Mechanism of strong mine pressure in composite hard roofs and identification of the primary control layer

Multilayered hard roofs overlie coal seams, and the instability mechanism of roof structures under combined stress conditions is highly complex. Through integrated approaches including physical similarity simulation, theoretical analysis, and engineering tests, this study investigated the fracture patterns and energy evolution of composite hard roofs, elucidated the mechanisms behind strong mine pressure, and identified the key control layer for pressure relief. Results revealed that during the initial mining phase, roof caving is primarily concentrated in the lower hard roof, with periodic fractures leading to regular weighting events. The higher hard roof, supported by the underlying fractured gangue, initially remained stable. However, as mining advanced to a square position, the higher hard roof underwent coordinated fracturing with the interbedded rock strata, resulting in significant mine pressure. Compared to pre-splitting the lower hard roof, this approach reduced the frequency of acoustic emission signals at the working face square position by 38.40% and confined the energy range of microseismic events to 1000–2000 J. This study provided valuable insights for the prevention and control of dynamic disasters in mining operations with composite hard roofs.