Effect of freeze-thaw cycles on engineering properties of nano-SiO₂ enhanced microbially induced calcium carbonate precipitation in kaolinite clay

Microbially Induced Calcium Carbonate Precipitation (MICP) is a nature-based soil stabilization technique, that has substantially lower environmental impacts compared to conventional chemical-based methods. However, its application in fine-grained soils, such as clay, remains challenging due to the soil's plasticity and saturation levels, which can hinder the effectiveness of MICP. Furthermore, the performance of MICP-treated soils under extreme environmental conditions, such as cyclic freeze-thaw (FT) processes common in cold regions, has not been fully explored. This study addresses these challenges by investigating the enhancement of MICP using nano-SiO₂ in kaolinite clay subjected to FT cycles, proposing a novel nano-bio soil stabilization method for cold regions. Samples treated with 30 % bacterial (e.g. Bacillus Pasteurii) and cementation solutions, supplemented with 1.5 % nano-SiO₂ over four weeks of curing time, were subjected to cyclic FT and triaxial compression tests. Treated samples demonstrated significantly higher peak shear strengths compared to untreated samples under varying confining stress conditions. A reduction in strength was observed in the treated samples as the number of FT cycles increased. However, by the sixth FT cycle, the treated samples showed a significant improvement in strength compared to the untreated samples, with increases of 4.00, 4.96, and 3.49 times under confining pressures of 50, 100, and 150 kPa, respectively. These findings highlight the effectiveness of the stabilization method under cyclic FT conditions. Microstructural analyses revealed increased calcium carbonate content and altered soil texture in treated samples, which affirms the effectiveness of the nano-bio stabilization approach.