## 3 Monitoring Results and Analysis
### Contact Pressures between Surrounding Rocks and Primary Liniings  
The time history plots of the contact pressures between surrounding rocks and primary linings in the K89+729 section are presented in Figure 13. As shown in the figure, after the earth pressure cells had been installed in the right places, the contact pressures between surrounding rocks and primary  
Figure 11: Time history plots of the internal displacements in class V rock masses at position C in the K89+729 section. Figure 12: Final distribution of the internal displacements in three different classes of rock masses (class III, IV and V) and the values in the figure represent the internal displacements in class V surrounding rocks. Figure 10: Time history plots of the internal displacements in class V rock masses at position B in the K89+729 section. lings increased rapidly and the increasing tendency did not slow down until the completion of the primary lining construction. The pressures reached up to 90% of the final stable-state values about 5 days after the construction of primary linings. When the construction of secondary linings was finished, the contact pressures almost stopped increasing. At this time, the state of surrounding rocks could be considered stable. Figure 14 presents the final distribution of the contact pressures between surrounding rocks and primary linings in the cross sections corresponding to different classes of surrounding rocks (class III, IV and V). It can be seen from Figure 14 that at different locations in the same cross section, the pressures at the tunnel vault are higher than that at the spandrel, while the pressures at the spandrel are higher than that at the side wall. As an important bearing structure of the double-arched tunnel, the existence of the middle wall is able to effectively ease the surrounding rock pressures on the primary linings near the middle wall, resulting in the decreasing of the contact pressures on that side. In addition, the contact pressures between surrounding rocks and primary linings are greater in the cross section with lower-level surrounding rocks. For instance, the contact pressure at the tunnel vault in the cross section K89+949 (class III) is about 0.15 MPa while the pressure in the section K90+105 (class IV) is about 0.25 MPa and the pressure reaches up to 0.30 MPa in the section K89+729 (class V). Since there is no definite calculating formula concerning the surrounding rock pressures of the double-arched tunnel, the surrounding rock pressures can be estimated through [PERSON]'s theory. According to the theory, the span length and the height of the tunnel balance arch as well as the surrounding rock pressures can be calculated by those formulas as shown below:  
\[a_{0}=a+2b\tan(45^{o}-\frac{\varphi_{0}}{2}) \tag{3}\]  
\[b_{0}=\frac{a_{0}}{2f} \tag{4}\]  
Figure 14: Final distribution of the contact pressures between surrounding rocks and primary linings in the cross sections in different classes of rock masses (class III, IV and V). Figure 13: Time history plots of the contact pressures between surrounding rocks and primary linings in the K89+729 section. \[q=\gamma b_{0} \tag{5}\]  
where, \(a_{0}\) and \(b_{0}\) respectively represent the span length and the height of the tunnel balance arch, a and b respectively represent the designed span length and height of the tunnel, \(\varphi_{0}\) stands for the calculating internal friction angle of surrounding rocks, \(f\) stands for Protodyakonov's firmness coefficient of surrounding rocks, \(\gamma\) is the rock density and \(q\) is the surrounding rock pressure at the tunnel vault. \[q_{1}=\gamma\left[\frac{a+2b\tan\left(45^{\mathrm{o}}-\frac{\varphi_{0}}{2} \right)}{2f}\right]=31.5\times\left[\frac{12.39+2\times 6.5\times\tan\left(29^{ \mathrm{o}}-\frac{29^{\mathrm{o}}}{2}\right)}{2\times 1.8}\right]\approx 0.14\mathrm{MPa} \tag{6}\]  
The cross section K89+729 in class V surrounding rocks (\(\varphi_{0}=29^{\mathrm{o}}\), \(\gamma=31.5\)\(\mathrm{kN}/\mathrm{m}^{3}\), \(f=1.8\)) is taken as a case.