## 4 Numerical Realization of Zero-Distance Construction Process
### Numerical Simulation Process  
In order to restore the stress state of the existing tunnel, the main structure of the station and surrounding rock and soil after being disturbed by the excavation before the construction of the undercrossing area in the project, the field model first completes the calculation of the initial ground stress equilibrium and the layered excavation of the foundation pit at the station location and then removes the support system in the foundation pit and imports the main structure model of the station, coupling it with the field model, to simulate a series of processes of the early station construction. At this time, because the soil mass of the undercrossing area was not excavated, the \"null model\" in FLAC is used to make the main structure of the undercrossing area invalid, and the linear elastic model is reset when the structure is constructed. In the previous simulations, all deformations and internal force indices of the foundation pit and the existing tunnel were within safe limits. The construction simulation of underground excavations in underground areas shall be carried out according to the following steps:  
(1) Reset the displacement of the model.  
(2) The soil mass to be excavated in the undercrossing area shall be reinforced by full-section grouting. The grouting range shall extend to 2 m outside the walls on both sides and 1 m downward at the bottom. This paper adopts the method of increasing the parameters of corresponding rock and soil mass by 30% to simulate ([PERSON] and [PERSON] [43]).  
(3) Break part of the retaining structure to construct the support of the existing tunnel. Due to the longer curing time of concrete compared to steel structures, a calculation process is necessary between soil mass excavation and concrete structure construction. In contrast, steel support structures can be immediately constructed after excavation. Since there are jacks on the top of the steel column, this paper uses the method of additional surface load to simulate the steel column, and the load size is determined according to the deformation of the existing tunnel ([PERSON] et al. [44]).  
Figure 3: Three-dimensional numerical model (taking Scheme I as an example).  
(4) The rock and soil mass of the undercrossing area shall be excavated according to the method required by each scheme, and the main structure of the undercrossing area shall be constructed when the working face meets the construction conditions; during excavation, reinforcement measures for side wall anchors shall be taken in time according to the construction plan, and the anchor bolts shall be simulated by using the \"cable\" of FLAC.  
Figure 4 shows a detailed modeling of the main structure of the existing tunnel and the new station and the construction simulation process. Figure 4i shows the situation after the completion of the tunnel support structure, and Figure 4 iii shows the situation after the completion of the excavation and the main structure. Figure 4 ii shows the excavation process of the undercrossing rock and soil mass, with the same colored rock and soil representing the same excavation steps. Unlike the other two schemes, Scheme III does not use roof and side walls to support the tunnel between the main structure and the tunnel, but instead uses steel columns, which are simulated by applying a surface load.  
Deformation and Stress Patterns of Existing Tunnel and Newly Built Undercrossing Tunnel under Three Construction Methods