## 4 Discussion  
Due to the particularity of the underground environment, the difference of hydrological conditions of coal seam affects the strength and gas seepage characteristics of coal seam to a certain extent. The above experimental results show that the ultimate strength of samples decreases gradually with the increase of water content. The reasons for this result may be as follows: (1) moisture affects the change of coal-rock own properties: water enters into pores, cracks and wets the particles on the free surface, weakens the relationship between the particles, weakens the cohesion and internal friction  
Figure 10: Fitting curves of effective stress-permeability curves during the process of post-peak loading and unloading confining pressure of the water content 0.65%. (**a**) First loading and unloading (**b**) Second loading and unloading (**c**) Third loading and unloading (**d**) Fourth loading and unloading  
coefficient of coal sample, and makes the strength of coal sample decrease obviously. At the same time, the lubrication of the water makes the coal sample more susceptible to deformation, and the deformation amount increases under the same stress increases. (2) Moisture affects the distribution of in-situ stress in coal and rock: under the action of loading and unloading stress field, the stress concentration effect on the inner part of coal and rock is caused by pore water pressure. The greater the water content, the greater the wedge effect of water on the fracture poreafter compression, the greater damage degree to the coal by stress. Therefore, the more water inside the coal and rock, the lower the strength of the coal and rock, the greater the deformation and the greater the degree of damage. Therefore, the water content of coal seam can be regarded as one of the important indicators for the determination of coal mine safety coal pillar strength. After coal seam mining, the coal and rock mass in the state of three-dimensional stress balance causes instability failure, and the stress redistribution of coal-rock mass around the working face. As the working face continues to move forward, the coal body in front of the working face undergoes a complex loading and unloading process. During the test, when the constant confining pressure is 8.6 MPa and the axial stress increases from 18.6 to 25.85 MPa, the axial strain increases by 59.58% when the moisture content is 0%, by 59.18% when the moisture content is 0.15%, by 58.60% when the moisture content is 0.32%, and by 57.84% when the moisture content is 0.65%. When the axial stress is loaded from 25.85 MPa to the sample failure, the axial strain of the sample increases by 300.16% when the moisture content is 0%, by 275.88% when the water content is 0.15%, by 249.19% when the water content is 0.32%, and by 236.04% when the water content is 0.65%. It can be seen that the moisture not only affects the peak strength of the specimen, but also has a great influence on the axial deformation of the specimen. That is, with the increase of moisture content, the axial strain increase of the sample gradually decreases. The permeability of samples is the result of combined action of axial stress, confining pressure and gas pressure. Therefore, it is more meaningful to adopt the relationship between effective stress and permeability. The sample was subjected to post-peak loading and unloading confining pressure tests. The sample had macroscopic cracks, and the axial direction was applied by the displacement control during the process of loading and unloading confining pressure, that is, during the post-peak loading and unloading process, and the axial was simultaneously subjected to axial stress, confining pressure and gas pressure. In the process of post-peak loading confining pressure, the effective stress of the sample increases gradually, which results in the closure of internal voids and cracks, weakens the ability of gas passing through the sample, and decreases the permeability of the sample.