## 4 On-Site Test Results and Analysis  
### Steel Arch Strain  
Figure 5 shows the strain-time distribution of the steel arch in the monitoring section of the tunnel, in which tension strain is represented by a positive value, and compression strain is marked by a negative value. The strain is assumed as zero when the upper steel arch is erected along the upper step. Combined with the arrangement of the measurement points in the tunnel section (see Table 3 and Figure 3) and the division of the construction cases (see Table 4), the variation of the strain at the steel arch is analyzed corresponding to different construction stages, as follows. 1. Construction stage I. The up-step of the left tunnel is located above the original hillside surface. Consequently, after the construction of the deflecting retaining wall, the primary lining at the up-step of the left tunnel is constructed (as shown in Figure 2). It is seen that after the completion of the primary lining, the strain at the steel arch shows a rapid growth trend at the early stage due to the effect of the concrete, formwork, and its own gravity. The strain development at each point of the steel arch gradually flattens out with time. It is worth noting that the strain at the remaining measurement point continues to increase while the strain at the LW and the LS of the steel arch decreases because the right side of the steel arch that \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Construction Stage** & **Construction Case** & **Specific Construction** & **Construction Time** \\ \hline \multirow{3}{*}{I} & 1 & Excavation and section of up-step in the left cavern & 24 Marh \\  & 2 & Excavation and section of mid-step in the left cavern & 19 April \\  & 3 & Construction of the invert in the left cavern & 6 May \\  & 4 & Excavation and section of up-step in the right cavern & 30 April \\  & 5 & Construction of the invert in the right cavern & 30 May \\  & 6 & Construction of the secondary lining in the left cavern & 21 July \\  & 7 & Construction of the secondary lining in the right cavern & 10 August \\  & 8 & The beginning of soil backfills & 26 August \\  & 9 & The ending of soil backfills & 5 October \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Main construction cases and construction stages. Figure 5: Strain distribution of the steel arch versus time. (**a**) Left cavern; (**b**) right cavern. overlaps with the mid-partition wall shares part of the right side of the surrounding rock pressure. However, when the mid-step of the left cavern is excavated, the strain at the LW and the LS increases simultaneously, and the increasing trend is smaller than at other positions. This indicates that the soil displacement of the middle-lower parts of both sides of the left cavern are caused by the excavation of the mid-step, driving a stress adjustment of the surrounding rock and squeezing the steel arch to a certain extent. Under the deflecting effect of right hillside body, the mid-partition wall on the right side shows a tendency to deflect to the left cavern after the removal of surrounding rock at the left side. Consequently, the pulling and squeezing effect at the steel arch on the same side is more obvious. During the construction of the lower step of the cavern, the above phenomenon becomes more obvious, and the strain growth increases significantly. It is seen that after the construction of the open cavern by the concealed method, the primary lining plays a certain role in the supporting action, especially for the hillside body on the mid-partition wall. In summary, regarding the construction of the open-buried double-arch tunnel, although the influence of the traditional (buried tunnel) up-step excavation on the tunnel supporting system and surrounding rock can be ignored, the soil excavation of the middle-lower part of the open cavern, which is equivalent to the unloading of the slope angle, has induced a stress adjustment of the rock around the left cavern and an attitude change of the mid-partition wall.