Mechanistic study and control of coal pillar deformation and failure in gob-side roadways with extra-thick seams and hard roof.

To address the large deformation failure of coal pillar ribs in gob-side roadways under the geological conditions of extra-thick coal seams with hard roofs, their mechanical instability mechanism was systematically revealed through comprehensive theoretical analysis, numerical simulation, and field measurements, and an innovative coordinated control system was developed accordingly.Theoretical investigations identified a coupled disaster-inducing mechanism combining intensive dynamic loading effects from periodic roof fracturing with high-stress gradients in extra-thick coal seams, inducing significant stress migration in coal pillar ribs. Based on the dynamic evolution of the overlying strata structure and mining-induced stress field, a reinforcement system using constant-resistance energy-absorbing (NPR) anchor cables was established to enhance the stability of the coal pillar rib. Numerical simulations showed that the NPR anchor cable reinforcement reduced the coal pillar plastic-zone depth by 62.5%-78.6% and decreased the coal pillar stress peaks by 8.9%-17.5%. Field measurements further showed that, compared with the original support section, the peak coal stress in the reinforced section decreased by 20.9%-26.3%, rib bulging was reduced by approximately 52%, and the borehole fissure depth was reduced by about 50%. The developed coordinated control scheme provides a reference for coal pillar rib stability control in gob-side roadways under similar extra-thick coal seam and hard-roof conditions.