# Analysis of Shield Tunnels Undercrossing an Existing Building and Tunnel Reinforcement Measures
## 8 Discussion  
### Discussion on the Effect of the Excavation of Double Shield Tunnels on Soil, Segment and Building  
From Figure 11, it can be seen that the excavation of double shield tunnels caused the soil around the tunnel to approach the tunnel in the longitudinal direction and stay away from the tunnel in the lateral direction, causing the soil at the midpoint of the connection between the two tunnels to be compressed, thus offsetting some of the subsidence of the soil. As a result, the subsidence of the soil above the tunnels was distributed in an \"M\" shape with the distance from the centerline, as shown in Figure 12. The displacement of the soil around the tunnel caused the tunnel to be compressed by the soil, resulting in an elliptical shape of the segments, as shown in Figure 14. The maximum subsidence value of 7.19 mm, the maximum bulge value of 3.34 mm, and the maximum lateral displacement of 4.27 mm were located at the vault, upset, and waist of the segment, respectively. As shown in Figure 36, due to the gravity effect of the upper building and the squeezing effect of the soil, and the subsidence of the soil above the tunnel is greater than the uplift of the soil below the tunnel, the tunnel generated a certain degree of contraction along the excavation direction and a certain degree of expansion along the horizontal direction. As shown in Figures 15 and 16, this effect caused pressure to be applied to the segments in all longitudinal directions, and the segments at the top and bottom of the tunnel arch were subjected to positive bending moments, while the segments at the arch waists on both sides are subjected to negative bending moments. As shown in Figure 22, shield tunneling caused uneven subsidence of the building, resulting in lateral displacement of the pile. In addition, shield tunneling excavation caused the lateral displacement of the soil around the tunnel. Due to the interaction between piles and soil, the pile can also experience lateral displacement. Building subsidence increased with the increase of shield tunneling distance, and the building subsidence increased from 0.09 mm at the beginning to 7.22 mm at the end of the excavation. The lateral displacement of the pile increased from 0.09 mm at the beginning to 3.82 mm during the excavation of the right tunnel. Discussion on the Effect of Grouting Pressure on Mechanical Behaviors of Soil, Segment, and Building  
As shown in Figure 24, the maximum subsidence of soil reduced linearly with the increase of grouting pressure. For the grouting pressures of 50 kPa and 550 kPa, the  
Figure 36: Tunnel under compression and tension: (**a**) Tunnel contraction along the longitudinal direction; (**b**) Tunnel expansion along the lateral direction. corresponding maximum subsidence of soil were 23.93 mm and 6.37 mm, with a reduction of 73.38%. According to the calculation, the maximum subsidence value of the soil mass would decrease by 3.512 mm when the grouting pressure increased by 100 kPa. This is because an increase in grouting pressure would increase the support effect on the soil around the tunnel, resulting in a decrease in the subsidence of the soil above the tunnel. Figure 26 shows the relationship curves between the maximum longitudinal and transverse stresses of the segment and the grouting pressure. From Figure 26, it can be found that when the grouting pressures were in the range of 50-250 kPa, the maximum longitudinal and transverse stresses reduced linearly with the increase of grouting pressure. When the grouting pressures were in the range of 250-550 kPa, The maximum longitudinal and transverse stresses increased linearly with the increase of grouting pressure. This was because when the grouting pressure increased appropriately, it reduced the subsidence of the soil above the tunnel, thereby reducing the compression effect of the soil on the tunnel, resulting in a decrease in the longitudinal and transverse stresses of the segment.