## 5 Conclusions  
In this work, the failure and seepage characteristics of gas-bearing coal under accelerated loading and unloading conditions were studied, and a permeability model that considered changes in the loading and unloading rates was established. In addition, the intrinsic relationship between the accelerated advancement of the working face and the occurrence of coal and gas dynamic disasters was found. The following conclusions can be drawn from the completed work:  
(1) The strength of the coal increased and the reduction ratio of the peak stress was remarkably improved under an accelerated loading path. The strength of the sample decreased linearly as the gas pressure increased, whereas it increased linearly as the initial confining pressure increased. The damage to the coal was more violent and impactful and the time required for its failure was shortened by approximately half. The coal was more broken after failure and the failure form was shear failure. When the permeability increased rapidly, the corresponding axial strain was larger and the increasing magnitude of the permeability was greater.  
(2) The unloading ratio decreased and the coal sample was more susceptible to failure under the accelerated unloading path. The damage to the coal was more violent and impactful, and the time required for failure was significantly shortened. The coal was more broken after failure, and the failure mode consisted of tensile and shear failures. The corresponding axial strain when the permeability increased sharply was smaller, and the increasing magnitude of the permeability was greater. The unloading ratio of the radial stress decreased linearly with an increase in the gas pressure, whereas it increased linearly with an increase in the initial confining pressure.  
(3) The influence degrees of the accelerated loading and unloading paths on coal were both weakened with an increase in the gas pressure, whereas there were enhanced with an increase in the initial confining pressure. The time required for the failure of the coal under the accelerated loading and unloading paths gradually decreased with an increase in the gas pressure, whereas it increased with an increase in the initial confining pressure.  
(4) A permeability model that considered changes in the loading and unloading rates was established and verified. The fitting results indicated that the permeability model was relatively reasonable.  
(5) It is found that accelerating the advancement of the working face is likely to induce coal and gas dynamic disasters. For deep high-gas mines, it is extremely important to maintain a reasonable advancing rate at the working face.  
**Author Contributions:** Investigation, X.Z.; Writing--original draft, X.Z.; Writing--review & editing, K.W. and F.D.; Funding acquisition and Supervision, K.W.; Resources, Z.L. and F.D; Data curation, G.W.  
**Funding:** This research was received no external funding.  
**Acknowledgments:** This research is financially supported by the National Natural Science Foundation of China (51874314), the Open Funds of Hebei State Key Laboratory of Mine Disaster Prevention (KJZH2017K02), and the Yue Qi Distinguished Scholar Project, China University of Mining & Technology, Beijing.  
**Conflicts of Interest:** All the authors declare that they have no conflict of interest.