Abstract: The effectiveness and mechanism of MICP (Microbial Induced Calcium carbonate Precipitation) improving the expansion characteristics of expansive soil were investigated via test of expansive soil mixed with urease-producing bacteria CGMCC 1.368 7. The effects of reaction solution ratio and Ca²⁺ molar concentration on the free expansion rate, unloaded expansion rate, and percentage generation of CaCO₃ of expansive soil treated with MICP were studied by orthogonal experiments, and the microscopic properties of MICP-modified expansive soil were revealed. The swelling characteristics of expansive soil treated with MICP can be improved remarkably: when the ratio of reaction solution is 1∶1 and the concentration of Ca²⁺ is 2.0 mol/L, the free expansion rate of expansive soil is reduced by 44.4% at most; when the ratio of the reaction solution is 1∶3 and the concentration of Ca²⁺ is 2.0 mol/L, the volumetric expansion rate is reduced by 92.2%, and the expansion moisture content is reduced by 24.9%. MICP technology reduces the hydrophilicity of expansive soil particles, hinders the repulsion between soil particles, and weakens the expansion potential of expansive soil by cementing soil particles, filling soil pores and ion replacement. The research results verify the feasibility of improving the swelling properties of expansive soil by mixing based on MICP technology.

Key words: expansive soil, free swelling rate, unloaded swelling rate, MICP, microanalysis