## 3 Experimental Analysis
### Displacement Analysis
#### 3.2.4. Tunnel Cross-Section Point Cloud Displacement Visualization  
The displacement analysis of tunnel point clouds provides a clear view of structural stability through a color-coded scale from 0 mm (blue) to 0.170 mm (red) (Figure 9). In the top view, most areas appear stable, with green representing low displacements (0.043-0.085 mm). Yellow and orange patches near bolds and seams suggest slight deformations (0.106-0.127 mm), while isolated red regions indicate higher displacement. Similar patterns can be observed in the bottom view, where blue and green dominate, reflecting low displacement. Yellow and orange areas align with structural seams; occasional red spots suggest localized deformation, potentially from nearby equipment; and green represents the displacement ranging from 0 to 0.021 mm. The right-set view shows consistent stability, with green areas indicating minimal displacement and minor yellow and orange patches near structural joints highlighting slight deformations. Red regions appear in the left-set view, indicating isolated higher displacement points. This analysis confirms the general stability of the tunnel, with moderate displacements focused around expected stress points. The color-coded scale efficiently identifies areas needing inspection, supporting a proactive tunnel monitoring and maintenance approach.  
Figure 8: Shield ring ovality change. Comparison between ovality of P\({}_{i}\) and P\({}_{0}\).  
The displacement analysis of the tunnel cross-section structure, visualized from both front and back perspectives, reveals spatial variations in displacement magnitude across the tunnel lining in 3D view, as shown in Figure 10. Utilizing point cloud data, we observed a range of displacement from 0 mm (green) to 0.170 mm (red). The green-dominated areas suggest that the majority of the tunnel lining remains stable, indicating minimal deformation. However, isolated red and yellow zones signify higher displacement magnitudes, potentially identifying areas of structural stress or deformation hotspots. This highlights the importance of multi-angle analysis for a thorough understanding of tunnel integrity, as each view provides unique insights into localized stress areas.  
The 3D scatter plot visually represents the deformation magnitudes across the tunnel cross-section, offering a detailed view of how different areas have been affected, as shown in Figure 10. The color scale on the right, ranging from blue to red, indicates the varying degrees of deformation, with blue representing low deformation and red indicating higher levels. Most of the points are in the green to yellow range, suggesting that most areas of the tunnel cross-section have experienced low to moderate deformation. This indicates that the tunnel has maintained its structural integrity overall, with only a few localized regions showing slightly higher deformation, as marked by the yellow to red points. Despite these variations, the tunnel's cylindrical shape remains largely preserved, reinforcing the conclusion that the overall deformation is minimal and within acceptable limits.  
Figure 10: Single cross-section point cloud displacement magnitudes (3D view). (**a**) Front view. (**b**) Back view.  
Figure 9: Single cross-section point cloud displacement magnitudes. (**a**) Top view. (**b**) Bottom view. (**c**) Left side. (**d**) Right side.