本研究旨在揭示MICP（Microbially Induced Calcite Precipitation）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，系统开展强度特性研究。通过固结快剪试验，重点探究了钙离子浓度对其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。膨胀土经改良后的粘聚力与内摩擦角随Ca2+浓度变化均呈现先上升后下降的非单调趋势，并在Ca2+浓度为1.5mol/L时达到峰值（粘聚力42.5kPa，内摩擦角18.4°），相较于未处理土样，粘聚力增幅高达269.6%，内摩擦角提升了10.2%。碳酸钙生成量同样在1.5mol/L时达到最大值。相关性分析进一步证实，碳酸钙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，其机制在于碳酸钙含量与强度指标呈显著正相关。

[Objectives] This study focuses on the mechanism of action of Microbially Induced Calcite Precipitation (MICP) technology in improving the strength characteristics of expansive soil. Expansive soil, characterized by high water sensitivity and significant swelling-shrinking behavior, often causes engineering hazards. Traditional modification methods have issues such as high cost and easy contamination. The MICP technology, which utilizes microorganisms to induce calcite precipitation, can fill soil pores and cement soil particles, and is expected to enhance soil strength in a green and efficient manner.[Methods] In this experiment, Sporosarcina pasteurii (CGMCC 1.3687) was selected, and the microbial solidification was carried out using the mixing method. This study systematically explored the mechanism of the effect of different calcium ion concentrations on the strength characteristics of modified expansive soil, among which calcium ion concentration is the core variable. Through a series of consolidated quick shear tests, the internal relationship and action path between calcium ion concentration and strength indicators were analyzed, and the key mechanism by which it improves soil strength by regulating calcium carbonate formation was clarified. [Results] This study shows that MICP technology significantly enhances the shear strength of medium expansive soil, altering its mechanical behavior from the single strain 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 exhibits higher shear strength than the untreated group. Specifically, cohesion (c) and internal friction angle (ϕ) follow a single-peak trend with increasing Ca²⁺ concentration, peaking at 1.5 mol/L (c = 42.5 kPa, ϕ = 18.4%)—representing a 269.6% increase in cohesion and 10.2% increase in internal friction angle. 1.5 mol/L is identified as the optimal Ca²⁺ concentration, as it maximizes calcium carbonate yield and cementation effect. Too low a concentration leads to insufficient Ca²⁺, while too high a concentration inhibits bacterial activity; both scenarios reduce calcium carbonate production and soil strength.[Conclusions] The research results verify that MICP technology can significantly enhance the strength of expansive soil, and the mechanism lies in the significant positive correlation between calcium carbonate content and strength indexes.