# Research on the Characteristics of Seepage Failure in the Surrounding Rock (Coal) of the Goafs
## 4 Conclusions  
In summary, the experimental and numerical analysis of coal sample permeability and fracture dynamics provided significant insights into the complex interplay between fracture width, confining pressure, and fluid flow. The results demonstrated that coal samples exhibit a range of permeability values, influenced by their inherent microstructural characteristics. The study identified distinct stages of permeability behavior, including stable and mutation phases, with abrupt changes often driven by alterations in confining pressure. These fluctuations were closely tied to the dynamic behavior of fractures within the coal, which responded to external forces by either expanding or contracting, thus affecting permeability. Narrower fractures were found to cause greater pressure drops and higher local flow velocities, leading to significant variability in fluid dynamics. The identification of a threshold width becomes more pronounced provides a valuable criterion for predicting sudden changes in fracture seepage behavior. Therefore, testing the characteristics of microfractures in the coal pillars of old mined-out areas, especially identifying and determining the fracture threshold that may cause a sudden change in permeability, is crucial for prevention. Although coal properties differ significantly across various mining regions, and their response to changes in physical properties due to water infiltration also varies, this study indicates that microfracture size plays a key role in controlling pressure changes and fluid dynamics. Thus, while 0.11 mm is considered a critical threshold in this study, it is not universally applicable as a sole standard. The critical fracture width may vary in different mining regions, and adjustments and evaluations need to be made according to the specific characteristics of the coal. This study emphasizes the key role of microfracture size in controlling pressure dynamics, providing an important reference for assessing and preventing water hazards \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Number** & **1-1** & **1-2** & **1-3** & **1-4** & **2-1** & **2-2** & **2-3** & **2-4** \\ \hline Width of the fissure \(w\) (mm) & 0.20 & 0.09 & 0.16 & 0.14 & 0.10 & 0.18 & 0.11 & 0.09 \\ The value of pressure reduction \(p^{\prime}\) (MPa) & 0.19 & 1.33 & 0.28 & 0.20 & 0.62 & 0.18 & 0.20 & 0.59 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The strength of pressure reduction at different crack width positions. in coal pillars in old mined-out areas, while also underscoring the necessity for different regions to determine their own critical fracture thresholds. Conceptualization, B.L. and C.Z. ; methodology, B.L. ; software, B.L. ; validation, B.L. and C.Z. ; formal analysis, C.Z., P.Z. and J.H. ; investigation, C.Z., P.Z. and J.H. ; resources, B.L. ; data curation, B.L. ; writing--original draft preparation, B.L. and C.Z. ; writing--review and editing, B.L. and C.Z. ; visualization, P.Z. and J.H. ; supervision, B.L. ; funding acquisition, B.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the Basic Research Program of Shanxi Province, China, grant number 20210302124438. Not applicable. The raw data supporting the conclusions of this article will be made available by the authors on request. We would like to thank the Xi Shan Coal Electricity Group Co. for their help and support in on-site sampling and technical exchanges. We thank the editors and reviewers for their time and effort, as well as for their suggestions and comments. The authors declare no conflicts of interest. References  
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