## 3 Results and Discuss
### Mechanical Properties of Treated Soils  
The UCS test results for the S-0 to S-5 soil samples are shown in Figure 4. As the cementation solution concentration increased, the UCS increased from 0.24 MPa to 1.18 MPa,  
Figure 3: Unconfined swell ratio test results.  
Figure 2: Free swell rate test results.  
an increase of 391.67%. The rate of change in the UCS exhibited a rapid increase at first, followed by a slower increase. The S-3 soil sample, corresponding to a cementation solution concentration of 1 M, was the turning point for the change in the compaction characteristics of the MICP-treated expansive soil. When the cementation solution concentration was less than 1 M (S-0 to S-3), the UCS increased from 0.24 MPa to 0.96 MPa, an increase of 300%. When the cementation solution concentration was greater than 1 M (S-3 to S-5), the UCS increased from 0.96 MPa to 1.18 MPa, an increase of 91.67%.  
The direct shear test results for the S-0 to S-5 soil samples are shown in Figure 5. As the cementation solution concentration increased, the cohesion increased from 20.67 kPa to 27.65 kPa, an increase of 33.76%, and the internal friction angle increased from 18.53\({}^{\circ}\) to 35.52\({}^{\circ}\), an increase of 91.67%. The rate of change in the cohesion and internal friction angle exhibited a rapid increase at first, followed by a slower increase. The S-3 soil sample, corresponding to a cementation solution concentration of 1 M, was the turning point for the change in the compaction characteristics of the MICP-treated expansive soil. When the cementation solution concentration was less than 1 M (S-0 to S-3), the increases in cohesion and the internal friction angle were 17.77% and 58.33%, respectively. When the cementation solution concentration was greater than 1 M (S-3 to S-5), the increases in cohesion and the internal friction angle were 15.99% and 33.34%, respectively.  
The CBR test results for the S-0 to S-5 soil samples are shown in Figures 6 and 7. The CBR value increased from 2.05% to 10.33%, an increase of 403.90%. A minimum value point occurred at a cementation solution concentration of 1 M, with the S-3 soil sample having a CBR value of 8.56%, which already meets the requirement for the highway subgrade filler CBR value in China. The load expansion rate results show that the S-1 to S-5 soil  
Figure 4: Compression test results.  
Figure 5: Shear test results.  
samples had a significantly lower load expansion rate compared to the S-0 soil sample, with a reduction of ranging from 13.40% to 61.44%. The overall change rate of the load expansion rate for the S-0 to S-5 soil samples exhibited a rapid decrease at first, followed by a slower decrease. When the cementation solution concentration was greater than 1 M, the decrease in the load expansion rate slowed down significantly.  
These results indicate that the mechanical properties of the expansive soil were significantly improved after MICP treatment. This is likely due to the formation of calcium carbonate precipitates through the interaction between _Sporosarcina pasteurii_, the cementation solution, and the expansive soil. Some of the precipitates were adsorbed onto the surrounding fine particles, forming aggregates, while others filled the pores between the soil particles [33]. This resulted in a more compact internal structure, with reduced voids and increased particle interlocking and frictional effects, allowing the improved soil to withstand higher stresses.  
Furthermore, the substantial increase in UCS, cohesion, and the internal friction angle, along with the significant reduction in the load expansion rate, highlights the effectiveness of MICP treatment in enhancing the load-bearing capacity and stability of expansive soils. The findings suggest that a cementation solution concentration of 1 M is optimal for achieving significant mechanical property improvements.