## 4 On-Site Test Results and Analysis
### Peripheral Displacement  
Peripheral displacement refers to the displacement of measurement points in surrounding rock within the tunnel boundary after excavation. It is difficult to accurately measure the absolute value of displacement of spatial points. Consequently, it is common to determine the relative displacement of two points along the direction of inner wall of the tunnel. The peripheral displacement of the surrounding rock of the tunnel is reflected by monitoring the relative displacement of the points along the upper and lower horizontal lines. The peripheral displacement vs. time on the left and right caverns of the tunnel is shown in Figure 7, in which the negative value indicates an inward convergence, and the positive value refers to an outward extension deformation. The positions of the upper horizontal line (UH) and the lower horizontal line (LH) are shown in Table 3 and Figure 3.  
1. The peripheral displacement around the tunnel shows an inward convergence, and then experiences an outward extension. After the construction of the up-step of the tunnel, the peripheral displacement of the left and right caverns tends to move inwards. The main reason for this phenomenon is that the mid-partition wall moves towards the shallowly buried side (left cavern) subjected to the deflection of slope. Regarding the right cavern, after the construction of the up-step of the right cavern, the mid-partition wall moves to the right because of the construction deflection pressure caused by tunneling construction between the caverns. A similar phenomenon is also discussed by [PERSON] et al. (2016) with regard to a bias-pressure double-arch tunnel, where the bias effect caused by the asymmetric construction is not considered. When the plastic zone of the surrounding rock is initially formed, the primary lining begins to work, and the relative displacement of the cavern gradually moves toward both sides.  
Figure 7: Peripheral displacement and displacement rate versus time. (**a**) Peripheral displacement; (**b**) peripheral displacement rate.  
2. The displacement development pattern of the tunnel cavern is different when subjected to different construction stages. For the first half of the month, because of a rapid stress release of the surrounding rock, the maximum peripheral displacement rate occurs at the early stage, and it moves toward the inner side. Compared with the construction of the lower step, the mid-step construction shows a greater effect on the accumulated displacement and the displacement rate at the UH. The effect of the invert construction on the accumulated periphery displacement and the displacement rate at the UH is greater than the construction of the mid-step. The periphery displacement rate of the left and right caverns converges gradually after the invert construction, which indicates that the timely construction of the invert plays an important role in controlling the displacement of the primary lining and surrounding rock.
3. The accumulated peripheral displacement around the UH is larger than the LH because the deflecting retaining wall and the mid-partition wall restrict the horizontal displacement of the lower line structure and surrounding rock. The accumulated peripheral displacement around the UH is smaller than the LH since there is no deflecting retaining wall on the right side of the right cave, which means that no restraint exists outside the right cave.