## 3 Field Monitoring of Shallow Four-Track HSR Tunnels  
### Monitoring of a Shallow Four-Track HSR Tunnel  
The XBS Tunnel of the Hangzhou-Shaoxing-Taizhou (HST) railway consists of Tunnel 1 and Tunnel 2, with lengths of 166 m and 430 m, respectively. Considering factors such as track merging near stations, the tunnel was designed as a single-bore four-track tunnel--the first of its kind in China. With a span ranging from 26.3 m to 27 m and a burial depth of 7-53 m, it represents a rare example of a super-large-span tunnel. The tunnel portal section passes through silty clay and tuff with well-developed joints and fractures, resulting in extremely fragmented rock masses classified as Grade V rock [16]. The main tunnel alignment traverses tuff formations, predominantly of Grades IV and V. Based on engineering analogy and geological conditions, a composite support system of \"advanced support + double-layer primary support + secondary lining\" was adopted for the XBS Tunnel, as illustrated in Figure 5. The construction site is shown in Figure 6. The XBS four-track HSR tunnel has been in operation since January 2022 and is currently performing well.  
Field monitoring of the XBS four-track HSR tunnel was conducted over a period of more than eight months from February to November 2019. The monitoring program included observations of rock deformation, deformation of the support structure, contact pressure between the rock and the primary support, stresses in steel arch frames, stresses in lattice girders, concrete stress within the primary support, and concrete stress in the temporary support. A selection of monitoring instruments is summarized in Table 1. The layout of monitoring points is shown in Figure 7, and the on-site monitoring setup is presented in Figure 8.  
Figure 4: Distribution of thinning defect parameters: (**a**) survey lines layout; (**b**) positional distribution of defects; (**c**) distribution of defect lengths; (**d**) average thickness of thinning zones; (**e**) minimum thickness distribution; (**f**) thinning–thickness ratio.  
Figure 5: XBS Tunnels: (**a**) support system; (**b**) geological condition.  
Figure 6: Construction site of the XBS Tunnel: (**a**) temporary support removal; (**b**) completion of secondary lining casting.  
Figure 8: On-site monitoring setup of the XBS Tunnel.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Monitoring Item** & **Instrument Type** & **Measurement Range** & **Accuracy** & **Quantity Per Section** \\ \hline Contact pressure & XYJ-2 pressure cell & 1 MPa & \textless{}0.5\% F.S. & 18 units \\ \hline Stress in steel arches & Surface strain gauge & \multirow{2}{*}{3000 \(\mu\varepsilon\)} & Compression: \textless{}0.5\% & \multirow{2}{*}{12 units} \\ of temporary support & & (steel) & F.S. Tension: \textless{}0.1\% & \\ \hline Stress in lattice & \multirow{2}{*}{XZ-B type \(\varphi\)22 rebar} & \multirow{2}{*}{200 kN} & \multirow{2}{*}{\(\pm\)0.1\% F.S.} & \multirow{2}{*}{36 units} \\ girders of primary support & & & & \\ \hline Shotcrete stress in primary support & \multirow{2}{*}{XJH-2 strain gauge} & \multirow{2}{*}{2000 \(\mu\varepsilon\)} & \multirow{2}{*}{\(\pm\)0.1\% F.S.} & \multirow{2}{*}{36 units} \\ \hline Concrete strain in temporary support & \multirow{2}{*}{XJH-2 strain gauge} & \multirow{2}{*}{2000 \(\mu\varepsilon\)} & \multirow{2}{*}{\(\pm\)0.1\% F.S.} & \multirow{2}{*}{18 units} \\ \hline Axial force of anchor & \multirow{2}{*}{Anchor bolt load cell} & \multirow{2}{*}{100 kN} & \multirow{2}{*}{\(\pm\)0.1\% F.S.} & \multirow{2}{*}{7 sets} \\ bolts & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Selected monitoring instruments used in the XBS Tunnel.  
Figure 7: Layout of monitoring points in the XBS Tunnel.