## 3 Results and Discussion
### Effect of Confining Pressure
When p\({}_{\text{mid}}\) increased to a certain level, the stress balance of the coal was disrupted due to damage, leading to collapse and the termination of the solution process. When comparing the stress models of the \(X\)-axis and \(Y\)-axis under the same pressure distribution, a significant difference in collapse intermediate pressure and time was observed (Figure 22). Under non-uniform pressure (A), the collapse intermediate pressure was the same, but there were slight differences in the collapse time. Additionally, under both uniform and non-uniform pressure (B), the \(Y\)-axis stress model exhibited significantly higher collapse intermediate pressure at collapse and a shorter collapse time. This was closely related to the heterogeneity of the coal body, including the distribution of pyrite and the Young modulus of the coal matrix. Figure 21: Percentage of damage area in S\({}_{0}\)M and RM under different p\({}_{\text{CS}}\). Figure 22: Intermediate pressure and heating time when the parallel pressure model collapses. In comparison to the model with no confined pressure, there were more damage paths along the \(X\)-axis and fewer along the \(Y\)-axis under UXP, while there were more damage paths along the \(Y\)-axis and fewer along the \(X\)-axis under UYP (Figure 23). The reason is that, after applying a parallel pressure, the coal matrix experiences compressive forces in the direction of the pressure. In other words, it acquires tensile stress resistance perpendicular to the pressure direction, which inhibits the development of damage paths along this direction. Therefore, the damage of the coal matrix caused by the thermal expansion of pyrites is more likely to occur in the direction parallel to the pressure, and the damage is prone to develop along the same direction. Compared to UXP, the damage fracture F\({}_{\text{XI}}\) disappeared in NXP (a), while F\({}_{\text{X2}}\) developed more extensively. In addition, in comparison to UYP, F\({}_{\text{Y2}}\) and F\({}_{\text{Y3}}\) disappeared in NYP (a), while F\({}_{\text{Y1}}\) developed more extensively. Conversely, in NYP (b), F\({}_{\text{Y1}}\) disappeared, and F\({}_{\text{Y2}}\) and F\({}_{\text{Y3}}\) developed more extensively. This was because of the different pressure distributions. In NXP (a) and NYP (a), the pressure mainly concentrated in the central part of the pressure direction, exerting a stronger inhibitory effect on damage in the central region and a weaker inhibitory effect on damage on both sides. In contrast, in NYP (b), the pressure concentrated on both sides, leading to a stronger inhibitory effect on damage on both sides. As shown in Figure 24, the percentage of damaged area in the parallel pressure models under 0.2 MPa was lower than under no confining pressure (0 MPa). This indicated that the inhibitory effect on damage development was greater than the promoting effect. After the pressure increased to 0.4 MPa, the changes in the percentage of damaged areas under NXP, UYP, and NYP (a) were not obvious, while NYP (b) showed a significant increase due to localized crushing on both sides of the pressure direction. Under 0.6 MPa, NXP, UYP, and NYP (a) all exhibited significant increases in the percentage of damaged areas, surpassing the values recorded at 0 MPa. Under 0.8 MPa, the percentage of damaged areas in UYP further increased due to the local boundaries of the coal being crushed by a high pressure. The above observations indicate that, at low pressure levels, parallel pressure has a greater inhibitory effect on damage development, while, when the pressure increases to a certain level, the promoting effect on damage development becomes stronger. Figure 23: Coal damage distribution at each intermediate pressure after 15 s.