Mechanism of reverse deformation increase in the virgin coal rib compared to the pillar rib of the gob-side entry in an extra-thick coal seam

The advantage of applying gob-side entries (GSEs) with narrow coal pillars in extra-thick coal seams to improve the recovery rate is particularly significant. However, the ground pressure is intense and overlying strata caving is severe under fully mechanized top-coal caving mining (FMTCM) in extra-thick coal seams. The ground pressure environment of the surrounding rock in GSE with narrow coal pillars is complex, leading to the phenomenon of reverse deformation increase (RDI) where the failure of the virgin coal rib is greater than that of the coal pillar rib. To reveal the mechanism of RDI, this study takes the GSE of the Panel 8211 under typical FMTCM in an extra-thick coal seam as the engineering background. Through on-site measurements of the surface convergence of the GSE ribs, damage of support structures, coal mass damage depth, and abutment stress, as well as numerical analysis of the supporting characteristics of gob gangue on coal pillars, the coal pillar width, and the excavation timing, the following findings are obtained. The support of gangue on the ribs of the coal pillar is the direct cause of RDI. RDI only occurs when the gangue's contact height with the coal pillar exceeds 20 m, while the degree of RDI no longer increases when the contact height exceeds 30 m. On the premise that the gangue's contact height exceeds 20 m, the smaller the coal pillar width or the shorter the lag time between the excavation of the GSE and the completion of mining in the upper panel, the more significant of the RDI. However, coal pillar width and GSE excavation timing only affect the degree of RDI, rather than being preconditions for RDI. To address RDI in GSE with narrow coal pillars under FMTCM in extra-thick coal seams, a support strategy is proposed: reinforcing coal pillars with anchor cables to share the ground pressure on the virgin coal rib, while simultaneously using anchor cables to strengthen the control of deformation in the virgin coal rib. This measure has achieved satisfactory results in practice, and the overall deformation of the GSE meets the requirements of safe production.