## 5 Conclusions  
This study comprehensively investigated the influence of different cementation solution concentrations on the swelling and mechanical properties of MICP-treated expansive soils. Based on the experimental results, the following main conclusions can be drawn:  
1. MICP treatment can significantly improve the swelling characteristics of expansive soils. As the cementation solution concentration increased, the free swell ratio and unconfined swell ratio of the soil samples both exhibited a decreasing trend. When the cementation solution concentration reached 1 M, the swelling properties of the soil samples were substantially suppressed. This can be attributed to the precipitation of calcium carbonate during the MICP process, which fills the pore spaces between soil particles and alters the internal structure, thereby enhancing the anti-swelling performance.
2. MICP treatment also significantly enhanced the mechanical properties of the expansive soil. As the cementation solution concentration increased, the unconfined compressive strength, cohesion, and internal friction angle of the soil samples all exhibited notable improvements. When the cementation solution concentration reached 1 M, the mechanical properties of the soil samples were significantly enhanced, meeting the technical requirements for highway subgrade fill materials. This is primarily due to the filling and cementing effects of the calcium carbonate precipitates, which strengthen the inter-particle bonding within the soil.
3. Under continuous wet-dry cycling, the mechanical properties of the MICP-treated soil samples experienced degradation. The unconfined compressive strength showed the most significant reduction, decreasing by 62.24%. The decreases in cohesion and internal friction angle were relatively smaller, at 18.92% and 26.59%, respectively. The CBR value also decreased by 39.05%. This performance degradation mainly occurred within the first nine wet-dry cycles, after which it tended to stabilize. This is likely due to the partial dissolution and loss of the calcium carbonate precipitates under the repeated wet-dry cycles, leading to a reduction in the strength and stiffness of the MICP-treated soil.  
In summary, MICP technology can effectively improve the performance of expansive soils, but in regions with hot and humid climates, the mechanical properties of the treated soil samples may undergo a certain degree of degradation under long-term wet-dry cycling. Therefore, further optimization of the MICP method is needed to enhance its durability under complex environmental conditions, providing a more reliable solution for the engineering construction in expansive soil-prone regions.  
**Author Contributions:** Conceptualization, F.H. and B.J.; methodology, S.W.; software, S.W.; validation, F.H., B.J. and F.Z.; formal analysis, F.Z.; investigation, F.Z.; resources, X.L.; data curation, X.L.; writing--original draft preparation, B.J.; writing--review and editing, B.J. and F.H.; visualization, X.L.; supervision, S.W.; project administration, F.Z.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Science and Technology Research Project of Chongqing Education Commission of China (Grant numbers: KJQN202305802, KJQN202403413), Chongqing Natural Science Foundation (Grant number: CSTB2024 NSCQ-MSX0006).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this paper will be made available by the authors by request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.