## 3 Research Methods
### Field Data Monitoring Methods  
To monitor the surface settlement changes during shield tunnel excavation in real time, the research team strategically placed monitoring points along both sides of the railroad (as shown in Figure 3). The placement of these points took into account critical factors, such as the orientation of the railroad line, the depth of the tunnel, and the extent of soil reinforcement, to ensure the representativeness and reliability of the monitoring data. Analysis of the collected data will further validate the effectiveness of the grouting reinforcement through flower tubes and provide a scientific basis for subsequent construction.  
The core of the surface settlement monitoring method lies in the construction of a dynamic monitoring system with hierarchical deployment and control, focusing on covering the scope of the influence of shield construction disturbance. Adopting a fully automatic detection system, the monitoring scope extends 30 m outward from the tunnel axis as the center, in which high-density points (3 m spacing) are used within 6 m of the outer wall of the tunnel structure, and measurement points are set up in the peripheral sections at a spacing of 5 m, forming a gradient monitoring network from dense to sparse. The monitoring object covers the rail surface, contact network pillars, and other key facilities; the rail surface measurement points are marked with red paint and numbered uniformly, and the contact network pillars are implemented with refined monitoring (3 measurement points per pillar), as shown in Figure 4. The monitoring frequency is dynamically adjusted  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **Density/(kg/m3)** & **Modulus of Elasticity/MPa** & **Poisson’s Ratio** \\ \hline \multirow{4}{*}{Grouting materials Shield} & Tube sheet & 2500 & 3.45 \(\times\) 10\({}^{4}\) & 0.3 \\  & pre-hardening & 2000 & 1.20 & 0.3 \\  & post-hardening & 2000 & 6.00 & 0.4 \\  & & 7850 & 2 \(\times\) 10\({}^{5}\) & 0.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical parameters of reinforcement materials.  
according to the construction stage: the conventional frequency is adopted before crossing (1 time/30 min), the peak frequency is encrypted to the peak frequency (1 time/10 min) during the period of crossing, and the attenuation adjustment period is experienced after the crossing (1 time/30 min is maintained for 1 week, and then it is changed to 1 time/h), which forms a three-level response mechanism of 'prevention-warning-tracking'. In the process of implementation, it is necessary to establish a coordinated monitoring mechanism with the operator of the existing line, covering the whole process of construction disturbance before the 30 m impact zone to the stability period of 3-6 months after the work, through the automated monitoring system to continuously collect the tunnel settlement, convergence, and track surface deformation data until all the monitoring indicators converge to the norm-allowed thresholds. The system realizes the visual control of settlement risk in the whole cycle of shield construction by means of spatial hierarchical deployment and control, time-frequency dynamic adjustment, and multi-source data fusion.