## 4 Performance Degradation of MICP-Treated Soil Under Wet-Dry Cycling  
### Degradation of Mechanical Properties  
The results of the UCS tests conducted on the specimens after various wet-dry cycling intervals are presented in Figure 9. The results show that the UCS of the MICP-treated soil samples gradually decreased with an increasing number of wet-dry cycles, exhibiting a trend of a rapid initial decline followed by a more gradual reduction. After nine cycles of wet-dry exposure, the UCS of the specimens tended to stabilize. When the specimens were subjected to 15 wet-dry cycles, the UCS decreased from the initial value of 0.96 MPa to 0.36 MPa, representing a total degradation of 62.24%. However, even after 15 cycles, the UCS of the MICP-treated soil remained above the minimum requirement of 0.35 MPa specified in the Chinese standards. The majority of the UCS degradation, 93.87% of the total, occurred within the first nine wet-dry cycles, where the UCS decreased from 0.96 MPa to 0.39 MPa, a reduction of 58.43%. Meanwhile, the UCS of the untreated expansive soil decreased from 0.24 MPa to 0.12 MPa, representing a total degradation of 50%. Although this degradation rate was notably lower than that of the treated soil, the final strength of untreated soil fell significantly below the standard requirement of 0.35 MPa. The results of the direct shear tests on the specimens after wet-dry cycling are shown in Figure 10. Similarly to the UCS results, the shear strength parameters, namely the cohesion (c) and the angle of internal friction (\(\varphi\)), gradually decreased with an increasing number of wet-dry cycles, also exhibiting a rapid initial decline followed by a more gradual reduction. After 15 wet-dry cycles, the cohesion of the MICP-treated soil samples decreased from 24.34 kPa to 19.74 kPa, a total degradation of 18.92%, while the angle of internal friction decreased from 29.34\({}^{\circ}\) to 21.54\({}^{\circ}\), a total degradation of 26.59%. The degradation rate was most pronounced during the first nine wet-dry cycles, with the cohesion decreasing from 24.34 kPa to 20.11 kPa (91.86% of the total degradation) and the angle of internal friction decreasing from 29.34\({}^{\circ}\) to 22.45\({}^{\circ}\) (88.37% of the total degradation). Despite experiencing greater degradation in cohesion and internal friction angle under wet-dry cycles than untreated soil, the improved soil maintained its superior shear strength--surpassing untreated soil by 23.9% in cohesion and 71.8% in internal friction angle after 15 cycles. Figure 9: Degradation of UCS results. The results of the CBR tests after wet-dry cycling are presented in Figure 11. The CBR value of the MICP-treated soil samples also decreased with an increasing number of wet-dry cycles, following a similar trend of a rapid initial decline and a more gradual reduction. After 15 wet-dry cycles, the CBR value decreased from 8.56% to 5.21%, a total degradation of 39.05%. The majority of the CBR degradation, 91.14% of the total, occurred within the first nine wet-dry cycles, where the CBR value decreased from 8.56% to 5.51%. Meanwhile, the CBR of the untreated expansive soil decreased from 2.05% to 0.77%, representing a total degradation of 62.43%, and the final strength of untreated soil fell significantly below the standard requirement. These results indicate that the wet-dry cycling had a significant negative impact on the mechanical properties of the MICP-treated expansive soil. The degradation of the mechanical properties, including then UCS, shear strength, and CBR, followed a trend of a rapid initial decline followed by a more gradual reduction. After nine wet-dry cycles, the degradation of the mechanical properties tended to stabilize. Using untreated expansive soil as a control, the experimental results demonstrate significantly less mechanical property degradation after wet-dry cycles compared to the improved soil. This difference primarily stems from the untreated soil's initially poor mechanical properties, which leave little room for further deterioration.