## 4 Monitoring Data Analysis  
### Settlement Development of the Existing Large-Diameter Tunnel  
The deformation law of the shield tunnel of Line 12 between the Jimen Bridge and Dazhongsi interval under the existing Beijing-Zhang High-Speed Railway of Qinghua Park was analyzed, and the vertical deformation ephemeral curve of the existing tunnel is shown in Figure 9. According to the construction sequence and control measures of Line 12's undercrossing construction, the settlement ephemeris curve of the existing tunnel was divided into the following six stages. MSC11-B and MSC12-B, which were positioned on the rail plate, had similar settlement tendencies. Depending on the monitoring data, six stages of vertical deformation development in the existing large-diameter shield tunnel were identified in the location of the new tunnel face and building measures, as shown in Figure 9, and the trend of settlement changes in each stage was categorized as follows:  
Stage I: Far-field excavation (>2 D)  
During this phase, the maximum settlement of the existing large-diameter tunnel was 1.2 mm. This was due to the new right tunnel face being excavated to the edge of the existing large-diameter shield tunnel by 12 m.  
Stage II: Forepoling reinforcement  
During this phase, the existing tunnel experienced approximately 2.1 mm of uplift due to the construction aid of overrunning deep hole grouting, which was caused by the use of deep hole grouting technology in sand, and the pebble strata could effectively squeeze  
Figure 8: The on-site installation and layout. (**a**) Diagram of reinforcement gauge installation. (**b**) Installation diagram of the earth pressure box. (**c**) Fiber optic cable box. (**d**) Appearance of part.  
the soil and water body, enhance the strength and stiffness of the soil, and form a certain strength arch shell in the grouting area to reduce the ground deformation caused by tunnel excavation. It could also achieve the effect of grouting jacking to reserve a certain space for the deformation and settlement caused by subsequent excavation. The design's grouting pressure was 0.2 to 0.8 MPa, and the actual grouting pressure was 1.4 MPa.  
Stage III: Right line underpass construction  
The settlement of the existing tunnel was approximately 3.4 mm at this stage. The excavation and unloading of the new tunnel resulted in a significant increase in the settlement of the existing large-diameter tunnel during tunnel construction. To reduce the deformation caused by the unloading of the strata, the excavation area could be reduced to minimize the disturbance of the strata.  
Stage IV: Deep hole grouting in the left line and post-wall grouting in the right line  
In the fourth stage, a bulge of approximately 1.2 mm appeared in the existing tunnel. According to the previous construction progress, it is known that at this time, the left line tunnel is undergoing deep hole grouting of the soil ahead as well as backfill grouting (pressurized grouting) behind the right line tunnel's lining to reduce the deformation caused by the unloading of the ground after the excavation of the right line tunnel.  
Stage V: Left line under crossing construction  
During this phase, the existing tunnel settlement was approximately 1.7 mm. In the excavation phase of the upper steps of the new left line tunnel, the excavation of the soil below the existing tunnel led to certain unloading below, which in turn increased the displacement of the existing large-diameter tunnel in the direction of the new tunnel.  
Stage VI: Grouting and stabilization behind the left line wall  
When the newly built left line tunnel's lower steps led down across the existing tunnel, the new tunnel excavation on the existing large-diameter tunnel deformation was also not significant. The second lining of the right tunnel increased the stiffness of the support structure, so the deformation gradually tended to be a stable state. The new left tunnel was then grouted behind the wall, and the existing tunnel bulged slightly.