## 4 Monitoring Data Analysis
### Joint Openings of the Existing Large-Diameter Tunnel  
The opening width of the segment joints is critical for the tunnel's structural safety, and an excessive opening size might result in groundwater leaking. The opening width of the joint was measured at each longitudinal joint position in this work, as illustrated in Figure 7.  
Figure 12 depicts the variations in the longitudinal joint displacement of the existing large-diameter tunnel during the twin tunnels' advance. The monitoring cross-section can be found at the 785 th ring of the existing large-diameter shield tunnel (see Figure 6). In Figure 12, the opening displacements of the segment joints are considered positive, whereas compressive displacements are considered negative. Depending on the observations, the displacement evolution tendencies of 1, 2, 3, 4, 5, and 6 on the existing large-diameter tunnel were similar. However, the magnitudes were different. Each stage depicted in the image corresponds to the stages of construction indicated in Section 4.1 (see Figure 9). In Stage 3, the longitudinal joint displacement of the existing large-diameter tunnel rapidly increased during the right tunnel of subway Line 12's advancement. The displacement of the longitudinal joints was approximately 0.2-1.2 mm in this stage. Backfill grouting behind the lining and multiple additional grouting (the first nonpressure backfill grouting and the final four pressure grouting) were performed in Stage 4 to decrease distortion of the existing tunnel. The left line tunnel grouted and reinforced the soil beneath the existing tunnel, which could improve soil strength and stiffness, reduce displacement induced by construction, and achieve the grout jacking effect. The injection pressure was designed to be 0.5 MPa, but it was 1.2 MPa. The displacement of the longitudinal joints was significantly reduced in this stage compared with the first stage, with the maximum change in displacement of the longitudinal joints in the existing tunnel being 0.7 mm (B1-A1 longitudinal joint displacement), which may be due to the joint action of the secondary grouting and the overgrowing range. The longitudinal seam displacement grew rapidly in Stage 5 and was roughly 2.9 mm after the conclusion of the left tunnel crossing construction (28 December 2018), indicating that the existing tunnel's longitudinal seam displacement was more susceptible to the new tunnel underpass development. Figure 11: Induced hoop stress of the existing tunnel in the transverse section. (**a**) Far-field excavation; (**b**) Forepoling reinforcement; (**c**) Right line underpass construction; (**d**) Deep hole grouting in the left; line and post-wall grouting in the right line; (**e**) Left line under crossing construction; (**f**) Grouting, and stabilization behind the left line wall. In Stage 6, secondary grouting of the left line was carried out, and the secondary lining structure was gradually poured, strengthening the support structure's integrity and rigidity. As a result, the displacement of the longitudinal joints in the existing tunnel first decreased rapidly and then decreased slowly and steadily. From the above four stages experienced by the longitudinal seam displacement in the 785-ring tunnel, the settlement occurring in the first stage accounted for most of the total settlement, so this stage needs the attention of engineers. Figure 12 depicts the displacement curve of the existing tunnel's longitudinal joints in ring 776. The development process of the longitudinal seam displacement caused by the new tunnel excavation below could also be separated into several phases: right line crossing and left line crossing (stage 1), the grouting recovery phase (stage 2), the left line crossing phase (stage 3), and the restoration of stability phase (stage 4). They correspond to each construction process of the new cut-and-cover two-lane tunnel. The evolution of the longitudinal seam displacement in the 776-loop tunnel is similar to that in the 785-loop tunnel (Figure 13). We will not elaborate too much on this here.