## 4 Discussion  
Based on the comparative study of numerical simulation and weak reflectivity fiber grating field monitoring, the evolution law and distribution characteristics of vertical and horizontal settlement of shield tunneling composite strata are comprehensively analyzed. On this basis, according to the influence area of surrounding buildings (structures), targeted control measures are put forward. From the longitudinal perspective, the temporal and spatial evolution law of layered settlement of composite stratum with shield tunneling can be divided into three stages according to different stages of shield tunneling. (1) Shield approaching stage: The angle between the maximum principal stress axis and the x-axis remains basically unchanged. The angle between the maximum principal stress axis and the y and z-axes continues to increase, but the amplitude is small and the frequency is slow, the direction of the maximum principal stress is mainly rotated along the vertical plane parallel to the tunnel axis. A small settlement is caused by the influence of shield thrust, ranging from the cutter head (\(-5\)D\(\sim\)0); (2) Shield crossing stage: Affected by factors such as excavation gap and continuous disturbance, the maximum principal stress increases sharply, but the angle with the y-axis is basically unchanged, the minimum principal stress continues to increase, and the direction also gradually changes, the stratum settlement gradually increases, and the range is from the cutterhead (0-1D); (3) Shield tail out stage: The direction of the minimum principal stress changes obviously. The angle between the minimum principal stress and the x and y-axes increases continuously, and the angle between the minimum principal stress and the z axis increases slowly. It mainly rotates in the horizontal plane along the tunnel axis, rapid increase in formation settlement, later settlement rate gradually decreased and stabilized, mainly by the creep of soil or compression consolidation, range from the cutterhead (1D-5D). In addition, the longitudinal main disturbance area of layered settlement is about 3D range after shield crossing and shield tail stripping [53; 54; 55; 56], as shown in Figure 11.  
From the horizontal perspective, according to the influence degree of shield tunneling disturbance, the composite stratum above the tunnel can be roughly divided into the main disturbance layer and the secondary disturbance layer. Among them, the main disturbance layer is located in the range of 0.5D above the tunnel, and its settlement accounts for about 80% of the total settlement of the composite strata, as shown in Figure 12.  
Figure 11: Vertical disturbance area division diagram. Based on the vertical and horizontal zoning (layers) of existing structures such as buildings (structures) at different stages of shield tunneling, the following layered settlement control measures and suggestions for composite strata are proposed:  
1. For the existing buildings (structures) located in the main disturbance layer and the main disturbance area, the above research shows that the soil unloading in this area is obvious in the shield approaching stage. [PERSON] and [PERSON] [57] through the field measurement and finite element simulation of the shield construction of the San Francisco tunnel in the United States, it is found that properly increasing the cutterhead pressure to make the ground in front of the excavation face slightly uplift in advance can reduce the total ground loss during the construction period, and then better control the settlement during the construction process. This experience is confirmed by most projects. Therefore, the support pressure of excavation face should be appropriately increased to balance the overload effect and reduce the settlement caused by unloading of excavation face before crossing; During the shield crossing stage, the shield tunneling speed should be kept stable to reduce the adverse effects on the surrounding strata and existing buildings.