Study on Overlying Strata Bearing Characteristics of Mining via Strip Slice Filling for Super-Thick Isolated Working Face Coal Seams

In this study, to address the overlying strata control issue for the isolated working face of a super-thick coal seam confined by surrounding goafs and open-pit boundaries, we focus on mining via upward slicing strip paste filling in a Ningxia coal mine. We adopt strip coal pillar stability theory for safety factor analysis and conduct FLAC3D three-dimensional numerical simulations to quantitatively reveal overlying strata displacement, stress redistribution, plastic zone evolution, and surface subsidence response. Our theoretical calculations show that, when the mining width is 5 m, the safety factors of retained coal pillars with widths of 5, 10, and 15 m are <1, 1.3, and 1.8, respectively. Our numerical results indicate that the overlying strata of the first slice are dominated by continuous bending subsidence, with a maximum vertical displacement of 21.4 cm, increasing to 55.5 cm after four slices without through damage. High stress is mainly controlled by mined-out area boundaries and inter-face coal pillars, with the maximum principal stress of the fourth slice reaching 18.9 MPa. The surface subsidence center stably corresponds to the underlying backfill goaf, with a maximum value of 11.4 cm. Our research demonstrates that slicing strip filling can suppress deformation and stress concentration risks by reconstructing load transfer and realizing synergistic bearing, as reflected by the limited surface subsidence of 11.4 cm, the controlled maximum principal stress of 18.9 MPa, and the improved coal pillar safety factor from <1 to 1.3–1.8 under wider retained pillars, providing a basis for optimizing strip pillar width and mining–filling parameters.