Experimental study on overlying strata migration law in fully mechanized top coal caving mining of extra-thick coal seam with interburden

In response to the issues of large block size and poor caving performance of top coal caused by uneven distribution of interburden and high strength of interburden in top coal during fully mechanized top coal caving （FMTC） mining of extra-thick coal seams in Datong mining area, this paper takes the geological conditions of the 3-5# extra-thick coal seam in Panel 8230 of Tashan Mine as the background. Using similarity simulation and theoretical analysis methods, the study investigates the influence of interburden and roof breakage on the overlying strata migration law during FMTC mining of extra-thick coal seams with interburden. The research results show that when the working face advances to 64 m and 92 m, the immediate roof and main roof experience their first weighting, respectively. The peak abutment pressure during the first weighting of the immediate roof and main roof reaches 940.51 MPa and 1 027.86 MPa, respectively. After the weighting, the abutment pressure drops sharply to its lowest point, reaching 626.81 MPa and 702.96 MPa, respectively. Meanwhile, during the coal caving process, the high-strength interburden in the top coal tends to form a “short cantilever beam” structure, which undergoes periodic breakage similar to that of the immediate roof or main roof. This leads to synchronous breakage of the immediate roof and main roof, resulting in intense local weighting phenomena, significantly affecting the safety and production efficiency of the working face. The periodic breakage of the “short cantilever beam” structure formed by the interburden in the top coal causes the lengths of the “cantilever beam” structures formed by the immediate roof and main roof to increase sequentially, leading to periodic synchronous weighting of the immediate roof and main roof. Additionally, the closer the interburden is to the top coal and the thicker the interburden layer, the more likely it is to form a “short cantilever beam” structure. The ultimate breakage length of the “short cantilever beam” is directly proportional to the rock properties and the thickness of the interburden layer, and inversely proportional to the thickness of the coal seam above the “short cantilever beam” structure. The research results provide a theoretical basis for mines with similar coal seam conditions as Tashan Mine, as well as for other mining areas with different coal seam conditions.