## 4 On-Site Test Results and Analysis
### Reinforcement Stress of the Secondary Lining  
The variation regulation of the reinforcement stress is consistent with that of the concrete strain as for the secondary lining in the open-buried double-arch tunnel. The reinforcement stress of the secondary lining is shown in Figure 9, in which the positive value indicates tension stress and the negative value refers to compressive stress. The initial data derive from the test data when the component is installed. 1. The mechanical properties of the reinforcement in the secondary lining are measured throughout the process of concrete casting and maintenance. After the secondary lining concrete is cast in the left and right caverns of the tunnel, the secondary lining forms a temperature gradient from the inside to the outside due to a large amount of heat released from the hydration heat reaction of the concrete. When the concrete cools down and shrinks from a high temperature, this results in tensile stress (temperature stress) [36], causing a significant increase in the tensile stress in the secondary lining. When the maintenance of the secondary lining concrete is completed, it presents a dominated compressive stress. 2. Compared with the left cavern, the distribution of stress in the right cavern is more discrete, and a great difference is observed among different measurement points. Figure 9: Steel stress of the secondary lining versus time. (**a**) Left cavern; (**b**) right cavern. The left shoulder of the right cavern is dominated by tensile stress. The supporting effect of the mid-partition wall leads to a relatively concentrated reinforcement stress at the location of the arch waist near the side of the mid-partition wall in the left and right caverns. If the mid-partition wall is taken as a central axis, the value of the reinforcement stress in the left and right caverns can be sorted as follows: vault > outer arch foot > inner arch foot > outer arch waist > inner arch waist. The reinforcement stress at the arch waist of the right cavern is smaller than that in left cavern, while the reinforcement stress at other measurement points of the right cavern is larger than those of the left cavern. Due to the supporting effect of the deflecting retaining wall and the mid-partition wall, the topographic bias will mainly affect the secondary lining of the buried cavern, which effectively controls the bias instability in the open-buried double-arch tunnel. 3. Soil backfill presents different effects on the forces of the secondary lining of the left and right caverns. After soil backfill, the surrounding rock pressure above the left cavern increases, which causes the pressure at the V, LF, and RF of the left cavern to increase, especially for the points at the V. The pressure at the LW and the RW of the left cavern shows a decreasing trend. After soil backfill, the vault is firstly subjected to the surrounding rock pressure, which will be transferred to the lower part of the structure (i.e., the arch foot). The inner arch effect produced the waist of the cavern due to a supporting effect from the deflecting retaining wall and the mid-partition wall. Unlike the left cavern, the pressure of the right cavern decreases briefly, and then increases continuously at all measurement points after soil backfill (the soil backfill is above the left cavern), especially for the points at V and RF. The deflecting loading of slope is balanced to some extent after soil backfill above left cavern. A stress adjustment occurs in surrounding rock of right cavern near the right side of slope, and the direction and the size of secondary lining of right cavern change at the same time. When the secondary lining is located at the surrounding rock of right arch foot, the force at the RF and V is most concentrated.