Mechanism and criterion of water burst in thick plate zone with pressure mining

As the eastern coal mines in the North China Coalfield extend deeper year by year, the water pressure of Ordovician limestone increases. Although the thickness of the water-resisting layer at the bottom of the coal seam in some mines is as high as 100 meters or more, and some mines have even carried out ground area treatment, water inrush from the floor of Ordovician limestone still occurs from time to time. In response to the water inrush problem under the condition of thick water-resisting layers, an in-depth study on the “fracturing and lifting” mechanism of thick plate initial water inrush has been conducted, and a supporting mechanical criterion has been proposed. Based on the elastic mechanics model of circular holes, a complete mechanism of water inrush from thick plates has been proposed, which focuses on the initial stage of water inrush. The mechanism includes "fracturing and lifting in the lower intact section, and free lifting in the upper damaged section". A supporting mechanical criterion has also been proposed, which considers the stress of surrounding rock, the mechanical properties of rock, and the pressure of confined water. The calculation formula for the water resistance coefficient of floor rock strata, which is centered on fracturing and lifting, has been improved. The water resistance coefficient reflecting the water resistance ability of rock strata is a variable value affected by geostress, rock tensile strength, and mining. It corrects the previous understanding that the water resistance coefficient of the same lithology rock strata is a fixed value or range. It explains the phenomenon that microseismic monitoring shows that the microseismic event gathering zone of the floor extends to the confined aquifer but does not occur water inrush. A technical process for risk analysis and evaluation of floor water inrush based on the “fracturing and lifting” mechanism has been designed. Taking the working face of Linxi Mine with high pressure and thick floor affected by faults as an example, numerical calculation methods have been used to analyze the variation law of pressure field reopening and the water resistance performance of various rock layers affected by mining under the conditions of floor integrity and fault influence. The “fracturing and lifting in different zones” mechanism has improved the water inrush mechanism of thick water-resisting layers, laying a theoretical foundation for water prevention and control in floor mining with pressure. It is also of reference significance for regional governance and floor monitoring.