## 2 The Types of Slide Surface and the Influence of Immersion
### Slide Surface in Shallow Buried Loess Tunnel
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Tunnel Name** & \
\begin{tabular}{c} **Buried** \\ **Depth/m** \\ \end{tabular} & **Construction Method** &
\begin{tabular}{c} **Excavation Location** \\ **When Collapse** \\ \end{tabular} & **Failure Point** \\ \hline
1 & Fengjiakai Tunnel & — & Three-steps method & Middle step & arch waist \\
2 & Erzhuangke Tunnel & — & Two-steps method & Lower steps & arch foot \\
3 & Baojiao Tunnel & — & Three-steps method & Middle step & arch waist \\
4 & Shekouao Tunnel & 15–40 & Three-steps method & Lower steps & arch foot \\
5 & Panshicha Tunnel & — & Two-steps method & Lower steps & arch foot \\
6 & Erlangcha Tunnel & 12–15 & Two-steps method & Lower steps & arch foot \\
7 & Changshanlining Tunnel & 15–22 & Two-steps method & Lower steps & arch foot \\
8 & Size Tunnel & 60 & Two-steps method & Lower steps & arch foot \\
9 & Qundong Tunnel & 7–26 & CD & Upper step of right guide & arch waist \\
10 & Nanzhuang Tunnel & — & Three-steps method & Upper steps & arch waist \\
11 & Zhaojiashan Tunnel & 34 & Three-steps method & Upper steps & arch waist \\
12 & Unreported & — & Three-steps method & Lower steps & arch foot \\
13 & Dingyuan No. 1 Tunnel & 13.5 & CRD & Upper step of left guide & arch waist \\
14 & Yangqu No.1 Tunnel & 19–21 & CRD & Upper step of left guide & arch waist \\
15 & Yuanyanghui Tunnel & 33.3 & Three-steps method & Upper steps & arch waist \\
16 & Xinzhuang Tunnel & 60 & Three-steps method & Lower steps & arch foot \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistics of construction methods and surrounding rock failure points of shallow buried loess tunnel. Figure 3: Common construction methods of loess tunnel: (**a**) The two-steps method; (**b**) The three-steps method; (**c**) The center diagram method (CD); (**d**) The center cross diagram method (CRD). In addition, the difference in the buried depth at both sides of the tunnel is referred to as stratum bias, which is closely related to the location of the slide surface. Table 3 shows the statistics of the stratum bias and the location of the slide surface of the shallow buried section of twelve loess tunnels [22]. It can be seen from Table 3 that the slide surface under the stratum bias appears on one side of the tunnel, while the slide surface under the unbiased stratum is distributed on both sides of the tunnel. Therefore, there are two possibilities for the location of the slide surface, which are unilateral distribution and bilateral distribution. The vertical joints of loess are usually well developed. The vertical crack of the Gaoqiao tunnel was surveyed with the pit exploration method, which was found to be at a value of 1.2 m [23]. The length of the vertical slide surface is much less than the tunnel buried depth, so the surface vertical slide surface can be ignored. The shape of the slide surface should still be represented by the oblique line. In conclusion, considering the influence of construction methods and stratum bias, the new types of slide surface of shallow buried loess tunnel were proposed, which were summarized into the following four categories, as shown in Figure 4. The failure points of the slide surface may occur at the arch foot and the arch waist. Therefore, the first type of slide surface is the bilateral arch foot slide surface, including slide surfaces 1 and 2 (Figure 4a). The second type of slide surface is the bilateral arch waist slide surface, including slide surfaces 3 and 4 (Figure 4b). There are two possibilities of unilateral distribution and bilateral distribution of the slide surface location under the effect of stratum bias. The third type of slide surface is the unilateral arch foot slide surface, including the No. 2 slide surface (Figure 4c). The fourth type is the unilateral arch waist slide surface, including the No. 4 slide surface (Figure 4d).