为了揭示微生物诱导CaCO₃沉积（MICP）技术改良膨胀土强度特性的作用机制，试验选用菌株CGMCC1.3687，采用拌和工艺，系统开展膨胀土强度特性研究。通过固结快剪试验，重点探究了钙离子（Ca²⁺）浓度对膨胀土强度特性的影响机制。结果表明：MICP处理可显著提升膨胀土的抗剪强度；膨胀土经改良后的黏聚力与内摩擦角随Ca²⁺浓度增加均呈现先上升后下降的非单调趋势，并在Ca²⁺浓度为1.5 mol/L时达到峰值（黏聚力42.5 kPa，内摩擦角18.4°），相较于未处理土样，黏聚力增幅高达269.6%，内摩擦角增幅10.2%；CaCO₃生成量同样在1.5 mol/L时达到最大值；相关性分析进一步证实，CaCO₃含量与抗剪强度指标之间存在显著正相关关系。研究成果验证了MICP技术可以显著增强膨胀土强度，其机制在于CaCO₃含量与黏聚力及内摩擦角呈显著正相关。

[Objective] The strength characteristics and underlying mechanical mechanisms of expansive soil treated with microbially induced calcite precipitation （MICP） remain insufficiently understood. Investigating the effect of varying calcium ion concentration on the strength of MICP-treated expansive soil can provide critical guidance for its engineering applications. [Methods] Sporosarcina pasteurii （CGMCC 1.3687） was selected for microbial solidification using the mixing method. This study systematically investigated the mechanism by which different calcium ion concentrations—the core variable—affect the strength characteristics of treated expansive soil. Through a series of consolidated quick shear tests， the relationship between calcium ion concentration and strength parameters was analyzed to elucidate the mechanism by which regulating calcium carbonate formation enhances soil strength. [Results] The results indicate that MICP treatment significantly enhances the shear strength of expansive soil， altering its mechanical behavior from the strain-hardening mode of untreated soil to a dual-mode response： strain-softening under low confining pressure and strain-hardening under high confining pressure. Soil treated with Ca²⁺ solution exhibited higher shear strength than the untreated control. Specifically， cohesion （c） and the internal friction angle （ϕ） followed a unimodal trend with increasing Ca²⁺ concentration， peaking at 1.5 mol/L （c=42.5 kPa， ϕ=18.4°）. This represents a 269.6% increase in cohesion and a 10.2% increase in the internal friction angle. A concentration of 1.5 mol/L was identified as optimal， maximizing calcium carbonate yield and cementation. Lower concentrations resulted in insufficient Ca²⁺， while higher concentrations inhibited bacterial activity； both scenarios reduced calcium carbonate production and soil strength. [Conclusions] This study confirms that MICP technology significantly improves the strength of expansive soil. The strengthening mechanism is attributed to a significant positive correlation between calcium carbonate content and strength indices.