*4.1. Analysis of Overall Flatness*

In the experiment, the point cloud data of a 2-m-long tunnel surface cross-section was taken along the direction of the central axis of the tunnel. After preprocessing the point cloud data, a total of 40,000 points are selected by random sampling, and their normal vector distance is calculated, and a 200 × 200 normal vector distance scalar matrix is formed at the same time. Then, through the coordinates of the point cloud data, there are one-to-one correspondence in the *X*-axis central axis direction and the *Y*-axis cross-sectional direction. Finally, the normal vector distance scalar matrix is colored into a graph to analyze the overall flatness of the tunnel surface, as shown in Figure 15.

**Figure 15.** Overall smoothness distribution of tunnel primary branch surface.

According to the calculation formula for the overall flatness of the surface of the initial support of the tunnel, the overall flatness of the surface of the first support of the tunnel is 19.9 mm. According to the "GB-T 50299-2018 Construction Quality Acceptance Standard for Underground Railway Engineering" [33], the flatness of the sprayed concrete is allowed the deviation should be 30 mm, and the overall flatness of the initial support surface of the tunnel meets the requirements of the specification.
