## 6 Influence Analysis of Shield Tunneling Parameters
### The Influence of Grouting Pressure on Surface Settlement
#### 6.2.3 Influence of Grouting Pressure on Mechanical Properties of Segment  
During the excavation process of the shield tunnel with the small curve radius, due to the uneven force on the inside and outside of the tunnel and the over-excavation on the inside, the force on the segment may be further increased. The grouting pressures were taken as 50 kPa, 100 kPa, 150 kPa, 200 kPa, 300 kPa, 400 kPa, and 500 kPa, respectively. The variation curves of the maximum longitudinal tensile stress and the maximum circumferential stress of the segment with grouting pressure are shown in Figures 15 and 16. Figure 15: Variation curve of the maximum longitudinal tensile stress of the segment with the grouting pressure. It can be seen from Figure 15 that the grouting pressure has a great influence on the longitudinal tensile stress of the segment when the tunnel is excavated. The longitudinal tensile stress of the segment increases with the increase in the grouting pressure, and the two have an obvious nonlinear relationship. When the grouting pressure is constant, the maximum longitudinal tensile stress of the segment increases with the decrease in the curve radius; especially when the tunnel radius is 100 m, the longitudinal tensile stress of the segment increases rapidly. When the grouting pressure is 200 kPa, the maximum longitudinal tensile stress with a curve radius of 100 m, 350 m, 600 m, and the straight tunnel is 3.29 MPa, 2.12 MPa, 2.05 MPa, and 1.98 MPa, respectively. The longitudinal tensile stress of the segment corresponding to the curve radius of 100 m, 350 m, and 600 m increased by 66.16%, 7.07%, and 3.54%, respectively. The reason is that when the curve radius is smaller, the jack force in the shield tunneling process is not related to the segment. The larger the direction angle is, the more the uneven force on the segment will cause additional stress on the segment. Figure 16 also shows that the maximum circumferential stress of the four tunnel segments increases with the increase in grouting pressure. When the curve radius is 100 m, the circumferential stress changes more obviously, and the circumferential stress of the segment increases with the curve radius. When the grouting pressure is 200 kPa, the maximum longitudinal tensile stress with curve radius of 100 m, 350 m, 600 m, and the straight tunnel is 3.30 MPa, 2.10 MPa, 1.83 MPa, and 1.81 MPa, respectively. Compared with the longitudinal tensile stress of the segment of the tunnel, the longitudinal tensile stress of the segment corresponding to the curve radius of 100 m, 350 m, and 600 m increased by 82.32%, 16.02%, and 1.10%, respectively. It can be seen that the curve radius of the shield tunnel has a certain influence on the force of the segment. To sum up, it can be seen that appropriately increasing the grouting pressure helps to control the surface subsidence, the deformation of the surrounding rock, and the stress of the segment, and the radius of the curve also has a certain influence on the three. Therefore, the radius of the tunnel should be increased as much as possible in the design stage. In this project, it is recommended that the grouting pressure be 200-250 kPa. In cities with dense engineering buildings, limited by the complex surrounding environment, it is necessary to build small radius curved tunnels to meet the line requirements of urban subway. The over break in the arc section of small radius arc tunnel shield construction is more obvious, and not only causes greater disturbance to the ground, but also has a greater impact on the stability of the surrounding rock and the stress of segments in zone II. According to the research results, a smaller curve radius will cause greater soil de  
Figure 16: Variation curve of the maximum circumferential stress of the segment with the grouting pressure. formation and segment stress. It is suggested that engineers and designers should not only consider the surface deformation, but also pay attention to the mechanical characteristics of surrounding rock and segments.