## 4 Analysis Results and Discussion
### Digital Image Correlation Analysis
At 1415 s, a rightward-inclined crack was clearly visible in the lower left corner of the goaf, extending through the floor rock of the specimen. Over the next minute, the horizontal deformation difference near the crack in the floor rocks increased significantly. By around 1488 s, these cracks suddenly expanded, tearing the speckles sprayed on the specimen surface, and forming a white area along the crack direction in Figure 5. Simultaneously, the horizontal deformation difference near the crack in the upper left corner of the goaf also increased. Within one more minute of continued loading, the specimen experienced macroscopic failure, with the surface horizontal displacement reaching approximately \(-0.38\) mm. The locations and propagation directions of these cracks are clearly visible in Figure 3d. In this test, the changes in the strain field in the X direction of the specimen during the loading process are shown in Figure 5. By comparing the horizontal deformation at the same time points in Figure 4, it was found that during the loading process up to 654 s, there were noticeable differences in horizontal displacement at different positions, but the differences in lateral strain were very small. By 830 s, a region of horizontal strain accumulation began to appear in the top plate of the goaf, indicating the start of a crack, corresponding to the results in Figure 5; there were still no significant differences in lateral strain at other locations. As loading continued to 848 s, a crack also initiated in the middle of the surrounding rock at the bottom plate of the goaf, with lateral strain accumulating near the crack. By 1236 s, the lateral strain near the cracks in the top and bottom plates of the goaf increased, and a difference in lateral strain began to emerge in the surrounding rock at the bottom left and right corners of the specimen. In the following minute, the accumulation of lateral strain in the lower left corner of the goaf intensified rapidly, more evident than the comparison at 1332 s in Figure 5. Shortly thereafter, a linear region of lateral strain accumulation appeared near the right side of the crack in the lower left corner of the goaf, indicating another potential crack initiation. By 1415 s, the crack in the lower left corner of the goaf gradually expanded, and the crack propagation path became more apparent. At 1480 s, the expansion of cracks on the left side and middle of the bottom plate accelerated, with lateral strain reaching 5% to 6% near the cracks. Soon, at 1488 s, sudden cracking occurred in the surrounding rock of the bottom plate of the goaf, with several cracks forming and connecting almost simultaneously, and strain accumulation rapidly increased. Additionally, a linear accumulation of lateral strain quickly appeared in the upper left corner of the goaf, with a sudden occurrence of an inclined crack. Compared to the results in Figure 5, the sensitivity of lateral strain to the formation of this crack was much lower than that of deformation, with the appearance of the crack in the lateral strain field lagging far behind that in the lateral deformation field. As loading continued, the cracks in the specimen rapidly expanded, leading to the overall failure of the specimen within one minute. Similarly, the longitudinal strain and deformation analysis were performed. The calculation results of the longitudinal deformation field in this experiment are shown in Figure 6. Since the INSTRON 1346 testing machine loads upwards from the lower loading block, i.e., moving in the positive \(Z\)-axis direction, the overall deformation of the sample is positive, that is, in the positive \(Z\)-axis direction. As shown in Figure 7, at the initial stage of loading, the sample begins to undergo slow longitudinal deformation. During the loading process up to 654 s, the differences in deformation size at different positions of the sample gradually appear.