# Study on the Co-Evolution Mechanism of Key Strata and Mining Fissures in Shallow Coal Seam Mining
## 1 Introduction  
Mining fissures are an unavoidable problem in the process of coal mining. At the same time, as the dominant factor of wind leakage, water intrusion, and sand intrusion disasters, it has become a precursor to the occurrence of complex mine disasters [1; 2; 3]. Mining operations cause the fracturing of the key bearing layer, which in turn weakens the control capacity of the overlying load layer and accelerates the complete development of mining fissures [4; 5; 6]. On the one hand, the formation of mining fissures is affected by mining parameters; on the other hand, it is controlled by the occurrence conditions of key strata [7; 8]. The fracture characteristics of the key stratum and the bearing structure formed to determine the shape of mining fissures [9]. The mining fissure is the embodiment of the fractured form of the key strata, and the fracture of the key strata is the prerequisite for the formation of the mining fissure; moreover, there is a strong causal relationship between the two [10; 11; 12]. It can be seen that the research on mining fissures should be a process of the comprehensive analysis of key strata fracture structures and mining fissures. Therefore, this study started from the fracture of the key strata in the mining face and led to the mining fissure, forming a research system of the key strata-mining fissure co-evolution. According to fissure mechanics, there are three types of fractures formed by objects, which are opening type (type I), sliding type (type II), and tearing type (type III). There are dynamic fissures and shearing mining fissures, and the tearing type is less likely to appearduring the mining process [13]. The fracture characteristics of overlying rock in the mining face can be divided into a caving zone, fissure zone, and curved subsidence zone, and there is no curved subsidence zone in the mining of typical shallow coal seams [14]. For the distribution of fissures, after the mining of the working face, the overall fracture shape formed on the surface or its profile is defined, and related theories such as O rings, high-level annular fissures, and elliptical belts are proposed [15; 16; 17]. On the basis of the above studies, many scholars have formed different research directions to study the evolution characteristics of fissures and formed the empirical formula prediction method and the key strata theoretical discrimination method mentioned in the three regulations to study the development height of fractures [18; 19; 20; 21; 22]. At the same time, in order to more comprehensively develop the influence of mining parameters on fissure evolution characteristics, the main influential factors of fissure evolution characteristics were obtained through corresponding research on mining height, mining depth, and working face length [23; 24; 25]. In the field production process, the mining fissure is not completely a disadvantage factor. For the gas drainage, the accurate determination of the fissure height and distribution range can reasonably arrange the location of the drainage hole, effectively extract the gas, and ensure the safety of the mine recovery [26; 27; 28; 29]. The formation of mining fissures on the surface is inseparable from the characteristics of the soil. Due to the characteristics of compressive but not tensile properties of the soil layer, the soil layer is prone to tensile failure after mining. When the overlying rock is mined in shallow coal seams to produce a stepped rock beam structure, the soil layer will be sheared and damaged, prompting the formation of mixed mining fissures in the soil layer [30; 31; 32]. Taking the vertical section of the working face as the dividing line, the mining fissures can be divided into advanced mining fissures and co-located mining fissures. The advanced mining fissures basically coincide with the surface subsidence movement angle position, and the horizontal deformation value of the rock and soil mass is greater than its limit tension.