## 5 Results and Discussion
### Mechanical Behavior of Caisson before and after Loose Soil Reinforced Using Permeable Polymer  
The soil above the caisson is categorized as miscellaneous fill soil, exhibiting an elastic modulus of 37.8 MPa. Assuming this modulus represents dense soil, the soil with elastic moduli of 30 MPa, 15 MPa, and 5 MPa is considered to be loose and very loose soil,  
Figure 10: Maximum horizontal displacement and stress of the caisson structures under multiple water level fluctuations for 3 years.  
Figure 9: Mechanical behavior of caisson structures under different groundwater levels: (**a**) the horizontal displacement and (**b**) the maximum principal stress.  
respectively. The calculated results indicate that with increasing looseness, the settlement of the surrounding soil is 5.67 mm, 8.28 mm, and 11.39 mm, which are 1.31 times, 1.92 times, and 2.63 times the settlement under dense soil conditions (4.32 mm).  
As the left-side soil of the caisson undergoes subsidence and collapse, the unbalanced force causes the well to shift to the left. Considering the outer side of the left caisson as the path, with the settlement of the dense soil as the reference, the relative horizontal displacement and stress at different soil density along this path were extracted. As shown in Figure 11a, the maximum tilting of the caisson under the different soil density of loose soil is 0.65 mm, 1.0 mm, and 2.05 mm, respectively. The greater the looseness, the more severe the deformation towards the loose side of the caisson. Meanwhile, due to the restriction of the caisson on the bottom displacement of the jacking pipes, the deformation curvature in the middle of the caisson decreases as the soil becomes less dense. The position with the maximum displacement changes from the middle of the caisson to the upper part.  
As illustrated in Figure 11b, the bottom concrete structure compresses due to inclined deformation, resulting in higher compressive stress. In the middle of the caisson, deformation increases tensile stress due to soil constraint. However, as soil density decreases, the constraint weakens, reducing tensile stress. When soil modulus decreases from 15 MPa to 5 MPa, the most significant decrease in tensile stress is 0.1 MPa.  
A permeable polymer has the ability to infiltrate the pores of the soil, consolidating it and enhancing the density and strength of the soil. The soil strength increases by approximately 2.8 times after reinforcement [22]. The density, elastic modulus, Poisson's ratio, and permeability coefficient of the permeable polymer material used in this study were 1.1 g:cm\({}^{-3}\), 10 MPa, 0.3, and 7.42 \(\times\) 10\({}^{-7}\) cm:s\({}^{-1}\), respectively.  
Using the loose soil with an elastic modulus of 15 MPa as an example, the elastic modulus of the soil after permeable polymer grouting increases to 57 MPa. A comparison of the caisson's maximum horizontal displacement and stress before and after reinforcement was conducted, as shown in Figure 12. It is evident that the leftward inclination trend of the caisson is effectively mitigated after reinforcement. Furthermore, owing to the small mass density of the permeable polymer, the grouting expansion not only enhances the strength of the soil but also reduces the inward lateral pressure of the soil on the caisson compared to dense soil, resulting in more effective repair effects.  
Figure 11: Mechanical behavior of caisson structures with different soil densities: (**a**) the horizontal displacement and (**b**) the maximum principal stress.