## 5 Sensitivity Analysis
### Tunnel Leakage  
The long-term settlement of the soft soil and the embedded tunnel is significantly affected by the seepage conditions of the lining. According to the \"Technical Specification for Waterproofing of Underground Engineering\" (GB50108-2008), the first-level standard for tunnel waterproofing is that no water seepage is allowed, the second-level standard is that the average seepage volume of the tunnel is not more than 0.05 L/(m\({}^{2}\cdot\)d), and the fourth-level standard is the average seepage of the tunnel, which is no more than 2 L/(m\({}^{2}\cdot\)d). For urban subways tunnels, the waterproof requirement is the second-level standard. However, due to the poor posture control of the shield machine and the poor quality of the segment assembled during the construction period, the shield tunnel may experience greater water leakage, which will aggravate the long-term settlement of the ground and the tunnel. This section divided the leakage of the tunnel into full-peripheral seepage and partial seepage and discussed the influence of tunnel seepage conditions on the long-term deformation of the ground and the tunnel. Figure 12 shows the surface and tunnel settlements during the consolidation period under different tunnel seepage volumes, as shown in Table 6 in the full-peripheral seepage condition. The results indicated that the tunnel seepage volume had a significant impact on the long-term displacement of the surface and tunnel. When the waterproof level  
Figure 11: Displacement of surface and tunnel under the different thickness of the soft underlying layer. (**a**) Development of surface displacement with time; (**b**) Development of segment displacement with time. of the tunnel reaches Grade II or above, the surface and the tunnel will not undergo consolidation settlement, on the contrary, a slight uplift will occur. This can be explained as follows: the soil around the tunnel cannot undergo consolidation settlement in the case of small tunnel leakage, because the tunnel seepage volume was too small to form an effective seepage path to change the pore water pressure and effective stress state of the soil around the tunnel. In this case, the displacement of the stratum and the tunnel was mainly controlled by the unloading rebound of the soil. In addition, from the perspective of the duration of consolidation, the time required for stabilizing increased with the tunnel leakage volume increasing, and the consolidation time for the maximum seepage volume was nearly 60 days. In practice, local leakage is often more likely to occur than full-peripheral seepage. According to the requirements of \"Technical Specification for Waterproofing of Underground Engineering\" (GB50108-2008), the water leakage of a single leaking point is not more than 2.5 L/d. Figure 13 showed the settlement of the stratum and the tunnel at different seepage positions. As shown in Figure 13a, the surface consolidation settlement caused by the tunnel top leakage was the largest, and the surface settlement caused by the tunnel bottom leakage was the smallest. However, the tunnel consolidation settlement was the opposite of ground settlement. As shown in Figure 13b, the tunnel settlement caused by the tunnel bottom leakage was the largest, and the tunnel settlement caused by the tunnel top leakage was the smallest. A possible reason was the redistributed pore water pressure and effective stress caused by different tunnel seepage points were different. For the conditions of tunnel top leakage, the stress redistribution area of the soil was above the tunnel, where the largest consolidation settlement of the ground occurred. Therefore, in this case, the surface settlement was the largest. For the condition of the tunnel bottom leakage, the stress redistribution area of the soil was mainly under the tunnel, which led to a larger consolidation settlement of the soil under the tunnel and induced a greater displacement of the tunnel.