## 3 Test Results and Analysis
### Seenge Characterization of Coal in Multiple Protected Mining Fracture Zones of the Coal Seam Group  
Seven sets of repeated loading and unloading seepage tests were conducted on the coal body of the protected seam in the fracture zone, following the test scheme in Figure 4 and Table 3. The test results are shown in Figures 7 and 8. (1) Permeability characteristics of repeated loading and unloading under the real stress path of the coal body within the rift zone (initial hydrostatic pressure 10 MPa). As shown in Figure 7, when the coal body in the fracture zone is not affected by protective layer mining, the permeability is extremely low under the initial rock stress. When the protected seam is affected by the first protective mining, the axial pressure and confining pressure change. Before the coal body is destroyed, the permeability increases very slowly and remains almost unchanged. Once the coal body is destroyed, the permeability rises sharply. Afterward, during the axial pressure unloading stage, the confining pressure remains constant, and the permeability of the coal samples gradually increases. Finally, as the axial and confining pressures gradually recover to a certain stress level due to the overlying rock strata, the permeability of the coal body decreases sharply. When the second protective layer mining affects the protected layer, the permeability characteristics are similar to those of the first mining event. However, the permeability of the coal body in the second mining is lower than that in the first mining throughout the process. This is because, during the first protective layer mining, the permeability of the coal samples increased by hundreds to thousands of times when they were fractured, indicating severe damage and the formation of many large fractures. After the loading stage, due to the compaction of the overlying rock layer, two things occurred: first, the internal penetrating fractures were closed by external forces, leading to a substantial reduction in fracture openings; second, the coal fragments on the rough fracture surfaces were prone to breaking off and blocking the fracture channels during the loading process, further reducing the fracture openings. This resulted in a significant reduction in permeability, as shown in Figure 7.  
Therefore, during the second unloading, the permeability of the coal samples is lower than during the first unloading, as it is difficult to reopen the closed fractures, and coal dust continues to accumulate and block the fracture channels. It can also be concluded that the permeability improvement and stress release effects from the first protective layer mining are more pronounced. The first mining event has a greater impact on the coal body, causing more significant damage, while the second mining event has a less noticeable effect on stress release and permeability improvement. (2) Permeability characteristics of repeated loading and unloading under the real stress path of the coal body in the fracture zone (initial hydrostatic pressure 5 MPa). As shown in Figure 8, the permeability characteristics of the coal body in the fracture zone with an initial hydrostatic pressure of 5 MPa under the real stress path of repeated loading and unloading are similar to those of the coal body in the rift zone with an initial hydrostatic pressure of 10 MPa. However, the overall permeability after the first unloading is smaller than after the second unloading in the coal body with an initial hydrostatic pressure of 5 MPa. This is the opposite of what is observed in the coal body with an initial hydrostatic pressure of 10 MPa. The main reasons for this are as follows: during the first unloading stage, the permeability of coal samples with an initial hydrostatic pressure of 5 MPa increased only a few dozen times during the crushing stage, indicating that the crushing was not severe and only a few penetrating cracks were formed. When the coal samples underwent the second unloading, they were damaged again, and more penetrating fractures were created.