Abstract
Control of the grouting pressure within the critical grouting pressure for crack propagation in Ordovician limestone can not only ensure grout penetration length, but also prevent the risk of creating an artificial water channel. Based on the fracture mechanics theory, a formula was proposed to calculate the critical grouting pressure of mixed mode I-II cracks in Ordovician limestone. The necessary conditions for tilted crack opening, the rationality of the existing empirical value of the maximum allowable grouting pressure was investigated based on the mechanical model. The RFPA2D-Flow numerical simulation software was used to evaluate the deduced theory. The research results show that the deduced theoretical calculation formula of the critical grouting pressure agrees with the numerical simulation results; when the mixed mode I-II fracture initiation occurs, the grouting pressure exceeds the perpendicular stress of the overlying rock; the greater the density of the overlying rock mass, the greater the value of grouting pressure for fracture initiation; when the side pressure coefficient was ≥1, crack dip angle increased and the grouting pressure for fracture initiation tended to decrease; and the empirical grouting pressure at the maximum allowable grouting pressure is 2.0–2.5 pw, which will not cause propagation and failure of the existing crack.
Highlights
As the mining depth increases, coal mines in the North China coalfields are mainly exploiting Carboniferous Permian coal seams, lying above the Ordovician karst aquifer, which is a massive threat to the top ultra-close seam coal mining [1,2]
The hydrostatic pressure in the crack is in equilibrium with the force exerted by the surrounding rock; a rock with a crack can still be considered as an intact rock, which can be analysed by rock mechanics, and a single inclined crack in a rock can be considered as a stress element [34]
The hydrostatic pressure in the crack is in equilibrium with the force exerted by the surrounding rock; a rock with a crack can still be consi3doefr1e4d as an intact rock, which can be analysed by rock mechanics, and a single inclined crack in a rock can be considered as a stress element [34]
Summary
As the mining depth increases, coal mines in the North China coalfields are mainly exploiting Carboniferous Permian coal seams, lying above the Ordovician karst aquifer, which is a massive threat to the top ultra-close seam coal mining [1,2]. To prevent water inrush from the Ordovician karst aquifer, the grouting reinforcement technology of the upper part of Ordovician limestone is widely applied [3,4,5,6]. Due to the high water pressure in the Ordovician karst aquifer, the traditional grouting reinforcement of the coal floor cannot prevent the water inrush. Many studies have been done on grouting materials’ characteristics in cracks [9,10,11,12]. Li [13] studied the grout transport law from grouting holes in the Ordovician karst. Qiu [14] explored the characteristics of grouting reinforcement engineering practice in the Ordovician top. The above studies never focused on studying the critical grouting pressure in the Ordovician aquifer
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