Field and Numerical Investigation of Water-Conducting Fractured Zone Development in a Deep Coal Seams with Variable Thickness at the 14,321 Working Face

This study characterizes the development of the water-conducting fractured zone at the 14321 working face of the Gubei Coal Mine, located in Fengtai County, Huainan City, Anhui Province, China, where segmentally differentiated mining was adopted in response to variations in coal seam thickness. The 4.2 m section was extracted by fully mechanized mining, whereas the 6.2 m section was extracted by fully mechanized top-coal caving with a cutting height of 4.7 m and a top-coal thickness of 1.5 m. Distributed optical fiber monitoring showed a four-stage strain evolution of steady response, slow increase, continuous increase, and abrupt change in the 4.2 m section. In the 6.2 m section, the strain response indicated progressive failure from the deeper part of the borehole toward the borehole opening. The measured caving zone heights were 13.4–13.6 m and 25.4 m, respectively. The DOFS-inferred upper boundaries of the potential water-conducting fractured zones were 41.4 m in the 4.2 m section and 58.0 m in the 6.2 m section. The 58.0 m result was further supported by surface-borehole drilling-fluid-loss observations, whereas the 41.4 m result was not independently verified by hydraulic testing. Numerical models constructed using the corresponding engineering conditions reproduced plastic zone heights of approximately 43 m and 66 m. These differences represent the combined influences of mining height, extraction configuration, mining intensity, and local geological variation. Because mining height and mining method changed simultaneously, their individual contributions cannot be separated in the present study. The results provide case-specific evidence for evaluating roof fracture development and water hazard prevention in deep coal seams with variable thickness.