Numerical simulation of mining-induced fracture evolution and water flow in coal seam floor above a confined aquifer

Coal mining above a confined aquifer involves the risk of water bursting into the mining excavation through fractured floor strata. Understanding the key mechanisms and processes of water inrush related to the fracturing evolution induced by the coupled stress–damage–flow interactions during the mining process is of vital importance for predicting and preventing the hazard accurately and in a timely manner. A micromechanics-based coupled damage and flow modeling approach is presented to simulate the progressive development of fractures and the associated water flow in the floor strata during mining above a confined aquifer. This approach combines a microcrack-based continuous damage model with generalized Biot poroelasticity, in which the macroscopic elastic stiffness, Biot effective stress coefficients and overall permeability are explicitly related to the microstructural microcrack kinetics. The numerical results successfully reproduce the stress re-distribution, acoustic emission (AE) evolution, fracture development, permeability changes and water inrush channel formation in the floor strata during the mining process. The deepest fractured zone with highly enhanced permeability appears under both sides of the mined-out area and extends rapidly downward with the increase of mining distance, eventually penetrating into the underlying confined aquifer to form the through-going water inrush channel where the hydraulic pressure and water flow velocity increase sharply. Moreover, the heterogeneity based on a Weibull distribution law is introduced in the numerical model, and the effects of the homogeneity index and confined hydraulic pressure on the process of water inrush in heterogeneous floor strata are examined. Results from such analyses indicate that for a lower homogeneity index (i.e., more heterogeneous floor strata) or a higher hydraulic pressure, the water inrush is more prone to occur at a shorter critical mining distance. Also, a higher homogeneity index or a higher hydraulic pressure will result in a sudden formation of water inrush, while a lower homogeneity index or a lower hydraulic pressure will cause a more gradual process of water inrush. The present study provides an improved understanding of the mechanisms and processes of water inrushes from underlying confined aquifers and will be helpful in practice to predict and prevent water inrush hazards.