## 1 Introduction  
Mining under a water body is a special coal mining condition in China [1]. In northern and eastern China, the bed rock is commonly covered by very thick alluvium with thickness of about 400 m. A kind of confined aquifer with 3-4 MPa water pressure and containing non-cemented sand and grit exists at the bottom of the thick alluvium. To prevent roof water flooding accidents when mining near the confined aquifer, coal mines were required to set safety pillars for isolating normal fracture zone from aquifer before underground mining [2]. Theoretically, mining below this aquifer is safe and the water from the aquifer could not rush into the working face. However, when mining under the aquifer, many water inrush disasters occurred, together with supports crushing disasters, called supports crushing and water inrush disasters (SCWIDs). This seriously restricts the safety and high efficiency of coal mining. These disasters not only caused significant economic cost, but also affected the normal mining scheduling and influenced the production situation [3, 4, 5]. After coal seam mining, the overburden is usually divided into the caving zone, the fracture zone, and the bending zone according to the different fracture states [6, 7, 8, 9, 10]. Among them, the caving zone has a high degree of fracture development, disordered fracture distribution, and strong permeability, while there are mainly horizontal fractures and interconnected vertical fractures in the fracture zone. Water source and water inrush passage are the necessary conditions for mine water inrush. Water flowing fracture is one of the main and common passages in the coal mine. Figuring out the evolution law of water flowing fracture is the foundation to reveal the mechanism of water inrush disasters. When interconnected vertical fractures directly develop into the aquifer, the coal mining faces suffer the threat of water inrush. Water inrush disasters usually are generally considered to be a problem in the domain of hydrogeology. Such water inrush presents many concerns for miners and researchers [11; 12; 13; 14; 15; 16]. For the problems of SCWIDs under the unconsolidated confined aquifer (UCA), previous studies mainly focused on the set of waterproof coal pillars. [PERSON] studied different types of UCA and their influences on leaving the safe coal pillar and proposed a new optimization method for coal pillar outcrops based on the variation rules of overburden's \"two belts\" growth altitude in the close unconsolidated aquifer [17]. According to the borehole results, [PERSON] attributes the disasters to geological factors such as fault, primary fractures, and so on [18]. However, the periodical disasters cannot be well explained, and it is hard to give a reasonable explanation for why the actual water flowing fracture breaks through the usual height. [PERSON] analyzed the characteristic of UCA and described the difference of aquifer characteristics and its impact on rock breaking, but the mechanism of the SCWIDs was not well revealed [19]. [PERSON] provided prediction results of water inrush areas based on GIS and analytic hierarchy process under UCA in the Qidong coal mine [20]. [PERSON] stated that the main reason for the disasters lies in the decrease of the bottom interface, which causes a shortage of waterproof coal pillar. They also proposed prevention measures of grouting and plugging, dewatering, and depressurizing [21]. [PERSON] and [PERSON] studied the relationship between water level variation of unconsolidated confined aquifer and roof weighting and put forward a method for warning of water inrush accidents, and then analyzed the water inrush mechanism [22; 23]. In addition, it is so strange that supports crushing disaster happened in the event of a water inrush disaster. In the traditional view, supports crushing is usually due to the abnormal appearance of underground mine pressure, which is a problem closely related to mining engineering.