## 6 Influence Analysis of Shield Tunneling Parameters
### The Influence of Grouting Pressure on Surface Settlement
#### 6.2.2 Influence of Grouting Pressure on Mechanical Properties of Surrounding Rock  
Tunnel excavation causes the redistribution of surrounding rock stress, resulting in the reduction in the surrounding rock strength and even damage [21], in particular, the profile of the excavation surface formed by the small curve shield tunnel is different, which leads to uneven stress on the surrounding rock, which may lead to the instability of the surrounding rock. In this paper, the shear stress and shear strain of the surrounding rock are taken as the evaluation indexes, the grouting pressure is, respectively, taken as 50 kPa, 100 kPa, 150 kPa, 200 kPa, 300 kPa, 400 kPa, and 500 kPa. The relationship curves of the maximum shear stress of the surrounding rock, the maximum shear strain of the surrounding rock, and the grouting pressure obtained by calculation are shown in Figures 13 and 14, respectively. Figure 12: Curve of surface settlement with simultaneous grouting pressure. It can be seen from Figure 13 that the variation trend of the maximum shear stress of the surrounding rock with the grouting pressure of the four tunnels is similar, that is, when the grouting pressure is less than 400 kPa, the maximum shear stress of the surrounding rock decreases, and when it is greater than 400 kPa, the maximum shear stress of the surrounding rock decreases. Then, it increases slowly, because the larger grouting pressure squeezes the surrounding rock or even destroys the surrounding rock. When the grouting pressure is 200 kPa, and the maximum shear stress of the surrounding rock with a curve radius of 100 m, 350 m, 600 m, and the straight tunnel is 202.6 kPa, 180.9 kPa, 176.6 kPa, and 169.6 kPa, respectively. Compared with the maximum shear stress of the surrounding rock with the straight tunnel, the maximum shear stress of the surrounding rock corresponding to the curve radius of 100 m, 350 m, and 600 m increased by 19.46%, 6.66%, and 4.13%, respectively. This is because the smaller the curve radius is, the more uneven the force on the surrounding rock is, which leads to greater stress on the surrounding rock. It can be seen that under the same conditions, the smaller the curve radius is, the easier the surrounding rock is to be unstable and damaged. Figure 14: Variation curve of maximum shear strain of surrounding rock with the grouting pressure. Figure 13: Variation curve of maximum shear stress of surrounding rock with the grouting pressure. It can be seen from Figure 14 that when the grouting pressure is less than 300 kPa, the maximum shear strain of the surrounding rock decreases with the increase in the grouting pressure. When it exceeds 300 kPa, the maximum shear strain of the surrounding rock increases rapidly with the grouting pressure. The reason is that when the grouting pressure is too large, the surrounding rock is squeezed excessively, and the surrounding rock will be split and large deformation will occur. At the same time, it can be found that when the grouting pressure is constant, the smaller the curve radius of the shield tunnel is, the greater the maximum shear strain of the surrounding rock is, and the easier the failure occurs. When the grouting pressure is 200 kPa, compared with the straight tunnels, the maximum surrounding rock shear strain of the three curved tunnels increases by 28.43%, 13.47%, and 2.42%, respectively. It can be seen that the tunnel radius and grouting pressure have a great influence on the stability of the surrounding rock. It can be seen that the grouting pressure and the radius of the tunnel curve have a certain influence on the stability of the surrounding rock. A moderate increase in the grouting pressure is helpful to improve the stability of the surrounding rock. When the grouting pressure is constant, the smaller the radius of the curve is, the greater the maximum shear stress of the surrounding rock and the maximum shear strain of the surrounding rock will be, and the more easily unstable the surrounding rock will be.