Experimental study on the start-up and transport evolution of mud-sand inrush in mining cracks under K1/J2 paleoweathered rock

The mechanism of mud-sand inrush during coal mining under paleoweathered rock (PWR) in the K1/J2 contact zone of the Ordos Basin is unclear, which seriously restricts the safety production of mines. Based on this, a physical model device of mud-sand inrush with adjustable crack width was developed, and the start-up and transport evolution experiments of PWR particles with different clay mineral contents (i.e., 21.03%, 66.20%, 77.41%) in mining cracks was carried out. Results show that the process of mud-sand inrush is divided into three stages: start-up, inrush and balance; as well as the inrush stage is characterized by four types: continuous inrush, gentle slope inrush, sudden drop inrush and fluctuating inrush. Under certain conditions, as the crack width increases from 1 to 4 cm, the critical hydraulic gradient decreases, the proportion of mud-sand and cumulative mass in inrush mixture increase, and it is easier to form a collapse pit throughout the PWR layer. As the initial water pressure increase from 12.45 to 40.10 kPa, the type of inrush in the later period of inrush stage changed from gentle slope inrush to sudden drop inrush, the duration of pore water pressure in continuous inrush decreased, and the change amplitude in sudden drop inrush increased, as well as the mass flow of water-mud-sand and cumulative mass of inrush mixture increased as a whole. These findings provide theoretical guidance for preventing mud-sand inrush during deep coal mining under thick bedrock in Ordos Basin.