## 3 Test Results and Analysis
### Displacement Analysis of the Key Position  
The displacement of the surrounding rock at the bottom of a tunnel is an important indicator of the failure mechanism of the tunnel bottom. Comprehending the displacement changes in vital sections of prefabricated inverted arch rock samples is essential for tunnel construction. It should be emphasized that the experimental displacement results cannot be directly compared with the actual deformation of the project. Therefore, it is also crucial to analyze key locations and failure times in lab tests. As shown in Figure 11, taking the typical samples SC-BI-10 and SC-BP-10 as an example, the measurement line \(L_{m}\) (\(X\) = 10 mm) and measurement points P1-P6 are set on the observation surface of the sample.  
Figure 12 shows that there is a significant increase in the displacement of the bottom surrounding rock within the range of 0-20 mm compared to the adjacent area. In addition, the displacement in the X-direction of the sample at measurement line \(L_{m}\) gradually enhanced with increasing load. Even if the loading method and the shape of the force were symmetrical, asymmetric deformation could occur. Similar experimental models and numerical simulations have previously shown this phenomenon [15; 36]. During the time period of 0-1064.4 s, the displacement of the measurement line \(L_{m}\) was very small, and the deflection of the sample was not significant. When \(t\) = 1064.4 s, the displacement measurement line \(L_{m}\) jumped. One second later, the maximum displacement increased from 0.01 mm to 0.42 mm at Y = 11.7 mm. It reached a maximum load at \(t\) = 1074 s, but the fragment size was small, and the displacement was too large, making it difficult to capture by the DIC technique. The change in displacement was abrupt, and uplift occurred within a very short period. Therefore, during the engineering support process, special attention should be given to the bulge at the center of the floor.  
As shown in Figure 13, for sample SC-BI-10, the displacement in the X-direction at the measurement point remained relatively stable from 0 to 1064.4 s. However, at \(t\) = 1064.4 s, there was a displacement mutation. Sample SC-BI-10 experienced tensile failure at points P0 and P1, resulting in an outward uplift and positive displacement in the X-direction. In contrast, it was under compression at points P2 and P3 and did not begin to heave. The displacement at point P0 started to change abruptly first, followed by points P1, P2, and P3, indicating bulging deformation. At \(t\) = 1065.2 s, the order of displacement magnitudes in the X-direction for the strain monitoring points was P0 \(>\) P1 \(>\) P2 \(>\) P3. For sample SC-BP-10, the displacement in the X-direction at the monitoring point remained relatively stable from 0 to 671.2 s. Subsequently, the displacement of the monitoring point moved toward the positive Y-coordinate, and a sudden change in displacement occurred at \(t\) = 671.6 s. This was considered to be a manifestation of arching, which slowed the occurrence of floor heave. This could also be seen as a precursor signal for the occurrence of floor heave. At \(t\) = 672.2 s, the order of displacement magnitudes in the X-direction for the strain monitoring points was P4 \(>\) P5 \(>\) P6. In general, floor heave first occurred in the outermost layer near the surface of the inverted arch and then transferred to the inner rocks. After the outermost  
Figure 11: Schematic diagram of monitoring points and lines.  
layer of rock was destroyed, the bearing area of the middle part decreased, leading to a decrease in bearing capacity. The stress was transferred to the interior, and the deep surrounding rock of the floor continued to rise upward. Repeating this process, floor heavy gradually occurred and could not be prevented. At the same time, it also increased the difficulty of supporting this part of the area. The maximum uplift displacement near the surface of the inverted arch was observed in the outermost region, gradually decreasing along the negative X-coordinate.