## 3 Test Results and Theoretical Analysis
### Analysis of the Permeability Properties during Post-Peak Loading and Unloading  
Based on the test results, the relationship curves deviatoric stress-axial strain-permeability of coal samples with different water content under loading and unloading confining pressure are shown in Figure 7.  
Figure 7: _Cont._  
According to the deviatoric stress-axial strain-permeability curve of the sample in Figure 7, the permeability of each sample under different stress states is shown in Table 2. The permeability-effective stress curves of specimens under different stress states were plotted, as shown in Figure 8.  
It can be seen that the permeability change of each sample during the post-peak loading and unloading process is as follows: for the dry coal sample, the permeability decreases by 63.54% after the first loading confining pressure, and the permeability increases by 6.43% after unloading confining pressure. After the second loading of confining pressure, the permeability decreases by 56.43%, and the permeability increases by 20.24% after the unloading confining pressure. After the third loading confining pressure, the permeability decreases by 58.99%, and the permeability increases by 24.75% after the unloading confining pressure. After the fourth loading confining pressure, the permeability decreases by 60.16%, and the permeability increases by 11.68% after the unloading confining pressure; for the coal sample with a water content of 0.15%, the permeability decreases by 21.55% after the first loading confining pressure, and the permeability recovered to 87.56% of the initial permeability after unloading confining pressure. After the second loading confining pressure, the permeability decreases by 56.43%, and the permeability increases by 35.61% after the unloading confining pressure. After the third loading confining pressure, the permeability decreases by 15.99%, and the permeability increases by 36.84% after unloading confining pressure. After the fourth loading confining pressure, the permeability decreases by 23.33%, and the permeability increases by 34.72% after unloading confining  
Figure 8: Permeability-effective stress curves of samples with different moisture contents. Figure 7: Post-peak deviatoric stress-strain-permeability curves. (**a**) water content 0% (**b**) water content 0.15% (**c**) water content 0.32% (**d**) water content 0.65%. pressure; for the coal sample with a water content of 0.32%, the permeability decreases by 45.1% after the first loading confining pressure, and the permeability recovered to 90.2% of the initial permeability after unloading confining pressure. After the second loading confining pressure, the permeability decreases by 23.91%, and the permeability increases by 23.91% after the unloading confining pressure. After the third loading confining pressure, the permeability decreases by 31.58%, and the permeability increases by 58.97% after unloading confining pressure. After the fourth loading confining pressure, the permeability decreases by 30.65%, and the permeability increases by 74.42% after unloading confining pressure; for the coal sample with a water content of 0.65%, the permeability decreases by 77.28% after the first loading confining pressure, and the permeability increases by 15.11% after unloading confining pressure. After the second loading of confining pressure, the permeability decreases by 76.06%, and the permeability increases by 15.4% after the unloading confining pressure. After the third loading confining pressure, the permeability decreases by 66.41%, and the permeability increases by 13.73% after the unloading confining pressure. After the fourth loading confining pressure, the permeability decreases by 62.63%, and the permeability increases by 10.11% after the unloading confining pressure.