## 4 Analysis of Deformation Characteristics of Loess Tunnel
### Spatiotemporal Deformation Characteristics of Each Monitoring Area in a Short Time
#### 4.1.1 The Spatiotemporal Evolution Law of Deformation of Initial Support in the Scanning Section  
The dynamic tracking scanning section for the initial support deformation in this test is the section from YK280+195 to YK280+214 at the entrance of the right line of the tunnel, with a total length of 19 m. Comparing the results of the second to fifth scans with the result of the first scan, the deformation value of the initial support of the tunnel within 24 h of the test section and the evolution process of the deformation with time is  
Figure 9: Three-dimensional laser scanning and data processing flow. obtained. The deformation values in the range of 4 m on the left and right of the vault are selected for analysis, and then the deformation contour map of the tunnel vault is obtained. Finally, a three-dimensional deformation map reflecting the deformation is produced by the surfer software (Figure 10). The area delineated by the dotted line in the figure is the range corresponding to the construction step, and the deformation of each construction step range can be visually compared. It can be analyzed from the figure that the deformation law of the tunnel is distributed asymmetrically from the same mileage section. The deformation value on the left side is generally larger than that on the right side, which is consistent with the fact that the left side of the actual construction sequence is excavated earlier than the right side. The existence of such distortion and deformation poses certain risks to the tunnel's safety. In actual engineering, it can be considered to reverse the left and right construction steps of the lower steps to balance the deformation amount. In addition, while maintaining the same construction sequence, the site can also consider using supporting means such as strengthening the strength of the locking foot bolt of the left step to adjust the deformation amount to ensure that the overall deformation of the tunnel presents asymmetrical distribution law. Figure 10: _Cont._  
Judging from the total deformation of different construction parts in 24 h, the deformation value near the tunnel face is the largest, and the maximum value reaches 130 mm. The excavation and monitoring of the upper steps are the key parts in the monitoring and measurement of large deformation of the tunnel. It can also be analyzed that the maximum deformation of the arch around the tunnel face is on the right side, not at the top of the arch, so it is necessary to strengthen the monitoring of this area. From the perspective of the overall deformation distribution of the tunnel, the range of the step phenomenon in the deformation change basically corresponds to the range of the actual construction step sequence. The construction process at this time can be judged by observing the deformation changes of each step range in the three-dimensional deformation map. From the monitoring results, the deformation around the tunnel face is the largest, followed by the left-middle step. It can be inferred that the excavation and support of the upper step and the left-middle step are being carried out in the tunnel during the monitoring period, which is consistent with the actual situation. Figure 10: Isoline and 3D deformation map of tunnel deformation (Monitoring time: (**a**). 2 h; (**b**). 4 h; (**c**). 20 h; (**d**). 24 h). 4.1.2. The Key Area of Tunnel Deformation Monitoring--The Spatiotemporal Evolution Law of the Initial Support Deformation of the Tunnel Face  
From the above analysis, it can be concluded that the deformation rate around the tunnel face is the largest during the entire excavation process, which is the key area in the monitoring process. Figure 11 is a three-dimensional subsidence diagram of the periphery of the tunnel face.