A Comparative Case Study on Stress Redistribution due to Extraction of Conventional and Split-Level Longwall Panels in Deep Inclined Coal Seams

Abstract

Through field observations, theoretical analysis, and a calibrated numerical model, a study of stress redistribution due to the extraction of longwall panels at depths ranging from 580 to 660 m with a 30° dip angle at Tangshan coal mine is presented in this paper. Conventional and new split-level longwall layouts are compared regarding their stress redistributions. The height of the caved zone is 21.7 m; angles of break of 55.6° on the left and 54.2° on the right side of the gob are observed using cross-measure boreholes. Structural models as well as numerical models are constructed based on the above field data to make the geometry of the gobs closer to the in situ situation and more realistic. Compared with the conventional layout, the theoretical analysis shows that the overall influence of the elevated split-level longwall gob on the lowest intact stratum increases by more than 5.07%, meaning that the split-level longwall layout is more likely to maintain the stability of the overlying strata. This is also corroborated by numerical modeling. Conventional longwall panels and split-level longwall panels with and without considering the gob are all simulated using FLAC3D. Instead of only backfilling the height of the coal seam or the height of the coal seam and the immediate roof, as in many numerical modeling studies in the past, in this study, the whole caved zone is backfilled with “double-yield” material. It is found that along the floor, the split-level longwall gob assumes 23.4% more load than the conventional longwall gob, and the split-level longwall abutment bears 6.2% less load than the conventional longwall abutment; stress arches are developed within the gob; concave-down stress beddings are more evident at higher locations of the gob; a self-supporting structure develops within the gob and surrounding rock mass around the lower end of the gob, forming a protective localized intact destressed zone around the location where the split-level tailgate is situated; the yield zone in the floor of the curved section tends to extends toward the center of the curved part, where the curvature is the maximum; the upper stress concentration zone is within the coal seam, while the lower one is above the coal seam; the upper one is more concentrated.