## 4 Simulation Results and Analysis
### Dynamic Evolution of Rock Layer Porosity  
#### 4.3.1 Global Porosity of the Surrounding Rock  
As shown in Figure 11, the global porosity measurements at various excavation stages correspond well with the fracture states of the surrounding rock layers. A comparison with the global stress distribution in Figure 7 reveals that, in fractured surrounding rock  
Figure 10: Stress evolution in siltstone interlayer with 43\({}_{\text{lower}}\)01 and 43\({}_{\text{lower}}\)02 working face excavation: (**a**) 43\({}_{\text{lower}}\)01 working face, and (**b**) 43\({}_{\text{lower}}\)02 working face.  
Figure 9: Stress evolution in immediate roof and floor with 43\({}_{\text{upper}}\)02 working face excavation: (**a**) Stress evolution of the roof, and (**b**) Stress evolution of siltstone interlayer.  
regions, high-stress zones do not fully align with high-porosity areas. In fact, they exhibit an inverse correlation. The stress level reflects the interaction intensity between rock blocks. Therefore, the distribution of macroscopic damage fractures provides an intuitive assessment of surrounding rock integrity. By combining the observed changes in rock mass porosity, it is possible to gain deeper insights into the interaction state between fractured rock blocks.  
Following the excavation of the 43\({}_{\text{upper}}\)01 working face, the overlying rock in the mined-out area did not fully collapse, resulting in a pronounced high-porosity region in Area A. At this stage, the porosity between the main and secondary key layers showed a slight increase, as seen in Area B, where delamination between the layers was evident. Area C experienced a localized rise in porosity, primarily due to the bending and failure of the main key layer.  
After the excavation of the 43\({}_{\text{upper}}\)02 working face, the porosity changes in the mined-out area mirrored those observed after the 43\({}_{\text{upper}}\)01 excavation. Specifically, high porosity in Area D was caused by the incomplete collapse of the overlying rock layers. Meanwhile, extensive collapse of the overlying rock in the 43\({}_{\text{upper}}\)01 mined-out area led to a significant decrease in porosity, as shown in Area E. Mining of the 3\({}_{\text{upper}}\) Coal Seam caused more pronounced fault slip and delamination between the main and secondary key layers, resulting in a noticeable increase in porosity in Area F, as indicated in the figure.  
After excavation of the 43\({}_{\text{lower}}\)01 working face, the mined-out area did not exhibit the same characteristics as Areas A and D. Instead of a significant increase in porosity, a widespread increase in porosity was observed in the surrounding rock mass, as shown in Area G. Simultaneously, the collapse of the overlying rock in the 43\({}_{\text{upper}}\)02 mined-out area led to a decrease in porosity in Area H. The movement of fractured rock blocks brought the main and secondary key layers into closer contact, eliminating the delamination, as seen in Area I.  
Following the excavation of the 43\({}_{\text{lower}}\)02 working face, porosity within the mined-out area showed a slight increase, as illustrated in Area J. However, with the expansion of the vertical mining range, fracture damage to the main and secondary key layers was intensified, leading to a significant increase in porosity in the damaged areas, as shown in Areas K and L.  
Figure 11: Porosity distribution of surrounding rock after each excavation period.