*4.2. Analysis of Test Results*

Scanning electron microscope imaging tests were carried out to obtain SEM images of the expanded soil, as shown in Figure 5.

**Figure 5.** SEM images of expansive soil: (**a**) fitted curve for unimproved expansive soil; (**b**) A1 fitted curve; (**c**) A2 fitted curve; (**d**) A3 fitted curve.

Scanning electron microscope imaging tests were carried out to obtain SEM images of the expansive soil, as shown in Figure 5. The shape of the unimproved expansive soil particles is mainly flat and flake, the particles are mostly in edge-to-face or face-to-face contact, with large variability in pore size. The *CaCO*<sup>3</sup> crystals appeared at the contact points of the expansive soil particles after the MICP improved, and the morphology was mostly spherical particles. The contact between the particles tends to be smooth and the pore size distribution is relatively uniform. In the expansive soil improved by higher cementation liquid content, there are more *CaCO*<sup>3</sup> crystals and larger porous spaces, and the porous spaces are evenly distributed. At low levels of cementation liquid, the expansive soils contain mostly small or medium porous spaces that are uniformly distributed. Li carried out XRD analysis on the improved expansive soils, the study found that the calcite characteristic peaks increased significantly, and the main peak values were enhanced in the expansive soils treated by the MICP method [39]. The calcium carbonate precipitate produced by the test was of a different shape to the typical rhombohedral calcite. This phenomenon might be caused by the shape of the particles being obscured by the soil, or by some bacteria and fine clay particles remaining on the surface of the crystals. Based on the variation of the particle porous structure, a microscopic perspective reveals the internal mechanism by which the air entry value of expansive soils gradually decreases with increasing contents of the cementation liquid. Compared to that of the cohesion of unimproved expansive soils, which are strongly influenced by matric suction and are more water sensitive, matric suction no longer has a restraining effect on the structure after saturation [40]. After improvement by the MICP method, the *CaCO*<sup>3</sup> crystals produced better cementation between the soil particles. The greater the content of the cementation liquid, the more calcium carbonate precipitation is induced and the greater the contact surface of the particles formed, which promotes the agglomeration of the soil particles, and the strength of the expansive soil is improved. Moreover, due to the low solubility of calcium carbonate, it is less affected by water, thus significantly improving the water stability of the expansive soil.

The swelling-shrinkage characteristics of expansive soils in the unsaturated state are mainly related to the water film thickness on the surface of the soil particles when they react with water. The process of water swelling in expansive soils is in essence a process whereby hydrophilic soil particles are wrapped in water molecules under the influence of electric field forces to form a water film. The thickness of the water film gradually increases as the expansive soil absorbs water, resulting in the expansion of the soil particle lattice and the swelling of the soil [41–43]. The unimproved expansive soil particles are mostly in the form of tightly packed or stacked flakes, the lattice can only expand longitudinally outwards when exposed to water. With the increase in the content of the cementation liquid, the flaky particles in the improved expansive soil gradually decrease and the pore space gradually increases. When reacting with water, the lattice of the improved soil particles expands in every direction, balancing the expansion potential in the internal space of the soil before expanding outwards, effectively reducing the expansion deformation of the expansive soil. At the same time, the microbially induced formation of calcium carbonate adheres to the surface of the expansive soil particles, reducing the interaction area between hydrophilic soil particles and water, reducing the water film thickness and interparticle spacing, tightening the soil structure and improving the strength of the soil effectively.

## **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.


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.

**Author Contributions:** 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.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 50978097.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The work described in this paper was supported by a grant from the National Natural Science Foundation of China (Project No. 50978097).

**Conflicts of Interest:** The authors declare no conflict of interest.
