Investigation on water inrush fracture mechanics model based on fracture mechanics and microseismic monitoring

North China coalfield is one of the most severe coal fields threatened by Ordovician limestone water (buried depth exceeding 1000 m). In this paper, the hydraulic mechanism of Ordovician limestone water in the complex geological structure are studied. An interesting phenomenon was found in the study, the water inrush from Ordovician limestone has a "latent period" and presents lag water inrush law. The hydrochemical characteristics of the early stage in Xingdong coal mine is non-Ordovician limestone water chemical characteristics, the later stage gradually shows the Ordovician limestone water with the continuous outflow. Meanwhile, the hyperbolic contour distribution characteristics are also observed in the spatial distribution law of microseismic in the floor working face. The minimum damage area is in the layer at the hyperbolic apex, which is the critical breakthrough point of water inrush. Furthermore, the water inrush fracture mechanics model (WIFM) of concealed fault with high confined water is established based on the fracture mechanics theory. The results show that the critical values of the length and angle of the water inrush of the conceal fault are about 50 m and 40°. It should be pointed out that the longer and larger angle conceal faults indicate the longer of the fracture expansion at the tip of the fault, and the probability of water inrush increases. Special attention should be paid to such concealed faults and methods such as grouting should be adopted to avoid the water inrush disaster caused by the activation of faults. This model provides a theoretical basis for the mechanical mechanism of the coal seam floor water inrush process.