*1.2. Research Background*

The traditional detection methods for the flatness of the initial support of the tunnel mainly include the 2 m leaning ruler combined with the tapered feeler gauge detection method and the total station detection method. Due to the randomness of the selection of detection points, the detection accuracy of this method is not high, the detection speed is slow, and the efficiency is low. When the total station is used to detect the flatness of the initial support surface of the tunnel, the randomness of the selection of detection points and the influence of the prism-free mode on the measurement accuracy will cause the fitted plane to deviate too much from the actual curved surface, which makes it impossible to obtain accurate tunnel flatness. Moreover, the selection of measurement points on the tunnel surface is subjective, so that it cannot comprehensively and truly reflect the condition of the initial support surface of the tunnel [7]. Compared with the traditional methods that have low efficiency, low accuracy, and inconvenient operation, the use of three-dimensional laser scanning technology to detect the surface flatness of the initial support of the tunnel can quickly and accurately obtain the data of the point cloud of the initial support of the tunnel. The operation is simple and does not need to touch the detected surface of the initial support of the tunnel. In addition, the accuracy of the instrument is high enough to meet the specification requirements.

At present, domestic and foreign scholars mainly apply 3D laser technology in the fields of tunnel monitoring and measurement, tunnel overall deformation analysis, etc. The application of 3D laser technology in the flatness detection of tunnel engineering is rarely studied. Duan [8] and others applied 3DLS technology to tunnel monitoring and measurement, and pointed out that when traditional detection methods are used, they are greatly affected by construction, points are easily destroyed, and data is manually recorded and inconvenient for long-term storage. Laser scanning technology has fast measurement speed and no dead ends, which effectively makes up for the shortcomings of traditional monitoring technology. Zhao [9] et al. proposed a dimensionality reduction grid deformation analysis method based on tunnel point cloud data, which can determine the tunnel deformation area and the magnitude of the deformation. Li [10] and others introduced 3DLS technology to collect 3D data of the full section of the tunnel, and qualitatively analyzed the overall deformation through chromatographic analysis, to obtain the deformation of each part of the tunnel more intuitively and automatically. Zhang [11] and others believed that the traditional method of tunnel convergence monitoring has obvious limitations and disadvantages, and clarified that the application of 3DLS technology to tunnel convergence monitoring has better advantages. The comparison with traditional methods shows the feasibility of its method. Weixing [12] and others believe that ground laser scanning technology has a huge development prospect in tunnel engineering, and clarified the advantages of ground laser scanning technology. Jong-SukYoon [13] et al. introduced a method for extracting features of tunnel concrete lining based on 3DLS technology, which provides a theoretical basis for structural health and safety inspection of tunnel construction. Manlin Xiao [14] and others introduced a tunnel surface smoothing algorithm based on mechanics correction, which can be used for 3D point cloud data collected by a 3D laser scanner. Using this algorithm, it is possible to detect and locate the damaged or deteriorated part of the inner wall of the tunnel based on the 3D laser point cloud data, effectively avoiding safety problems. Farahani [15] et al. proposed a three-dimensional laser scanning system, which can effectively obtain the contour of the 3D tunnel model, and set the deformation monitoring of the tunnel through a 3D digital image correlation system suitable for the tunnel structure. Peji´c [16] proposed an optimal scheme for effectively measuring the geometry of the tunnel surface through 3DLS technology. This scheme can reliably inspect railway tunnels and achieve the purpose of optimizing the railway tunnel monitoring and measurement system. Fekete [17] and others applied the 3DLS system to the drilling and blasting tunnel operation of railway tunnel projects, which has more advantages than traditional detection methods. At present, there is relatively little research on tunnel flatness detection methods based on 3DLS technology. Therefore,

research on tunnel flatness detection methods based on 3DLS technology can provide new technical ideas and methods for tunnel flatness detection.

The calculation method and analysis of the flatness are the keys to how to establish the method for detecting the flatness of the initial support surface of the tunnel based on the three-dimensional laser scanning technology. Many scholars at home and abroad have applied the 3DLS technology to flatness detection. Cheng [18] and others believe that in the engineering survey of the building facade, an electronic total station without cooperation target can be used to fit a plane and calculate the distance from the point to the fitting surface so that the fluctuation of the observation point can be observed. The overall situation can represent the flatness of the building facade. This method is feasible in the measurement of the building facade and meets the accuracy requirements. Li [19] obtained the point cloud data of the building wall according to 3DLS, obtained the accurate wall plane equation and the distance from each point to the fitting plane, and finally calculated and analyzed the wall flatness based on these distances. Based on these values, a distance statistical histogram and a distribution map of the leveling of the wall were made, and the distances were given different colors according to the threshold, which intuitively reflects the leveling of the wall. Li [20] et al. applied the three-dimensional laser scanning technology to the flatness detection of building concrete components, and also developed a color-coded deviation map to indicate the flatness of the components. At the same time, by scanning two different types of concrete components, comparative analysis shows that this method is feasible. Bosché et al. [21] proposed a new method for characterizing the flatness of building surfaces, which relies on the combination of ground laser scanning and continuous wavelet transform. It can provide accurate and extremely dense measurements on the surface of the building, while also providing a method for frequency analysis with high resolution in the spatial and frequency domains. Tang [22] et al. proposed three practical methods for evaluating the flatness of building surfaces through three-dimensional laser scanning technology. Kim [23] et al. proposed a method to detect the surface features and flatness of precast concrete components using building information modeling (BIM) and three-dimensional laser scanning technology. Based on this, a framework for evaluating the surface characteristics and flatness of concrete components is established. Fuchs [24] discussed the feasibility of introducing three-dimensional laser scanning technology into the inspection system of highway bridges, which also includes the inspection of the road smoothness of highway bridges. The research on flatness detection methods based on three-dimensional laser scanning technology is relatively complete. These studies also provide a theoretical basis and ideas for the establishment of the flatness detection system method of the primary surface of tunnel engineering.
