# Influence of Approaching Excavation on Adjacent Segments for Twin Tunnels
## 4 Numerical Analysis Results and Comparisons
### Comparison of Numerical and Monitored Results  
Figure 10 shows the contours of the circumferential strain and the deformations of the measured segment at three stages. When the second tunnel is far from the measured cross-section (Figure 10a), the deformed configuration of the segment, scaled with factor 200, presents a slight difference with that calculated by [PERSON] [46] with respect to a straight-line shield tunnel. The difference is that the deformations at the inner side of the curved tunnel are larger than those at the outer side. The reason for the asymmetry of deformations on both sides of the curved tunnel is that the resistance of strata at the inner side of the curved tunnel is weakened due to the overcutting. When the second tunnel approaches the measured cross-section (Figure 10b), a more asymmetric and increased ovalization is presented as compared to Figure 10a. This is a consequence of the approaching excavation, which generates a pronounced lateral pressing effect on the measured segment. As the second tunnel keeps away from the measured cross-section, the deformations of the segment at Cross-section C no longer changes significantly. In order to investigate the spatial effect of the approaching excavation of the second tunnel on the first tunnel, Figure 11 shows the variations of the longitudinal deformations in the measured segment during the approaching excavation. Positive values on the ordinate indicate the deformations in the direction of the TBM advance. During the approaching of the second tunnel to the monitored Cross-section B, the monitored segment moves toward the direction of the TBM advance. However, it moves toward the reverse direction during the TBM shield gradually keeps away from the monitored cross-section. Among them, the variation of the point at the inner side of the curved tunnel (\(\alpha=180^{\circ}\)) is more obvious than that at the outer side (\(\alpha=0^{\circ}\)), which is due to the influence of the overcutting. The variation of the point at the tunnel crown (\(\alpha=90^{\circ}\)) is more obvious than that at the tunnel invert (\(\alpha=270^{\circ}\)). These significant variations mainly occurred within about 30 m from the front of the  
Figure 10: Circumferential strain and deformation of the measured segment when the excavation face of second tunnel approaches: (**a**) Cross-section A; (**b**) Cross-section B; and (**c**) Cross-section C.  
monitored cross-section. In other words, the longitudinal range of the impact of the approaching excavation on the structure of the first tunnel is 30 m (5\(D\)). Taking the strain values when the second tunnel approaches Cross-section A (step 77) as the initial values, after each excavation step, the strain increments due to the approaching excavation are calculated as shown in Figure 12. The strain increments at each monitoring point remain small until the excavation of the second tunnel with a lagged distance of about 24 m (10 steps) behind the measured cross-section. After that, the strain increments develop gradually and reach the peak values when the second tunnel face arrives near the measured cross-section. Eventually, the strain increments are going to stabilize after the TBM shield passes over the measured cross-section. In particular, the fluctuation amplitudes of circumferential strain increments are larger than those of axial strain increments. Concretely, the variations of the strain increments nearby the measured cross-section are summarized as follows:  
* The circumferential strain increment at C1 firstly develops as tensile strain; then fluctuates towards the reverse direction; finally stabilizes in the compressive strain state. The circumferential strain increment at C2 mainly fluctuates within the range of compressive strain. These variations indicate that the impact of the approaching excavation is significant on the measured segment at the tunnel springline and shoulder regions, which are mainly subject to the circumferential compression.