# Experimental Study on Microstructure of Unsaturated Expansive Soil Improved by MICP Method
## 4 Microbial Improvement of the Microstructure of Expansive Soils
### Analysis of Test Results  
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 \(CaCO_{3}\) 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 \(CaCO_{3}\) 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  
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. 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 \(CaCO_{3}\) 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; 42; 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.