Stabilization of coarse-grained saline soil using microbially enhanced calcium carbonate deposition

Coarse-grained saline soil, considered as one of the problematic soils, is extensively distributed in China and causes serious damage to civil infrastructure built on or with these soils, such as road embankment failure, cracks of the buildings, and sliding failure of channel slope. This paper explores the feasibility of microbially induced calcite precipitation (MICP) for the improvement of coarse-grained saline soils, the optimal culture conditions, and the reinforcement mechanism. Laboratory tests including X-ray diffraction (XRD), scanning electron microscope (SEM), and unconfined compression tests were carried out. The results show that the preferable culture conditions are the optical density (OD₆₀₀) of 0.5 for the Bacillus pasteurii solution, the concentration of 0.5 M for the CaCl₂ and urea solution, respectively, and the urea–CaCl₂ molar ratio fixed at 1:1. Using this combination of parameters, the unconfined compressive strength (UCS) of the coarse-grained saline soils is significantly improved after the MICP treatment. The images of SEM also reveal that the calcium carbonate (CaCO₃) crystals are precipitated among the soil particles. For the MICP treatment, fine-particle contents have a negative effect on the improvement of MICP-treated soil samples. There is a little difference on the UCS for MICP-treated saline soils under various salt contents. In addition, the mechanism of coarse-grained saline soils improved by MICP technique is proposed. This study confirms the positive effects of MICP for improving the properties of coarse-grained saline soils.