Characteristics and application of separation in soft–hard interbedded overburden strata in thick coal seam mining

Given the geological conditions of thick coal seams with obvious soft-hard interbedded overlying strata (SHIOS), inclined longwall mining is likely to induce fracturing and subsidence in the overburden, which in turn leads to synergistic separation across multiple strata. This study focuses on the 12,030 fully mechanized caving face in a western Henan coal mine. Based on its specific geological and mining conditions, we established a mechanical model for fracture development in soft-hard interbedded overburden strata (SHIOS). By means of theoretical analysis, numerical simulation, and physical similarity simulation, the dynamic evolution law of the separation space in SHIOS under mining disturbance was systematically revealed, and the migration model of SHIOS beam was constructed. Drawing on the ‘multi-peak’ form of dynamic expansion of SHIOS, this study proposed the “three-stage theory” of separation fracture development, i.e., the slow development stage, the accelerated expansion stage, and the stable opening stage. Meanwhile, according to the geological and mining conditions of SHIOS, a 2D similarity simulation model for multi-layer separation grouting was constructed. The results disclose that the spatial development of overburden separation is controlled by the stiffness ratio and the mining thickness ratio, and the shear stress concentration at soft-hard rock interfaces is the main cause of separation initiation. Under the joint action of overburden load and grout-induced support, the maximum subsidence value observed along the overburden monitoring line under multi-layer separation grouting is 4160 mm; the monitoring line with the maximum subsidence was located 14.7 m away from the coal seam roof; and the maximum subsidence observed on the surface monitoring line of the upper overburden is approximately 860 mm. The research conclusions have important guiding significance for the practice of multi-layer separation grouting under SHIOS conditions in inclined longwall mining of thick coal seams.