## 1 Introduction  
With the rapid development of highways and railways in various regions of the world, the number of loess tunnels has increased. More and more problems are exposed in the design, construction, operation, and maintenance of loess tunnels. Large deformation is expected, and the resulting rock collapse and structural cracking account for most of the damage [1, 2]. Loess tunnels are extremely complex special soil tunnels. Researchers have carried out a lot of scientific research on the problem of large deformation and studied their deformation law in time and space. [PERSON] [3], [PERSON] [4], and [PERSON] et al. [5] used loess tunnels to track and monitor the deformation of multiple sections of the tunnel during the excavation process. The results show that a certain degree of overall subsidence occurs in the arch after tunnel excavation. The subsidence of the tunnel result is more significant than the convergence in the horizontal direction. The study by [PERSON] et al. [6] showed that the surrounding rock of a large-section deep-buried loess tunnel conforms to the spatial distribution law of slow deformation rate of the surrounding rock at the top of the arch and faster deformation rate of the surrounding rock at the sidewall. Through on-site monitoring and numerical simulation, [PERSON] [7] and [PERSON] et al. [8] revealed the complex displacement characteristics of loess tunnels under different excavation steps during the construction process of the \"sequential excavation method\" [9]. [PERSON] [10; 11] et al. monitored and analyzed the deformation of the tunnel during the excavation process by geodetic survey, and the results showed that the uncontrolled deformation of certain \"weak\" sections propagated bidirectionally to adjacent, previously stabilized sections producing a \"chain effect\". [PERSON] et al. [12] studied the time law of the deformation of water-saturated loess tunnels. They concluded that the deformation of the surrounding rock in the early stage after the tunnel excavation develops rapidly, and it is supposed that the deformation of the surrounding rock will tend to be stable until the tunnel face leaves a distance of 35-40 m. On the basis of similar monitoring and analysis methods, [PERSON] [13] and [PERSON] et al. [14] divided the deformation of the tunnel into three stages: rapid deformation stage, continuous deformation stage, and slow growth stage. In addition, the variation characteristics of which function the surrounding rock conforms to overtime mainly focus on two aspects: logarithmic function and exponential function. It can be seen from the above studies that the characteristics and mechanisms of the large deformation of loess tunnels are extremely complex. That is not only related to the complexity of the loess tunnel itself but also related to the backward monitoring methods and insufficient data on loess tunnel deformation. Most tunnel deformation monitoring still uses a level or total station to observe specific points at intervals. This kind of method is carried out by surveyors setting up instruments in the tunnel for observation, which has the characteristics of high environmental requirements, a small number of measuring points, slow data collection, and high safety risks, and cannot accurately obtain the spatiotemporal evolution law of tunnel deformation [15]. Three-dimensional laser scanning is automatic measurement equipment and technology that has emerged in recent years. It can quickly acquire massive point cloud data with a long measurement distance and low environmental requirements. It is applied to slope stability analysis, the geometric capture of bridges, and deformation monitoring of tunnels by relevant researchers, and good monitoring results are obtained [16; 17; 18]. To adapt to different monitoring environments, different excavation methods, and different section characteristics, the current application research of 3D laser scanning mainly focuses on two aspects.