Research and application of surrounding rock control technology for gob-side upper entries in fully mechanized caving faces of thick coal seams with large dip angles

• Revealed the asymmetric stress distribution and failure mechanism of the roadway roof under steeply inclined, thick coal seam conditions. Theoretical analysis elucidated the asymmetric distribution of surrounding rock stress in the roof of gob-side upper entries under steeply inclined, thick coal seam conditions, as well as the resulting mechanisms of asymmetric failure, slip and dislocation failure, and ‘X’-shaped shear failure, thus providing a theoretical basis for ground control in gob-side upper entries. • Proposed the 'cable bolt rooting' technique for the gob-side rib and established its instability criterion mathematical model. Based on the engineering characteristics of gob-side upper entries and the migration law of coal gangue, an innovative 'cable bolt rooting' support scheme was designed for the gob-side rib. A mechanical instability criterion for cable bolts was established through a mathematical model, which effectively addressed the stability challenges of the gob-side rib under steeply inclined conditions. • Developed a coordinated ground control scheme for gob-side upper entries. Addressing the ‘void effect’ and complex geological conditions, a coordinated control scheme of ‘reinforcement + blasting-induced roof cutting + rib backfilling’ was proposed. Through theoretical calculations, roof cutting parameters were optimized, effectively controlling surrounding rock deformation and achieving stability in the retained entry. The 13040 working face in Xinmi Hengtai Mine presents a thick coal seam, high dip angle, and a thick, hard roof, which challenge the upper gob-side entry with poor stability, support failure, and severe rib deformation. This study addresses these ground-control difficulties using theoretical analysis, mechanical modeling, and field tests. It reveals the mechanism of asymmetric stress distribution and associated failure in the steeply inclined roof and proposes a coordinated strategy of ‘reinforcement + blasting-induced roof cutting + rib backfilling.’ Key parameters such as cable-bolt length and cutting geometry are determined theoretically. An innovative ‘cable bolt rooting’ mechanism is introduced for the gob-side rib, accompanied by a mathematical model for bolt instability, enabling active anchorage and coupled control of U-shaped steel supports. A dedicated backfilling technique also counters the dip-induced ‘void effect.’ Monitoring data show maximum roof subsidence and floor heave of 130 mm and 300 mm, respectively, with no significant spalling and overall controllable deformation, confirming successful entry retention. The paper systematically addresses the ground control challenges of upper gob-side entry retention under complex geo-mining conditions by matching key control technologies with geological constraints and constructing a coupled synergistic control strategy. It provides a novel technical reference for such engineering projects under similarly challenging conditions.