## 3 Results and Discussion
### Analysis of Microstructure Characteristic Test Results  
The macroscopic engineering characteristics of soil are influenced by the state and variation of the microstructure, and its complex mechanical behavior is a specific expression of microstructure characteristics. To quantitatively analyze the influence of different cement content on the shear strength of the soil around piles, the image information of permeation grouting soil was obtained through a soil microstructure test system on the basis of the direct shear test. The changes in the internal characteristic parameters of grouting soils with different cement content are further analyzed, to reveal the strengthening mechanism of permeation grouting to the soil around piles from the mesoscopic level. Figure 10 shows the binarized image of the microstructure of permeation grouting muddy clay with different grouting parameters. Through the analysis of image information, the variation law of the microstructural characteristic parameters of muddy clay with permeation grouting can be obtained, as shown in Figure 11.  
From Figure 11, it can be seen that the proportion of particle area in the muddy clay of permeation grouting increased when compared with the non-grouted muddy clay, and the ratio of pore area decreased. This is because the cement in the soil around piles is hydrated after grouting, and the generated cementing substance fills the pores between particles. In Figure 11a, the particle roundness of muddy clay showed an upward trend after grouting. Roundness is used to represent the shape of the particles, and the higher the value, the closer the spatial arrangement. The compressibility of particles is negatively correlated with roundness. The compressibility is reduced, and thus the ability of the soil skeleton to resist deformation increases. In addition, the fractal dimension of the particle distribution  
Figure 10: Binarized image of microstructure of permeation grouting muddy clay. (**a**) Cement content of 0%. (**b**) Cement content of 5%. (**c**) Cement content of 10%. (**d**) Cement content of 15%. indicates the orientation degree of the particles. The higher the fractal dimension value of distribution, the worse the orientation of particles, the more chaotic and dispersed the particles, and the greater the density of soils. Comparing Figure 11a,b, the fractal dimension value of the particle distribution tends to increase overall after grouting, while the change in pore orientation is opposite to that of the particles. After grouting, the pore orientation of the soil improves and the pore density decreases, while the fractal dimension of the pore distribution does not have a relatively clear change law. This is because grouting makes the particle structure denser, which limits the change in pore arrangement and distribution, resulting in a reduction in the change in pore orientation and fractal dimension of pore distribution. Reflected at the macroscopic level, this is manifested as an obvious stress peak on the shear stress-shear displacement curve. Figure 12 shows the binarized image of the microstructure of permeation grouting silty clay with different grouting parameters. Through the analysis of image information, the variation law of the microstructural characteristic parameters of silty clay with permeation grouting can be obtained, as shown in Figure 13.  
Figure 13 shows that the particle area ratio of silty clay decreases and the pore area ratio increases after grouting. Since the cement hydration products grow along the outer edge of the particles, the equivalent diameter of the particles increases continuously, resulting in an increase in the particle roundness. Thus, the particles are arranged more closely, and the compressibility is reduced. However, the distribution fractal dimension of the particles showed a decreasing trend as a whole. This indicates that the degree of soil particle agglomeration increases and the pore connectivity decreases.