## 3 Results and Discussion  
### Analysis of Direct Shear Test Results  
#### 3.1.1 Evolution Rule of Shear Stress  
The variation curve of the shear stress-shear displacement of permeation grouting muddy clay under different normal stresses is shown in Figure 4.  
Figure 4 shows that the shear stress of the soil around piles after grouting is significantly higher than that without grouting. As the shear displacement increases, the shear stress of muddy clay without grouting first increases approximately linearly, then slows down, and finally tends to a stable value. In addition, the soil around the pile produces an obvious peak shear stress after grouting, and its value is slightly greater than the residual stress. For the permeation grouting muddy clay, the variation rules of shear stress-shear displacement under different cement content are consistent and can be divided into a rapid growth stage (\(0<\delta<1\) mm), peak stress stage (\(1<\delta<3\) mm), and stabilization stage (\(3<\delta<7\) mm). The cement content has a positive effect on increasing the shear stress of the soil around piles with permeation grouting. The peak shear stress of the soil around piles rises significantly with the cement content, and the shear displacement required to reach the peak strength gradually decreases. This shows that the soil around piles with high cement content is developing toward brittle failure. The shear strength of muddy clay and the mechanical properties of the soil around piles are improved by the permeated cement slurry. The shear stress-shear displacement curves of the soil around piles have obvious peak shear stress differences between 0% to 5% and 5% to 10%. This is because, as the cement content increases, more hydration products fill the pores in the soil, and the greater cement strength enhances the bonding force between particles. The soil skeleton is more compact under normal stress, and the shear strength is significantly improved. For the softening curve of muddy clay in Figure 4, the shear stress corresponding to the peak point is taken as the representative peak shear stress. Figure 5 shows the relation between the peak shear stress and grouting parameters of permeation grouting muddy clay under different cement content. Here, the peak shear stress of the soil around permeation grouting piles increases approximately linearly when the cement content is less than 5%. However, the rate of peak shear stress development slows significantly when the cement content is between 5 and 10%. The peak shear stress no longer increases basically under lower normal stress when the cement content is greater than 10% but still increases slowly under higher normal stress. This is because the greater the cement content, the more obvious the hydrolysis and hydration between it and the soil particles [34]. The generated cementing substance filled in the soil particles, which greatly improved the strength of the cement soil. Figure 6 shows the variation curve of permeation grouting silty clay with different normal stresses. For the silty clay without grouting, its curve has a short linear rapid growth initially and then grows slowly. The shear stress basically tends to be stable when the shear displacement approaches the maximum shear strain. It is shown that the silty clay  
Figure 4: Curves of shear stress and shear displacement of muddy clay. (**a**) \(\sigma=50\) kPa. (**b**) \(\sigma=100\) kPa. (**c**) \(\sigma=200\) kPa. (**d**) \(\sigma=400\) kPa. Figure 5: Relation between peak shear stress and grouting parameters. has elastic deformation at the beginning and then gradually changes to plastic deformation. Finally, the sample experiences shear failure, the shear strength reaches the maximum, and the shear stress tends to be stable and no longer increases. Under the same normal stress, the slope of the initial shear curve gradually increases, and the increase in cement content obviously improves the shear strength of the soil around the piles.