# Experimental Study on Microstructure of Unsaturated Expansive Soil Improved by MICP Method
## 5 Conclusions  
The SWCC of the expansive soils improved by the MICP method was obtained using the filter paper method under different conditions of cementation liquid content. Based on the theory of unsaturated soil mechanics, the experimental study and theoretical analysis were carried out on the water stability of improved expansive soils. At the same time, scanning electron microscope imaging technology was used to test on expansive soil samples. Based on the analysis of SWCC, the microscopic mechanisms affecting the water sensitivity and strength properties of the improved expansive soil were revealed. The following conclusions were obtained.  
1. The content of the cementation liquid has a significant influence on the SWCC of the expansive soils improved by the MICP method. The saturated and residual water content of the improved expansive soils gradually increases with the content of cementation liquid increased. The air entry value decreases, and the water stability increases gradually.
2. Scanning electron microscope imaging tests were carried out on the expansive soils before and after improvement. The change of particle morphology and porous structure of the expansive soils under different conditions of cementation liquid content was investigated. The study shows that, compared to that of the unimproved expansive soil, the particle composition of the improved expansive soil evolves from the mutual superposition of flattened and flaky particles to spherical particles in contact with flaky particles, which increased agglomerates significantly. At the same time, with the increase in cementation liquid content, the contact between the particles tends to be smoother and the soil porous tends to be uniformly distributed gradually.
3. Based on the analysis of macroscopic and microscopic test results, it was found that the mineralization process of microorganisms changed the particle size of soil particles and the porous structure of the soil. The microscopic mechanism affecting the water stability, swelling and shrinkage characteristics and strength properties of the improved expansive soil were revealed. After the improvement, the calcium carbonate formed by microbial induction precipitates on the surface of the soil particles and in the soil pores, enhancing the interparticle linkage, reducing the hydrophilicity and swelling-shrinkage of the soil, and improving the strength and water stability of the soil. The study also shows that the antierosion ability of the soil is improved significantly due to the increase of aggregates in the soil, the coarsening of the soil particles, and the decreasing water sensitivity of the soil.  
The MICP method of improving expansive soils is effective in increasing the strength and water-holding capacity of the soil, but the ammonium ions produced during the reaction can also contaminate the soil [44,45]. The management of ammonium ions in the improvement process is an issue worthy of further study.  
Conceptualization, X.Y. and H.X.; methodology, X.Y. and H.X.; software, X.Y. and H.X.; validation, X.Y. and H.X.; formal analysis, X.Y., H.X., Z.L., J.Q., S.L. and H.S.; investigation, X.Y., H.X., Z.L., J.Q., S.L. and H.S.; resources, X.Y. and H.X.; data curation, X.Y. and H.X.; writing--original draft preparation, X.Y. and H.X.; writing--review and editing, X.Y., H.X. and Z.L.; visualization, X.Y., H.X., Z.L., J.Q., S.L. and H.S.; supervision, X.Y., H.X. and Z.L.; project administration, H.X.; funding acquisition, H.X. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 50978097.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The work described in this paper was supported by a grant from the National Natural Science Foundation of China (Project No. 50978097).  
The authors declare no conflict of interest.