## 4 Numerical Realization of Zero-Distance Construction Process
### Analysis of the Deformation Control Effect and the Mechanical Response of the Existing Tunnel
It is worth noting that the presence of non-smooth points in the settlement curve at the center point of the tunnel and at the edge of the subsurface region in Scheme III is due to the presence of vertical jacks at these locations, which play a supporting role for nearby structures. It can be seen from Figure 6 that the vertical deformation curve of the tunnel also presents a large \"U\" shape deformation in the middle and small at both ends in the horizontal direction, which indicates that the existing tunnel support scheme applied by the three schemes before excavation played a good role. The five points in the figure are located near the side walls, the middle wall, and the train tracks of the existing tunnel, respectively. It can be seen that the maximum vertical displacement of the tunnel in Scheme I appears near the train track on the right side of the center point, and that the overall tunnel structure has a tendency to tilt slightly to the right. In Scheme II, the deformations of the three points in the middle of the existing tunnel are almost equal, and the existing tunnel as a whole remains horizontal. In Scheme III, due to the existence of jacks near the middle wall, the middle settlement is small, and the overall settlement is also the smallest, and the settlement curve is in a \"W\" shape. Figure 7 shows the development of the vertical displacement of the center point of the existing tunnel during the excavation of the underground area. The whole construction process can be roughly divided into two stages: the existing tunnel support (S1) and the undercrossing soil excavation (S2). The number of steps at the end of the construction is different in the three schemes due to the large difference in the method of excavation construction. It can be seen that the settlement process of Scheme II was the most gentle throughout the excavation, with the tunnel settling slowly with the excavation and the final settlement being small. In Scheme I, the settlement of the existing tunnel was relatively severe, with each excavation causing a large deformation within a few steps, and the final settlement being the largest. In Scheme III, the deformation curve fluctuates greatly, and  
Figure 6: Vertical deformation curve of the existing tunnel (transverse). the tunnel deforms violently during the excavation process, with the maximum settlement of about 3.0 mm (about 1.9 mm at last). The arrow in the figure shows the moment when the jacks are applied in Scheme III, which is the steel support and column at the entrance portal, the column near the middle wall of the tunnel, and the steel support and column at the exit portal from left to right, respectively. After the jacks are applied, the existing tunnel is jacked up under the action of the jacks, and its settlement value decreases. Figure 8 shows the minimum principal stress cloud map of the existing tunnel floor near the underground area at the end of the excavation, which reflects the pressure stress distribution of the tunnel floor under the three schemes. In the figure, the dotted line is the undercrossing area, and the dash dot line is the diagonal line of the undercrossing area. As can be seen, the stress state of the existing tunnel floor is similar under the three schemes, with a smaller minimum principal stress on the longer diagonal and a larger principal stress on the shorter diagonal, indicating that the area near the shorter diagonal bears a larger compressive stress. It is worth noting that in Scheme III there is a clear concentration of stress in the range of motion of the jack near the side walls and the middle wall, especially at the bottom of the middle wall of the tunnel. Excessive local compressive stress can easily cause nearby concrete to exceed its ultimate bearing capacity, causing local damage to the tunnel. Figure 8: Cloud chart of minimum principal stress of existing tunnel floor (unit: Pa). Figure 7: Vertical deformation of center point of existing tunnel.