## 3 Field Monitoring of Shallow Four-Track HSR Tunnels
### Analysis of Field Monitoring Data  
The distribution of rock pressure in the XBS Tunnel is illustrated in Figure 9, using sections DK215+105 and DK215+142 as representative examples. Based on the rock pressure curves and deformation profiles, the rock pressure and rock mass deformation under different burial depths and rock mass grades were extracted and summarized in Table 2.  
Field monitoring data from the shallow-buried four-track HSR tunnel indicate that the measured rock pressures were significantly lower than those calculated using the specifications outlined in the Chinese Tunnel Code [16], [PERSON]'s theory [20], and the full overburden weight method. Specifically, for Grade V rock, the measured pressures accounted for only 30.92% to 42.00% of the theoretical values derived from empirical formulas; for Grade IV, the values ranged from 54.21% to 61.73%; and for Grade III, they reached 85.18% to 90.48%. Additionally, the maximum vault settlement and horizontal convergence measured during monitoring were 49.5 mm and 28.9 mm, respectively--well below the design allowances of 150 mm and 200 mm. Similarly, the internal forces in the support system--including the concrete lining, steel arches, and anchor bolts--remained within acceptable limits. No excessive deformation or structural damage was observed in either the rock mass or the supporting elements. Furthermore, no signs of loosening or collapse were detected in either shallow or deep rock zones of the XBS Tunnel. The tunnel primarily passes through tuff, a relatively intact, water-free, and geologically stable rock type. The adopted support scheme--consisting of \"advance support + dual-layer primary support + secondary lining\"--was effective in enhancing the self-bearing capacity of the rock and promoting cooperative deformation between the rock mass and support structure. As of this writing, the tunnel remains in a stable and safe operational state. Based  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Burial** & **Rock** & **Sample** & \begin{tabular}{c} **Field** \\ **Monitoring** \\ **(kPa)** \\ \end{tabular} & \begin{tabular}{c} **Code-Based** \\ **Values (kPa)** \\ \end{tabular} & \begin{tabular}{c} **Protodyakonov’s** \\ **Theory (kPa)** \\ \end{tabular} & \begin{tabular}{c} **Full** \\ **Overburden** \\ **(kPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Settlement** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Convergence** \\ **(mm)** \\ \end{tabular} \\ \hline \(H\leq 1.0\) & V & 5 & 48.35–131.26 & 115.12–328.16 & - & 123.34–420.21 & 39.7 & 18.8 \\ \(B\) & IV & 13 & 110.01–196.16 & 189.09–339.73 & - & 211.06–440.26 & 49.5 & 28.9 \\ \hline \multirow{3}{*}{1.0–1.5 \(B\)} & V & 5 & 126.60–189.01 & 409.40–455.74 & - & 556.92–705.28 & 47.8 & 22.4 \\  & IV & 5 & 126.30–157.65 & 230.43–257.93 & 328.60–367.81 & - & 15.4 & 7.7 \\  & III & 6 & 116.19–124.72 & 132.12–140.23 & 184.93–196.29 & - & 18.1 & 10.1 \\ \hline \multirow{3}{*}{1.5–2.5 \(B\)} & IV & 8 & 124.24–156.92 & 229.19–254.21 & 326.83–360.74 & - & 24.6 & 16.2 \\  & III & 14 & 110.96–126.32 & 130.26–139.61 & 182.33–195.42 & - & 22.4 & 12.7 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of measured data.  
Figure 9: Rock pressure curves: (**a**) DK215+105; (**b**) DK215+142.  
on the rock pressure mechanism, field monitoring data, and operational performance, it can be concluded that the XBS Tunnel experienced a genuine pressure state characterized by cooperative deformation and shared load-bearing between the rock and the support system during the construction phase.