Key-stratum-controlled overburden fracture evolution and porosity reconstruction during repeated slicing mining of an extra-thick coal seam

To reveal the coupled evolution of overburden fractures and pore structure during repeated slicing mining of an extra-thick coal seam, the 172,307 working face in Lingquan Coal Mine was investigated through physical similarity simulation and PFC2D numerical modeling. The staged failure of key strata, fracture propagation, strata pressure response, floor stress redistribution, and porosity evolution during upper- and lower-slice mining were systematically analyzed. The results show that the overburden response is strongly stage-dependent and closely related to key-stratum instability. During upper-slice mining, the fractured-zone height increased abruptly from approximately 50 m to 174 m following failure of the sub-key stratum at an advance distance of about 338 m. During lower-slice mining, failure of the primary key stratum at approximately 366 m triggered further high-level fracture development, and the fractured-zone height ultimately reached approximately 256 m. The average periodic weighting intervals were 30.4 m and 29.8 m for the upper and lower slices, respectively. Floor stress along the strike shows distinct zonation, including pressure relief near the open-off cut, a compacted bearing zone in the central goaf, and a strongly disturbed zone behind the working face. After lower-slice mining, the porosity ranged from 0.196 to 0.293 in the caving zone and from 0.142 to 0.295 in the fractured zone. Porosity evolution indicates that high-porosity zones migrate upward and concentrate near the open-off cut, stopping line, and upper part of the goaf, showing pronounced spatial reconstruction of the pore structure under repeated mining disturbance. These findings clarify the linkage among key-stratum failure, fracture evolution, stress redistribution, and porosity reconstruction in repeatedly disturbed overburden.