Collaborative control mechanism of double-layer flexible thick anchor and its engineering application in deep large-height mining gob-side entry driving

To address the surrounding rock large deformation and support instability of deep large-height mining gob-side entry driving under high in-situ stress, strong mining-induced disturbance, complex roof strata and water inflow, the 20103 return airway in Dahaize Coal Mine was taken as the engineering background. Field investigation, mechanical testing of support components, FLAC3D numerical simulation and industrial tests were adopted to investigate coal pillar width optimization, asymmetric damage evolution, the thick-layer transboundary anchoring mechanism and the double-layer flexible thick anchoring control technology. The results show that the average burial depth of the 20103 return airway is 596.81 m, the maximum vertical in-situ stress reaches 16 MPa, and the roadway section is 6 240 mm × 4 550 mm. Under the superimposed influence of lateral abutment pressure from the adjacent goaf and advanced abutment pressure from the working face, the shallow roof damage is mainly concentrated within 0-4 m, while a deep damage zone still exists within 6-10 m, resulting in obvious asymmetric deformation. Orthogonal tests and the comprehensive evaluation of stress, displacement and plastic zone indicate that coal pillar width is the primary factor controlling surrounding rock stability, and the reasonable coal pillar width is 6 m. Different from conventional long cable support or simply strengthened bolt-cable support, thick-layer transboundary anchoring is based on surrounding rock damage zoning. Flexible high-strength members are used to cross the shallow fractured zone and plastic expansion zone and anchor into relatively stable deep strata, thereby forming a continuous bearing structure composed of a primary thick bearing layer and a secondary reinforcement layer. The optimized support scheme consists of 4.5 m flexible bolts and 8.3 m large-diameter cables in the roof, and 2.6 m threaded steel bolts combined with 4.5 and 3.8 m flexible bolts in the ribs, with the primary support spacing of 0.9 m. Field monitoring shows that the average roof subsidence, coal-pillar rib displacement and solid-coal rib displacement are 99.92 mm, 112.50 and 87.40 mm, respectively, and the deformation process can be divided into severe deformation, stabilization and stable stages without continuous late-stage increase. The proposed support system can effectively control the asymmetric large deformation of deep large-section gob-side entry driving.