## 7 Discussion  
In tunnel construction, the surrounding rock experiences three stages of stress changes. First, the surrounding rock has initial stress before tunnel excavation. For loess tunnel, the main initial stress is self-weight. At the second stage, the free surface formed during the tunnel excavation releases the stress and deformation of surrounding rock. The stress of the surrounding rock near the tunnel face is changed and redistributed. At the third stage, after the construction of the tunnel support structure, the support structure restrains the excavation surface, which restricts the deformation and stress release of the surrounding rock, and the surrounding rock stress near the tunnel is once again redistributed. The calculation method for the surrounding rock pressure via [PERSON]'s theory, [PERSON]'s theory, SDHT, and TCRTL apply to the second stage of surrounding rock. It was thought that a certain range of surrounding rock near the free surface forms a pressure arching after stress release, and the surrounding rock under the arch has a loose pressure. The loose pressure in this range could therefore be calculated as the surrounding rock pressure, such that the surrounding rock pressure would not change with cover depth.  
However, the conclusions from statistical analysis were the opposite. This was because the statistical analysis was based on the test data of the tunnel in the third stage. These data were the result of interaction between surrounding rock and supporting structure. In shallow tunnels studied, the surrounding rock pressure was dominated by loose pressure. Loose pressure was dispersed, which showed that the influence of cover depth on surrounding rock pressure was not significant in statistics. However, with the increase of tunnel depth, the initial stress increased gradually. After tunnel excavation and construction of support structure, the surrounding rock around the tunnel would produce limited deformation, which should not be regarded as loose pressure, but as deformation pressure.  
Additionally, the deformation pressure was related to initial stress and support structure parameters. When the cover depth was less than 500 m, the initial stress was mainly gravity stress, which was related to the tunnel depth [33]. Therefore, the surrounding rock pressure increased with the increase of cover depth. At the same time, the parameters of support structure determined the release degree of initial stress, which had a great influence on the surrounding rock pressure. In the follow-up study, the influence of support structure parameters on the surrounding rock pressure should be especially studied. As the sample and factor levels of statistical analysis data were not abundant, the results of this study are only applicable to similar tunnel engineering. The tunnel span was within 15 m and the tunnel depth was within 100 m. The amount of data and factor levels should be increased  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Methods for Cover Depth** & **Cover Depth** & **Methods of Pressure** & **On-Site** \\
**Depth Threshold** & **Threshold/m** & **Pressure** & **/kPa** & **Test/kPa** \\ \hline [PERSON] & 29.7 & [PERSON]’s theory & 226.8 & \\ [PERSON] & 47.3 & [PERSON]’s theory & 398.8 & \\ SDHT & 37.2 & SDHT & 221.6 & \\ TCRTL & 52.2 & [PERSON]’s theory & 424.8 & \\ Statistical analysis & 33.3 & Regression analysis & 391.68 & \\ \hline \hline \end{tabular}
\end{table}
Table 7: Contrast of different methods for vertical pressure of surrounding rock in Xinzhuangling Tunnel.  
in later studies to obtain a more accurate variation law of surrounding rock pressure with cover depth.