Catastrophic Mechanism of Delayed Water Inrush from Fault Fracture Zones in Coal Seam Floor of Deep Mine

Delayed floor water inrush can be triggered during deep coal seam mining in Northern China under the coupled effects of Ordovician limestone confined aquifers and fault structures. Taking a typical working face threatened by a confined aquifer as the engineering background, this study investigates the hydraulic erosion-induced instability of fault fracture zone fillings and the formation mechanism of water-conducting pathways by integrating compositional analysis of fault-zone fillings, laboratory seepage tests, and numerical simulations. The results show that the fault fillings are dominated by fine-grained clay minerals, mainly including kaolinite, illite, illite–smectite mixed-layer minerals, and montmorillonite. Under mining-induced disturbance and confined water pressure, these fillings are prone to pore-structure reconstruction and permeability enhancement. The seepage process in the fractured rock mass exhibits pronounced non-linearity and can be divided into three stages: initial seepage, abrupt seepage transition, and stable seepage. The migration and loss of fine particles are the key factors controlling the formation of water-conducting pathways and the increased risk of water inrush. As the fracture-zone width increases, fault dip angle and aquifer water pressure all enhance fault water-conducting capacity, promote the upward migration of confined water along the fracture zone, and aggravate the risk of floor water inrush at the working face. The research achievement can provide an important reference for elucidating and controlling floor water-inrush mechanisms in confined-aquifer working faces affected by faults in similar engineering conditions.