# Evolution of Permeability and Sensitivity Analysis of Gas-Bearing Coal under Cyclic Dynamic Loading
## 6 Conclusions  
This paper focuses on the evolution of coal permeability under the combined effects of dynamic loading, static loading, and gas adsorption. It considers the influence of critical factors such as the dynamic load frequency and amplitude, the static load stage, and the types of adsorbed gasses. The principal conclusions are as follows.  
1. As the frequency and amplitude of dynamic loading increase, the development of pore and fissure structures within the coal body becomes increasingly pronounced during the dynamic loading cycles, resulting in a gradual rise in permeability. Notably, as the coal approaches its yielding stage, the permeability can increase by up to 47%. However, with the enhancement of gas adsorption, the permeability of the coal body gradually diminishes.
2. The permeability curve is divided into four regions: the compaction reduction zone, the oscillation zone, the gradual recovery zone, and the abrupt increase zone of failure. It is noteworthy that the permeability and stress-strain relationship exhibit a phenomenon of resonance at the same frequency, with the permeability of the coal body gradually increasing during dynamic load cycling. Ultimately, in the failure phase, the permeability surges dramatically, potentially reaching values of four to five times the initial permeability.
3. A sensitivity analysis of permeability to dynamic and static loading and gas adsorption was conducted, defining two evaluation parameters: the rate of change in permeability and the dynamic loading sensitivity coefficient of permeability. When the static load stage was constant, the rate of change in permeability of the coal under dynamic loading decreased with increasing adsorption amounts. When the adsorption amount was constant, the rate of change in permeability of the coal under dynamic loading increased with the increase in the static loading stress stage, with the maximum increase reaching 75.2%. It can thus be concluded from the rate of change in permeability and the dynamic loading sensitivity coefficient that the permeability of coal is highly sensitive to cyclic dynamic loading, with an increased sensitivity associated with larger static loading stages and decreased sensitivity with greater adsorption amounts.  
In the realm of equipment designed for cyclic loading, it is imperative to undertake regular maintenance and upkeep to ensure its long-term stable operation, which incurs a certain maintenance cost. Although enhancing cyclic loading can improve methane extraction efficiency, the process may have a discernible environmental impact due to methane emissions. Therefore, additional funds must be allocated for environmental protection and remediation efforts. By employing cyclic loading fracturing technology, fractures can be induced within the coal seam, boosting methane extraction efficiency and increasing energy yield. An enhanced methane extraction efficiency contributes to reducing gas accumulation in coal mines, thereby mitigating the risk of gas explosions and safeguarding miners' safety.  
**Author Contributions:** Conceptualization, Z.L.; methodology, Y.L.; validation, Z.W.; and writing--original draft preparation, W.H. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This study was sponsored by the Basic Research Funding for Central Universities of China (grant no. 2024 KYJD1010).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors upon request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.