## 4 On-Site Test Results and Analysis
### Steel Arch Strain
It is necessary to adopt engineering measures (e.g., installing the deflecting retaining wall) to improve the stability of the whole tunnel and slope subjected to the open cavern construction load. 2. Construction stage II. After the excavation and erection of the primary lining at the up-step of the right cavern, the surrounding rock pressure is released within 3 days. The steel arch strain increases rapidly when it begins to bear the surrounding rock pressure. As the up-step of the right cavern continues to be excavated, the strain of the steel arch at the RW and the RS is essentially stable, while the other points continue to increase until the construction of the invert begins. The reason for the above phenomenon is that the right side of the right cavern is on a higher elevation of mountain topography, while the left side is reduced in height and linked with the mid-partition wall. The caverns are in a right-to-left skewed topography. After the excavation of the up-step and the invert in the right cavern, the surrounding rock deforms towards the left side with a stress adjustment, and the left side of the steel arch bears the surrounding rock pressure. It is seen that the location of the joint area (near the mid-partition wall) is a sensitive area during the buried-cavern construction of the open-buried double-arch tunnel. 3. Construction stage III. The strain at the steel arch increases rapidly in both caverns within 3 days after the secondary lining is applied, and reaches the maximum value during the monitoring period. After 5 days, the strain at the steel arch of the left and right caverns decreases rapidly until they are essentially stable. The reason for the above phenomenon is that contact pressure arises between the primary and the secondary linings due to gravity and the solidification of the concrete when the secondary lining is completed [33]. While the secondary-lining trolley is still acting on the secondary lining, the contact pressure between the linings causes the strain at the steel arch to increase rapidly. When the secondary-lining trolley is removed, there is a certain release of interaction force between the linings. Meanwhile, the secondary lining concrete at this time tends to be stable due to its solidification. It is worth noting that the strain of the steel arch at each measurement point of the right cavern did not increase repeatedly due to the soil excavation when the secondary lining of the left cavern was completed. The strain tends to be stable after it slightly decreases during the construction of the secondary lining of the right cavern. It is seen that the supporting strength at the open cavern side in the open-buried double-arch tunnel is effectively utilized, which is of great significance to guarantee the stability of the tunnel structure system. 4. Construction stage IV. When the secondary linings of the left and right caverns reach the design strength, the soil is back-filled on the left cavern (open cavern). It is seen that the backfill presents different effects on the strain at the steel arch in both caverns. Soil backfill increases the strain at the steel arch of the left cavern again, which is especially significant at the LW, while the strain at the steel arch of the right cavern decreases at the end of backfill. Soil backfill will increase the surrounding rock pressure around the left cavern, which indicates that it causes a transformation from an \"open\" cavern into a \"buried\" cavern. The bias pressure caused by the slope is balanced, which subsequently reduces the strain at the steel arch of the right cavern, and increases the strain at the left cavern. During the whole process, the strain at each measurement point is still within the safe range.