## 5 Conclusions  
A large number of engineering practices have shown that large-scale deformations with complex spatial and temporal distributions often occur in loess tunnels. Based on 3D laser scanning technology, this paper reveals the refined characteristics of loess tunnel deformation. According to the monitoring analysis results, the following conclusions can be drawn:  
1. The deformation of loess tunnels has the characteristics of rapid mutation, continuous development, and complex distribution. The monitoring of 3D laser scanning technology has the characteristics of real-time, fast, and dynamic exchange, which solves the problems of less measurement point data, slow data acquisition, and incomplete monitoring data under conventional monitoring methods, which is of great significance for analyzing the deformation characteristics of loess tunnels;
2. Different from the current application form of 3D laser scanning technology, it is no longer limited to the scanning frequency of once a day and adopts the form of short-term high frequency and long-term low frequency to focus on monitoring and analyzing the refined characteristics of the overall deformation of the tunnel after each construction process. The results show that in the construction process of three-bench and seven-step excavation methods, with the different excavation processes, differential deformation occurs on the left and right sides of the tunnel, and then certain distortion occurs, which may cause potential safety hazards. In the actual project, it can be considered to adjust the construction steps on the left and right sides or strengthen the supporting means, such as strengthening the locking anchor bolt on the side with a larger deformation, to ensure the coordination of the overall deformation of the tunnel. In addition, from the perspective of the deformation distribution in the mileage near the tunnel face, the general deformation distribution of the tunnel strongly correlates with the excavation process, showing a stepped deformation law;
3. From the long-term monitoring results of the loess tunnel section, the deformation rate of the initial support is mainly related to the excavation sequence and time. With the construction of the following excavation process, the deformation rate of the tunnel will always show the characteristics of large in the early stage and small in the later stage. During the whole monitoring period, the overall trend of the tunnel deformation rate gradually decreased and finally stabilized;
4. The monitoring data analysis results further verified that the variation of the loess tunnel deformation with time conforms to the distribution law of the exponential function. This characteristic can be used in practical engineering to predict the final deformation of surrounding rock at each position and provide a certain reference for the design of subsequent deformation reserves outside the specification.  
Conceptualization, Y.W.; methodology, Y.W.; formal analysis, Z.Z. ([PERSON]); investigation, K.H.; resources, K.H.; data curation, Z.Z. ([PERSON]); writing--original draft preparation, Z.Z. ([PERSON]); writing--review and editing, S.S.; visualization, S.W.; project administration, Z.Z. (Zhizhong Zhao), and S.W.; funding acquisition, Z.Z. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was funded by the China Railway Corporation Science and Technology Research and Development Program, grant number 2018K019.  
Not applicable.  
All data are available from the author.  
The authors declare no conflict of interest.