## 4 Analysis of Deformation Characteristics of Loess Tunnel
### Spatiotemporal Deformation Characteristics of Each Monitoring Area in a Short Time
#### 4.1.3 Spatiotemporal Deformation Characteristics of Representative Points of the Cross-Section  
Three-dimensional laser scanning obtains the point cloud data of the whole section. Still, the deformation changes of some representative points, such as vaults and spandrels, are often concerned in the deformation analysis process. These points can reflect the dynamic information of the stress and deformation of the surrounding rock and supporting structure to monitor the stable state and deformation characteristics of the cavern and provide a basis for analyzing the reliability of the surrounding rock and supporting system. Therefore, on the basis of the above research, the spatiotemporal deformation characteristics of the representative points of each section within a certain mileage of the tunnel face are analyzed.  
1. Deformation of the vault  
Figure 14 show the change of the vault deformation along the longitudinal direction of the tunnel within 24 h of the monitoring mileage. It can be analyzed from the figure that the deformation of the tunnel vault within 24 h decreases with the increase of the distance from the face, and the deformation near the tunnel face changes most significantly, which is the key area in construction monitoring. After a certain distance from the tunnel face, the difference in the deformation change in each monitoring is relatively stable, indicating that the deformation is slowing down. Compared with the results of 3D laser scanning, there  
Figure 13: Evolution diagram of deformation amount at each representative point.  
are fewer monitoring measuring points arranged when monitoring the total deformation of the vault in 24 h by the conventional method. According to the routine monitoring results, the change of the vault deformation is easy to be misjudged as the law of continuous linear increases with the advancing direction of the tunnel. However, the 3D laser scanning results show that the vault deformation actually presents a step-like decline. Such misjudgments will increase construction risks.  
(2) Deformation of left and right spandrels of the tunnel  
Figures 15 and 16 reflect the variation of the subsidence of the left and right spandrels along the longitudinal direction of the tunnel within 24 h of the monitoring mileage. The deformation of these two positions is related to the convergence of the tunnel circumference. It can be seen from the figure that the deformation law of the left and right spandrels is similar to that of the vault. The deformation of the right spandrel around the face is larger than that of the left spandrel, and there is distortion. The three-stage descending of the deformation of these representative points is also confirmed by the three-stage excavation method of the tunnel. Compared with the total deformation of the spandrels around 24 h monitored by the conventional method, the 3D laser scanning has more measuring points and more monitoring sections, which can better reflect the overall deformation trend of the left and right spandrels.  
Figure 14: Longitudinal distribution of total deformation of the vault at right line entrance in 24 h.  
Figure 15: Longitudinal distribution of total deformation of the left spandrel at right line entrance in 24 h.