# Study on the Co-Evolution Mechanism of Key Strata and Mining Fissures in Shallow Coal Seam Mining
## 3 Fracture Structure of Key Strata and Evolution Characteristics of Mining Fissures
### Analysis of the Fracture Structure and Bearing Characteristics of the Key Strata  
(1) Analysis of key strata fracture structure characteristics  
In this study, the overlying rock was a single key strata geological condition, and the relationship between the fracture structure of the key strata and the evolution of mining fissures was explored. According to field measurements and similar simulation experiments, when the working face was pressed for the first time, the lower layer of the key strata was broken, and the upper part of the key strata did not break. When the working face advanced to 84 m, the overlying bedrock of the key strata broke synchronously. During the mining process of the working face, the key strata were mainly composed of a hinged rock beam structure, supplemented by a stepped rock beam structure, forming the supporting system of the working face. The broken structure of the key strata led to differences in the shape of the mining fissures. When the key strata were a hinged rock beam structure, the mining fissures were similar, and the fissures had a small or no drop, as shown in Figure 4. When the key strata were broken to form a stepped rock beam structure, there was a large drop on both sides of the mining fissure, as shown in Figure 5.  
(2) Hinged rock beam structure-(I-I) type mining fissure  
When the key strata form a hinged rock beam structure, the mechanical characteristics of the key block can be simplified, as shown in Figure 6. The key block B was subjected to the mining action to carry the upper overlying rock and loose soil layer to produce counterclockwise rotation movement, and the key blocks A and B generated upward tensile fissures at point F1. At this time, the overlying rock located on the upper part of the key block was subjected to tensile stress acting on both sides, and the stress characteristic of the mining fissure was the opening type (type I) in fracture mechanics [13], resulting in upward tensile fissures in the overlying rock. When the cumulative tensile strength of the overlying rock and loose soil layer is less than the tensile stress generated by the rotation of the key block, the fissures will penetrate and develop to the surface. If the cumulative tensile strength is equal to the tensile stress generated by the rotation of the key block, the fissures will stop growing to the cumulative height of the overlying rock development. Figure 4: Mining fissure diagram of the key strata hinged rock beam structure. Figure 5: Mining fissure diagram of the key strata step rock beam structure. When the two are equal, the development of mining fissures in the rock formation will have stopped, so there will be no greater situation. At the same time, due to the rotation of the key block B, the overlying rock and soil layer produce a subsidence space, forming a certain deflection. According to the principle of curvature calculation, the curvature w is positively correlated with the radius of curvature \(r\), so the surface curvature is the largest, and it is easy to be damaged to produce downward type I mining fissures, which are collectively referred to as type I-I mining fissures. The depth of the descending type I mining fissures generated from the surface is affected by the geological occurrence conditions and the opening degree of the fractures [44]. For the relationship between fissure opening degree and fissure depth, Formula (1) can be used for calculations. \[h_{f}=\text{F}\sqrt{d} \tag{1}\]  
where \(h_{f}\) is the fissure depth, m; \(d\) is the fissure opening, m; F is the correlation coefficient, which is 8 for the loose soil layer and 15 for the bedrock layer.