## 4 Analysis Results and Discussion
### Digital Image Correlation Analysis
The results of the strain field evolution in the \(Z\) direction at different positions of the sample during the loading process of this set of experiments are shown in Figure 2:0 From the figure, it can be seen that at the initial stage of loading, the strain field on the surface of the sample changes uniformly, with slight longitudinal strain accumulation occurring in the surrounding rock on the top and bottom plates of the goat. By 830 s of loading, a faint line-shaped strain anomaly appears in the surrounding rock of the top plate, indicating the formation of a crack. When loading continues to 848 s, the longitudinal strain accumulation at the crack in the surrounding rock of the top plate in the goat becomes more pronounced, and a line-shaped longitudinal strain accumulation gradually forms in the surrounding rock of the bottom plate, indicating the emergence of a crack there. As loading continues to 1052 s, the longitudinal strain accumulation near the cracks in the surrounding rock of the top and bottom plates in the goat becomes increasingly evident, with regions of longitudinal strain accumulation also appearing in the rock around the cracks. By 1245 s, the longitudinal strain near the cracks in the surrounding rock of the top and bottom plates in the goat becomes more apparent, and a nearly horizontal longitudinal strain accumulation region appears below the crack in the surrounding rock of the bottom plate. As loading continues to 1323 s, the newly formed line-shaped longitudinal strain accumulation in the surrounding rock of the bottom plate in the goat becomes more distinct, and areas of longitudinal strain accumulation in the surrounding rock of the top and bottom plates continue to increase. As loading continues, around 1407 s, an inclined line-shaped longitudinal strain accumulation appears in the lower left corner of the goat, and longitudinal strain near other cracks continues to increase. When loading continues for another 23 s, the line-shaped strain accumulation in the lower left corner of the goat becomes more apparent, indicating the formation of an inclined shear crack. By 1480 s of continuous loading, the longitudinal strain accumulation in the surrounding rock near the inclined crack in the lower left corner of the goat increases rapidly, and another crack forms above it. Additionally, an approximately horizontal longitudinal strain accumulation region forms near the first crack in the middle of the bottom plate of the goat and continues to grow, while the line-shaped strain accumulation above the crack branch in the top plate of the goat also increases rapidly. After another 8 s of continuous loading, the existing longitudinal strain accumulation regions on the top and bottom plates of the goat expand rapidly. A large-scale inclined longitudinal strain accumulation region suddenly appears in the upper left of the goat, with an approximately horizontal longitudinal strain accumulation region forming above the strain bifurcation area. The surrounding rock of the bottom plate in the goat quickly develops two major macroscopic cracks and several smaller cracks, with the longitudinal strain accumulation rapidly increasing. When loading continues to 1550 s, the surrounding rock in the top and bottom plates of the goat rapidly expands, leading to the overall instability of the sample. Comparing the deformation and strain variation patterns between the transverse and longitudinal directions, it can be concluded that since the experiment is loaded along the longitudinal direction, the specimen undergoes compressive deformation in the longitudinal direction and tensile deformation in the transverse direction. Meanwhile, as there is no horizontal force, its deformation is not limited. Therefore, the deformation and strain variation in the transverse direction are more pronounced than those in the longitudinal direction. Through the above analysis, it is found that in the goat instability test, during the loading process, tensile cracks first appeared in the surrounding rock of the top and bottom plates.