoir Area is prone to natural disasters; microbial induced carbonate precipitation (MICP) technology is a soil consolidation technology with the advantages of low energy consumption, no pollution and sustainability. The clayey purple soil is the main soil type in the Three Gorges Reservoir Area with small soil pores, and the effect of MICP on its reinforcement is unclear. Therefore, the MICP-cured soil specimens, which conformed of different Bacillus bacterium concentrations (OD600=0, 0.5, 1.0, 1.5) and curing fluid concentrations (0, 0.5, 1.0, 1.5 and 2.0 mol/L) were subjected to unconsolidated undrained triaxial shear tests to studied the stress-strain relationship, elastic modulus and shear strength index (cohesion and internal friction angle) and their microstructures were analyzed by SEM tests. The results indicated that the shear strength, elastic modulus and cohesion increased and then decreased with the increase of the another solution under the same colloidal liquid concentration or the same curing fluid concentration. The best combination existed when bacterium concentration is OD600=1.0 and the concentration of cement solution is 1.5 mol/L. The average internal friction angle was characterized as increase followed by decrease with the increase of curing fluid concentration, the highest value of internal friction angle was obtained at the bacterium concentration with OD600=0.5 or OD600=1.0 under the same curing fluid concentration. Compared with the no MICP treatment, the maximum values of shear strength, modulus of elasticity, cohesion and internal friction angle of the cured specimens were increased by 62.59%, 64.29%, 119.55%, and 10.33% (226 kPa, 6.44 MPa, 48.30 kPa and 26.70°), respectively. The SEM revealed that the MICP-reinforced purple soil formed a large number of spherical and flaky calcium carbonate crystals, which distributed on the surface and in the interstices of soil particles. The crystals present on the surface of soil particles increased the surface roughness and soil particle size, which in turn increased the friction angle within the soil, while the crystals present in the interstices of soil particles produced cementation, which in turn also increased the soil cohesion. The MICP can effectively improve the strength of cohesive purple soil, and the optimal reinforce performance occurred at the combination of the bacterium concentration with OD600=1.0 and curing fluid concentrations with 1.5 mol/L.