Fracture evolution of a thick soft protection layer and the water inrush mechanism in overburden under longwall mining

Abstract In the Jurassic coal‐bearing strata of western China, thick soft protection layers (TSPLs) are widespread and play a key role in controlling roof water inrush hazards. Understanding their time‐dependent deformation and failure mechanisms is essential for hazard prevention. This study combines laboratory creep tests, mechanical modeling, and numerical simulations to investigate the fracture behavior of TSPL under mining. A modified triaxial testing system was developed to characterize both shear and tensile creep. A viscoelastic thick‐plate mechanical model and a numerical model were established to capture progressive deformation and failure processes. Results show that tensile creep leads to failure in the decelerate creep stage with limited deformation, whereas shear creep is dominated by large‐scale rheological deformation and failure in the accelerate creep stage. The rheological properties and the thickness of TSPL jointly control fracture patterns: thicker layers tend to develop inclined shear‐induced fractures connecting the high‐position separation zone to the working face, while thinner layers are prone to vertical tension‐induced fractures linking to the goaf. Further, four typical types of mine roof hydraulic hazards are identified, including aquifer water inrush via the water‐conducting fractured zone, water accumulation in separation layers without inrush, water inrush from separation layers into the goaf, and water inrush from separation layers at the working face. Mechanism‐based prevention measurements tailored to each water inrush type are proposed to enhance mine safety and ensure sustainable coal extraction.