# Study on the Co-Evolution Mechanism of Key Strata and Mining Fissures in Shallow Coal Seam Mining
## 2 Evolution Law of Surface Mining Fissures
### Development Law of Surface Mining Fissures  
(1) Static development law of surface mining fissures  
In order to obtain the development law of surface mining fissures, the evolution characteristics of surface mining fissures in the No.2-2 coal working face of Hanjiawan Coal Mine were investigated. Since the buried depth of the working face in this research was 79 m, according to the sufficient mining conditions proposed in the literature [43], it can be known that full mining can be achieved when the pushing length of the working face is 110.6 m. Through the analysis of on-site production conditions and surface characteristics, a working face with a strike length of 500 m was selected as the research area for surface mining fissures. According to the space-time relationship between mining fissures on the surface and the mining push in the working face, the mining fissures are divided into two types: permanent fissures and periodic fissures. A total of 31 periodic fissures and 11 permanent fissures were found in the study area. The outermost permanent fissure of the cut hole was 38.5 m away from the coal wall, and the width of the fissure was 29 cm. The maximum distance between the two ends of the face was 19.4 m, and the fissure drop was 0-52 cm. Periodic fissures appeared periodically as the working face advanced, and the distance between the two fissures was 13.2-19.7 m. The fissures produced step sinking, the drop was 0-25 cm, and the fissure width was 0-20 cm, as shown in Figure 2. According to the distribution characteristics of surface mining fissures, the fissures with large drops were mainly distributed in the cut hole and both ends of the working face, and were less distributed in the normal mining area. Moreover, the drop was small. The periodic fissures were distributed approximately in parallel, and the boundary fissures at both ends of the working face were discontinuously distributed. Figure 1: Stratum layout of the working face. Figure 2: Surface mining fissure distribution map. In the process of working face mining, periodic mining fissures are more frequent than permanent fissures. Combined with the research content of this paper, the evolution characteristics of periodic mining fissures were statistically analyzed. The periodic mining fissure drop of 0 cm accounted for about 25% of the total, the fissure drop of 0-5 cm accounted for 21.9% of the total, the fissure drop in 5-10 cm accounted for 21.8%, the fissure drop of 10-15 cm accounted for 15.6%, the fissure drops of 15-20 cm accounted for 9.4%, and the fissure drop of more than 20 cm accounted for 6.3%, as shown in Figure 3a. The fissure width of 0-5 cm accounted for 68.7% of the total, the fissure width of 5-10 cm accounted for 21.9% of the total, the fissure width of 10-15 cm accounted for 6.2% of the total, and the fissure width of 15-20 cm accounted for 3.2% of the total, as shown in Figure 3b. According to the evolution characteristics of surface mining fissures, it can be seen that the gap of periodic fissures was relatively small, and there were a large number of fissures with no gap or a small gap. The fissure width was mainly concentrated in the range of 0-5 cm, and only a small part of the fissure width was large. This shows that the periodic fissures had a strong healing ability, which can be closed during the mining process of the working face and can reduce the surface damage to a certain extent. (2) Temporal and spatial evolution characteristics of surface mining fissures  
The formation of surface mining fissures is the result of working face mining, so it is the key to determine the relationship between working face advancing distance and mining fissures. Periodic mining fissures span across the working face and become the main fissure channel for mine air leakage, water inrush, and sand inrush during the mining process of the working face, which poses an unpredictable threat to the working face.