## 5 Stability Evaluation for Existing Line Foundation during the Construction Period
### Track-Inspection Car Verification  
The current assessment of the condition of an operating railway in China was mainly based on a combination of the condition of the section track quality and local unevenness management, a scoring method for obtaining a deduction value per kilometer, and the standard deviation method for obtaining the track quality index (TQI) value for different  
Figure 12: Lateral stress distribution curve.  
sections. Both methods required a track-inspection vehicle for their implementation. The scoring method is simpler but does not reflect the rate of change in track unevenness, the length of overruns, or periodic continuous unevenness. The standard deviation algorithm usually determines the TQI to be 200 m for a section, taking into account the gauge, height, level, alignment, and track twist; on seven items in the standard deviation calculation, the TQI exceeded the limit value that should trigger maintenance and repair. For the Beijing-Shanghai Railway, it was determined that a TQI greater than 15 is over the limit, and the system must be maintained and repaired if this is exceeded, as shown in Table 2.  
The foundation excavation and piling stage of the project was completed in July, and the roadbed filling was carried out in August. Therefore, this period was selected to test the location of the section (Beijing-Shanghai K1243 + 100) of existing railway waveform data for analysis. The analysis used the scoring method and standard deviation method, and the results of the scoring method are shown in Figure 13, indicating that the deduction of points during the construction phase (July and August) did not differ from that in other normal periods.  
The TQI value obtained by the standard deviation method is shown in Figure 14. It can be seen that the TQI value increased significantly after the beginning of August, from 4.26 on 22 May to 9.78 on 22 August, an increase of 129.58%, indicating the obvious influence of construction on the existing lines. However, the standard management value was not exceeded, and maintenance and repair were not required, so the monitoring data were used as the basis for the existing line safety analysis.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Project** & **High** & **Alignment** & **Gauge** & **Level** & **Track Twist** & **TQI** \\ \hline \multirow{2}{*}{Management value} & V\({}_{\text{max}}\) \textless{} 160 km/h & \(2.5\times 2\) & \(2.2\times 2\) & \(1.6\) & \(1.9\) & \(2.1\) & \(15.0\) \\ \cline{2-7}  & V\({}_{\text{max}}\) \textgreater{} 160 km/h & \(1.5\times 2\) & \(1.6\times 2\) & \(1.1\) & \(1.3\) & \(1.4\) & \(10.0\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Railway management standard.  
Figure 13: Track geometrical transfinite single distribution per kilometer.  
From the inspection data of the track-inspection car, the linear change curve of the track during the construction period (April-August) was obtained. According to the orbit quality state TCI and local irregularity data, it was known that the existing line foundation remained in a stable state without excessive deformation during the construction period. Among these, the linear change was the most obvious in August. This period was close to the foundation excavation stage, which was mutually verified with the experimental analysis results. The existing line foundations adjacent to the foundation excavation stage are most affected and are in the most unfavorable state. Field test detection and track-inspection car detection data can complement and verify each other. The former obtains the internal deformation data of the subgrade, and the latter obtains the track deformation data of the subgrade superstructure to jointly ensure the operation safety of the existing subgrade during construction.