## 2 Field Monitoring Project
### Tunnel Structure  
The structures of the Gangkou tunnel vary a great deal in different classes of surrounding rocks. Figure 1 shows the geometric dimensions of the double-arched tunnel in different classes of rock masses. In the primary lining in the class III surrounding rocks, the grouted rock bolts which are made up of hot-rolled ribbed bar (HRB) round steel with length of 250 cm and yield strength of 33.5 MPa were fixed in surrounding rocks of main tunnels after excavating each one by spacing of 120 \(\times\) 100 cm\({}^{2}\) in longitudinal and circumferential directions, the diameter and length of which are 25 mm and 250 cm respectively. I-shape joint steel arch with weight of 16.9 kg/m and cross-sectional area of 21.5 cm\({}^{2}\) was arranged by longitudinal spacing of 250 cm and round bar with diameter of 8 mm was adopted as steel mesh by spacing of 20 \(\times\) 20 cm\({}^{2}\) in longitudinal and circumferential directions. The sprayed concrete was used with thickness of 25 cm and compressive strength of 25 MPa (C25). Moreover, the secondary lining adopted the model-building reinforced concrete with compressive strength of 35 MPa (C35) and thickness of 35 cm and the deformed steel bar with diameter of 22 mm was adopted as main reinforcement in the concrete by longitudinal spacing of 20 cm. Besides, composite waterproof layer was fixed between primary and secondary lining, with thickness of 1.5 mm. Detailed support parameters in class IV and V surrounding rocks are respectively shown in Tables 2 and 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Rock Mass** & **Cohesion** & **Internal Friction Angle** & **Elastic Modulus** & **Unit Weight** \\
**Classification** & **(MPa)** & **(\({}^{\circ}\))** & **(GPa)** & **(kN/m\({}^{3}\))** \\ \hline III & 1.3–1.4 & 36\({}^{\circ}\)–38\({}^{\circ}\) & 7–10 & 33.0–35.0 \\ IV & 0.5–0.7 & 32\({}^{\circ}\)–34\({}^{\circ}\) & 4–6 & 31.5–33.0 \\ V & 0.07–0.10 & 27\({}^{\circ}\)–29\({}^{\circ}\) & 0.8–1.9 & 27.5–31.5 \\ \hline \end{tabular}
\end{table}
Table 1: Physical and mechanical parameters of three different classes of rock masses (class III, class IV and class V respectively).  
Figure 1: Longitudinal and cross sections of the Gangkou double-arched tunnel.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline  & Rock bolt & Steel arch & Sprayed concrete & Secondary lining \\ \cline{2-5} class IV & Dameter: 2.5 cm; & No. 14 hot rolled just steel with a cross-sectional area of 21.5 cm\({}^{2}\); & Thickness of sprayed concrete: & Thickness of formwording concrete: 40 cm; \\ surrounding rocks & Longitudinal and circumferential spacing & cross-sectional area of 21.5 cm\({}^{2}\); & 20 cm. & Diameter of main reinforcement: 2.2 cm; \\  & 120, 50 cm & Longitudinal spacing: 200 cm. & Spacing of main reinforcement: 20 cm. \\  & Material: HRB 335 round steel. & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Detailed parameters of primary and secondary supports in class IV surrounding rocks.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline  & Rock bolt & Steel arch & Sprayed concrete & Secondary lining \\ \cline{2-5} class V & Dameter: 2.5 cm; & No.14 hot rolled just steel with a cross-sectional area of 21.5 cm\({}^{2}\); & Thickness of sprayed concrete: & Thickness of formwording concrete: 50 cm; \\ surrounding rocks & Longitudinal and circumferential spacing & cross-sectional area of 21.5 cm\({}^{2}\); & 20 cm. & Diameter of main reinforcement: 2.2 cm; \\  & 120, 50 cm & Longitudinal spacing: 150 cm. & 25 cm. & Spacing of main reinforcement: 20 cm. \\  & Material: HRB 335 round steel. & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Detailed parameters of primary and secondary supports in class V surrounding rocks.