Characteristics and Control of Severe Strata Behavior in Top-Coal Caving Gate Roads of Extra-Thick Coal Seams Beneath Goafs

In close-distance coal seam mining, the long-term static high-stress field induced by remnant pillars in the overlying goaf superimposes with the dynamic load disturbance from fully mechanized top-coal caving (TCC) in the underlying extra-thick seam. This superposition leads to severe strata behavior in gate roads, such as frequent large deformations and support structural failures, which severely restrict the safe extraction of the working face. To address this issue, taking the material gate roads of the 221303 and 221304 working faces in the No. 13 extra-thick coal seam beneath the No. 8 seam goaf of the Xiaojiawa Coal Mine as the engineering background, comprehensive methods including field monitoring, theoretical analysis, numerical simulation, and underground industrial tests were employed. This study revealed the disaster-causing mechanism induced by the superposition of three stress sources: the static stress of the overlying remnant coal pillar, the gob-side lateral abutment pressure, and the advance abutment pressure of the working face. Based on the limit equilibrium theory, the theoretical width of the plastic zone in the gob-side coal pillar was calculated. The influence laws of different pillar widths on the stress, deformation, and plastic zone distribution of the surrounding rock were systematically investigated to determine a reasonable width for the protecting coal pillar. Furthermore, a synergistic support system consisting of “rib reinforcement anchor cables + flexible formwork pumped concrete pillars” was proposed and tested in the material gate road of the 221304 working face. The results demonstrate that the theoretical width of the plastic zone in the gob-side pillar is 3.43 m; integrating the stress and deformation characteristics of the roadway under different pillar widths, the optimal protecting pillar width for the study area was determined to be 30 m. Under the original 20 m narrow pillar and conventional support conditions, the maximum roof subsidence, two-rib convergence, and floor heave in the severely mining-affected zone reached 619 mm, 722 mm, and 324 mm, respectively. After adopting the 30 m protecting pillar and the synergistic support system, the roof subsidence, rib convergence, and floor heave all decreased by more than 50%. The surrounding rock deformation in the compound high-stress section decreased by 38.2–39.3%, the extent of the plastic zone was significantly reduced, and the roadway stress environment was notably improved. The research findings can provide a theoretical basis and engineering reference for the design of coal pillar parameters and surrounding rock stability control in similar dynamic pressure gate roads of extra-thick coal seams beneath close-distance goafs.