## 4 Monitoring Data Analysis
### Induced Hoop Stress of the Existing Large-Diameter Tunnel  
This section analyzes the circumferential stress changes in the existing large-diameter shield tunnel during the new tunnel's construction. The circumferential stress changes induced by the tunnel construction are shown in Figure 11. The monitoring section is located in ring 785 of the existing large-diameter shield tunnel (see Figure 7 MSC12 monitoring section). As shown in Figure 7, only the data for the eight strain gauges numbered 1, 2, 3, 4, 5, 6, 7, and 9 are shown in Figure 11 due to the failure of strain gauge number 8. The circumferential stress was equal to the pipe segment's modulus of elasticity (34.5 GPa) multiplied by the strain. In Figure 11, the tensile stress inside the tube sheet is considered positive stress, while the compressive stress is considered negative stress. The phases depicted in the diagram correspond to the building phases described below:  
1. When the length between both the new tunnel face excavation and the edge of the existing tunnel exceeded 12 m, the circumferential tension at the strain gauge position remained almost unchanged.
2. The circumferential stresses at all strain gauge locations fell when the tunnel face of the right tunnel met the edge of the existing tunnel, except for the circumferential stress at numbered location 7, which increased. According to the preceding description, the stress decrease could be attributed to the existing tunnel's secondary grouting.  
Figure 10: Subsidence profiles of the existing large-diameter shield tunnel.  
When the right tunnel crossed the existing tunnel, the circumferential stresses at strain gauges 1 and 5 decreased dramatically, and the minimum extra stress was approximately 3.5 MPa at the tunnel's L-springline, whereas the circumferential stress at strain gauge 7 increased dramatically to approximately 0.4 MPa. This was most likely due to soil disturbance induced by the excavation of the new right tunnel, which resulted in partial unloading and hence an increase in the tensile stress at the arch's bottom.
4. Subsequently, in the left line tunnel crossing the existing tunnel (Figure 11d,e), the stress at number 7 decreased and then increased, probably as a result of the combined effect of deep hole grouting and excavation unloading. The progressive increase in circumferential stress at the other strain gauge locations could be attributed to the excavation's slow upward expansion of the soil arch above the initial tunnel, resulting in a gradual increase in compressive stress. The stress at each site, however, was significantly less than the compressive and tensile strength of C50 concrete, and therefore the pipe piece was safe.
5. With the completion of the new left line tunnel crossing for some time, the deformation and stress of the original tunnel gradually recovered due to the secondary grouting of the left line and part of the recovery capacity of the soil. After the new left line tunnel crossing has been completed for some time, the displacement and strain of the existing tunnel progressively recover due to secondary grouting of the left line and part of the soil's recovery capacity.