Structural characteristics of the slurry-filled receptive zone in longwall goaf filled with gangue slurry

To address the dual challenges of environmentally sustainable coal gangue disposal and the effective utilization of underground mined-out spaces, this study investigates the structural characteristics of goafs that govern the efficiency of gangue slurry filling. An integrated methodology combining theoretical analysis, physical simulation experiments, and field-scale industrial testing was employed to systematically examine the spatial architecture and void distribution within the goaf. The research elucidates the spatial evolution mechanism of overlying strata, which follows a sequential "arch-beam-shell" pattern, leading to the identification of the stable residual "shell" structure as the Slurry-Filled Receptive (SF-R) zone. The key stratum governing slurry migration is defined as the Slurry Movement Control (SM-C) stratum, for which a discriminant criterion is established based on mechanical stiffness, strength, and stratal control principles. The influence of the SM-C stratum on the formation of three distinct structural zones-namely, free accumulation, load-affected, and compacted zones-along both strike and dip directions is clarified. Furthermore, the void distribution within the goaf is quantitatively characterized, revealing that the bulking factor of caved rock decreases with increasing depth and height into the goaf, following a negative logarithmic function. The SF-R zone constitutes a quantifiable and exploitable space for slurry injection, with its structural configuration and porosity primarily regulated by the SM-C stratum. These findings were validated through an industrial application at Huangling No. 2 Coal Mine, where targeted filling was implemented in the overlapping region of the free accumulation and load-affected zones. This study establishes a robust theoretical framework and provides a practical methodology for the design and optimization of high-efficiency gangue slurry filling systems.