## 3 Results and Analyses
### Analysis of Damage Patterns  
The macroscopic damage of coal rock is a consequence of the continuous development, expansion, and penetration of internal fractures. Figure 7 illustrates the damage distribution characteristics of the specimen. The white scatters for DIC monitoring were prepared manually using a whiteboard pen. By observing the coal rock damage pattern and sketching larger cracks, we can use the DIC image detection system to determine the instantaneous changes in the strain and rupture characteristics during the stressing process of the coal rock specimen. This informs us of the main failure mode. Coal rock is brittle, and the damage mode of coal rock varies under different immersion durations. When the natural coal rock was immersed in water for 0 days, it displayed two pronounced cracks at its base, indicating that the primary source of damage was shear. The specimen presented slip damage along the shear surface in response to compressive stress. One crack was particularly obvious; it spanned from the lower center of the specimen to the right side, forming through damage at an approximately 60\({}^{\circ}\) angle about the edge of the specimen. This suggests that crack No. 1 was the predominant source of damage. A vertical crack arose along the sprouting position but did not result in a complete fracture. The two cracks exhibited characteristics of \"Y\"-type damage, and internal granular densification dominated the specimen's damage, as demonstrated in the stress-strain curves shown in Figure 4. The strain curves exhibit stages III and IV. During brief water immersion, the damage pattern of coal rock is relatively intact, and its degree of fragmentation is lower than that of natural coal rock. This occurs because coal rock's brittleness is reduced after brief water immersion, resulting in a weaker degree of destruction. After immersing the coal rock in water for one day, a longitudinal rupture occurred. The crack extended from the lower left to the middle but did not pass through, and there was only one main crack and fewer fine branches. When immersed for three days, the coal rock experienced tensile rupture. The crack position extended from the lower right to the middle but did not pass through, and there was only one main crack but more fine branches. After being submerged in water for 5 days, the coal rock underwent longitudinal ruptures. The crack's position extended from the bottom to the middle but did not pass through. The main crack had only two fine branches or fewer. Similarly, after 7 days in water, longitudinal ruptures occurred in the coal rock, extending its crack position from the bottom to the middle but not penetrating it. The main crack had two or fewer fine branches. When the coal rock was immersed in water for 7 days, it underwent tensile rupture, resulting in one main crack and two secondary cracks with fewer small branches. Long-term flooding caused significant damage to the coal rock, resulting in more surface cracks and shedding due to the relatively loose structure of the coal rock under the water softening effect. After being submerged in water for 10 days, there was a shear rupture on the left side of the observed surface of the coal rock. This induced the appearance of a single crack and shedding. When submerged for 15 days, there was a shear rupture on the top surface of the coal rock, resulting in the creation of two main cracks that extended through the surface, causing shedding to occur. After 20 days of immersion in water, there was shear damage on the top of the coal rock under pressure, accompanied by local shedding. After 25 days, there was a longitudinal rupture of the coal rock, with one main crack passing through the middle. Upon immersion in water for 30 days, the coal rock was damaged by water softening. After being immersed in water for 30 days, the coal rock exhibited multiple longitudinal fractures.