## 3 Monitoring Results and Analysis
### Internal Displacement of the Surrounding Rock  
Tunnel excavation can cause stress redistribution of the surrounding rock. When the stress is greater than the bearing capacity of the surrounding rock, the surrounding rock mass will enter a plastic state and produce microcracks. The propagation and coalescence of microcracks will make the physical and mechanical properties of surrounding rock change obviously, such as the decrease of strength and elastic modulus, and cause obvious internal displacement, which is usually called the damage of surrounding rock. Therefore, the range of damage of surrounding rock can be obtained by monitoring the internal displacement of the surrounding rock. Figure 13 has shown the curve of the duration of the internal displacement at the crown that was monitored by the multi-point extensometers. \(\delta_{1}\)-\(\delta_{5}\) represent the relative displacement between Monitoring Points 1-5 and the orifice, respectively. The following steps were used to analyze the damage of surrounding rock: (1) Special time points were selected using the following rules: 1 the time point at which the initial displacement rate was slow (Time Point 1). 2 The time point when the displacement began to mutate (Time Point 2). 3 The critical time point when the displacement began to change slowly after mutation (Time Point 3). 4 The critical time point at which the displacement increment between two adjacent monitoring points started to change suddenly (Time Point 4). 2 The time point when the displacement of each monitoring point began to converge (Time Point 5). 6 The cut-off time of the monitoring (Time Point 6). (2) According to the displacement data that was selected from the above time points, curves of the displacement at the special time points vs. depth were drawn, as shown in Figure 14. (3) A fixed point in space was found, that is, the dense point in the Figure 14. Within the depth range corresponding to this point, the deformation of the surrounding rock was obvious, and the damage of the surrounding rock was more serious. It can be considered that the distance between this point and the orifice is the range of damage of surrounding rock. The same method was used to obtain the range of damage of \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Displacement Parameter**} & \multirow{2}{*}{**Monitoring**} & \multicolumn{6}{c}{**Construction Stage**} \\ \cline{3-8}  & & **Stage** & **Stage** & **Stage** & **Stage** & **Stage** & **Stage** \\  & & **1** & **2-3** & **4-5** & **6-7** & **8-9** & **10-11** \\ \hline \multirow{3}{*}{Settlement Proportion (\%)} & DK68+290 & 35.1 & 29.9 & 11.6 & 9.3 & 3.8 & 10.3 \\  & DK68+295 & 54.0 & 21.6 & 10.7 & 5.7 & 1.7 & 6.3 \\  & DK68+300 & 41.4 & 35.2 & 7.1 & 11.0 & 3.2 & 2.1 \\ \hline \multirow{3}{*}{Convergence Proportion (\%)} & DK68+290 & — & 42.3 & 21.7 & 19.7 & 7.7 & 8.6 \\  & DK68+295 & — & 38.6 & 17.8 & 26.8 & 8.9 & 7.9 \\ \cline{1-1}  & DK68+300 & — & 30.6 & 31.5 & 14.3 & 4.1 & 19.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Proportion of the tunnel displacement in the different construction stages. the surrounding rock at the spandrel and haunch of the tunnel. Table 6 has shown the range of damage of surrounding rock at different positions. It can be seen from Table 6, that the range of damage of surrounding rock in this project is well controlled, and the maximum value is 8.1 m at the crown. Moreover, the range of damage of surrounding rock generally displays a trend of crown > spandrel > haunch. Research into the range of damage of surrounding rock is of great significance when designing a super-span tunnel's support structure. In order to avoid the slipping of anchoring end, the design length of the anchor cables and bolts were long enough to penetrate through the range of damage of surrounding rock. The tunnel displacements were controlled within 20 mm, which indicated that the support design was successful.