Mechanism of shear strength enhancement of expansive soil by using MICP

Expansive soils are widely distributed across China and pose considerable challenges to engineering construction because of their pronounced swelling and shrinkage behavior. Although microbially induced calcium carbonate precipitation (MICP) has been extensively applied to reinforce coarse-grained soils, its effectiveness and microscale reinforcement mechanisms in fine-grained expansive soils remain insufficiently understood. In this study, laboratory tests were conducted to investigate the effects of bacterial concentration, cementation solution concentration, and curing time on the shear strength of expansive soil treated with MICP. The microstructural evolution of the treated soil was characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). The results showed that the shear strength of the expansive soil increased with increasing bacterial concentration, with the most pronounced improvement occurring as OD 600 increased from 0 to 1.0. As the cementation solution concentration increased, the shear strength initially increased and subsequently decreased. When the cementation solution concentration exceeded 1.0 mol/L, excessive calcium carbonate precipitation caused pore blockage, thereby reducing the shear strength. The shear strength also increased with curing time, with the most pronounced improvement occurring during the first 7 days, after which the rate of strength gain gradually decreased. Microstructural analyses indicated that the calcium carbonate generated during MICP treatment was predominantly deposited as calcite. The precipitated calcite filled the pores between soil particles and formed coatings on the surfaces of clay particles. Unlike coarse-grained soils, in which relatively large pores allow bacteria to remain within interparticle spaces and induce calcium carbonate bridges between adjacent particles, expansive soils possess smaller pores and a greater capacity to adsorb and retain bacteria on clay particle surfaces. The adsorbed bacteria provide nucleation sites for calcite precipitation, thereby promoting calcite accumulation on clay particle surfaces and around clay aggregates. Consequently, the calcite coatings fill interparticle and interaggregate pores and enhance the stability of clay aggregates. This process substantially increases the cohesion between soil particles and aggregates, while the calcite coatings increase surface roughness and improve interparticle friction. Together, these effects restrict particle displacement and shear-band development during direct shear, thereby enhancing the shear strength of the expansive soil. These findings provide a scientific basis and practical guidance for the application of MICP in expansive-soil stabilization.