## 3 Test Results and Theoretical Analysis
### Analysis of the Permeability Properties during Post-Peak Loading and Unloading
The above analysis shows that during the post-peak loading and unloading confining pressure, the permeability of the sample gradually decreases with the loading confining pressure and increases with the unloading confining pressure, the permeability of the coal sample increases in wave shape with the increase of axial strain, that is, the permeability increases after each loading and unloading confining pressure compared with that after the previous loading and unloading confining pressure. The possible reason is that the specimen has been destroyed in the process of post-peak loading and unloading confining pressure, and macro-penetrating cracks are generated inside the specimen, and the axial stress is always applied by displacement control. At this time, loading confining pressure can make the macro-cracks produced by sample failure close slowly, resulting in a non-linear decrease in permeability; while unloading confining pressure can make the closed macro-cracks open slowly, but at the same time, under the combined action of axial stress, the cracks in the sample further expand, cracks increase and new cracks are generated, and the gas through the sample capacity increases, resulting in the permeability of the sample after each loading and unloading confining pressure being greater than that of the previous loading and unloading confining pressure. In conclusion, the permeability of coal sample is closely related to confining pressure during loading and unloading. Due to the limitation of the length of the space, this paper only draws the deviatoric stress-confining pressure-permeability curves of water content 0.65% sample during loading and unloading confining pressure post-peak as shown in Figure 9.  
Figure 9: _Cont._  
It can be seen from Figure 9 that with the loading of the confining pressure, the deviatoric stress increases linearly, and the permeability decreases nonlinearly. When the confining pressure is loaded to about 98% of the predetermined value, the deviatoric stress increases rapidly, and the permeability decreases rapidly. Perhaps it is because under the confining pressure of the raw coal sample, the internal crack and part of the raw coal sample are closed by pressure, which makes the gas flow passage narrow, resulting in a non-linear decrease of gas permeability. When the confining pressure of the specimen is close to the predetermined value of 9 MPa, the confining pressure loading rate automatically decreases under the influence of the test system, and the axial pressure is always in the displacement control uniform loading process, resulting in a rapid increase of the axial stress of the sample, the corresponding deviatoric stress of the specimen increases rapidly, and the permeability of the specimen decreases rapidly. With the unloading of confining pressure, the deviatoric stress of the specimen continues to increase. The deviatoric stress of the specimen reaches the maximum when the confining pressure is unloaded to 5.705 MPa during the first loading and unloading process. The deviatoric stress of the specimen reaches the maximum when the confining pressure is unloaded to 5.975 MPa during the second loading and unloading process. The deviatoric stress of the specimen reaches the maximum when the confining pressure is unloaded to 5.8 MPa during the third loading and unloading process. The deviatoric stress of the specimen reaches the maximum when the confining pressure is unloaded to 5.78 MPa during the fourth loading and unloading process, that is, when the confining pressure is unloaded to an average of about 5.815 MPa (64.61% of the initial confining pressure unloading value), the deviatoric stress of the sample reaches the maximum, and in this process, the permeability of the sample increases slowly, but the rate of increase is small. With the continuous unloading confining pressure, the deviatoric stress of the specimen changes from gradually increasing to decreasing gradually.