# Influence of Approaching Excavation on Adjacent Segments for Twin Tunnels
## 4 Numerical Analysis Results and Comparisons
### Influence of Approaching Excavation on Ground Displacements
#### 4.1.2 Horizontal Ground Displacement  
Figure 7 shows the evolutions of the horizontal displacement of the ground located at the middle position between twin tunnels (i.e., the MP axis located at Cross-section B in Figure 3) during the approaching excavation. Positive values on the abscissa indicate the ground inclines to the second tunnel, and negative values indicate the ground moves towards the first tunnel.  
When the second tunnel is far from the measured cross-section (at Cross-section A), the ground within a span of \(D\)/2 nearby the tunnel axis inclines to the second tunnel; while the ground nearby the surface moves towards the first tunnel. When the second tunnel approaches the measured Cross-section B, the ground moves towards the first tunnel, which is mainly affected by the construction loadings during the approaching excavation.  
Figure 6: Ground surface settlements at measured Cross-section B during the approaching excavation.  
As a result, the peak value of horizontal displacement that inclines to the second tunnel (at about 17 m below the surface) reduces by 82% as compared to that when approaches Cross-section A; while the peak value that moves towards the first tunnel (at about 12 m below the surface) raises by 159%. When the second tunnel approaches Cross-section C, the ground again moves towards the second tunnel, which is mainly affected by the ground loss [45] during the approaching excavation. As a result, the peak values that inclines to the second tunnel and the first tunnel are 0.5 mm smaller than those at Cross-section B.  
Figure 8 shows the evolutions of the horizontal displacement of the ground located at the left side and the right side of twin tunnels (i.e., LP and RP axes located at Cross-section B in Figure 3c) during the approaching excavation. Positive values on the abscissa indicate the ground inclines to twin tunnels, and negative values indicate the ground moves outward. When the second tunnel is far from the measured cross-section (at Cross-section A), the ground at both sides presents similar variations as compared to that at the MP axis. The peak value of the displacement that moves outward at the LP axis is larger than that at the RP axis.  
Figure 8: Horizontal ground displacements along the LP and RP axes during the approaching excavation.  
Figure 7: Horizontal ground displacements along the MP axis during the approaching excavation.  
When the second tunnel approaches the measured Cross-section B, the ground nearby the tunnel axis moves outwards. The increment of displacement that moves outward at the RP axis is fairly pronounced, the peak value of which is close to that at the LP axis. This is a consequence of the existing (first) tunnel, which obstructs the transfer of lateral pressing effect. When the second tunnel approaches Cross-section C, the ground nearby the tunnel axis again moves outward; while the ground nearby the surface moves towards the tunnels. The increment of the former movement is smaller than that of the latter one. Under the dual action of ground loss and ovalization of the tunnel structure, the peak value of the displacement that moves outward at the LP axis (nearby the tunnel axis) is 0.3 mm larger than that at the RP axis. However, the peak value that inclines to the tunnels at the LP axis (nearby the surface) is 0.8 mm smaller than that at the RP axis.  
By comparing the results when approaching cross-sections A and C in Figures 7 and 8, it can be seen that the approaching excavation reduces the horizontal displacements of the ground located at the middle position between twin tunnels; whereas increases those at both sides.