# 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 Mining FISSURE Activation Period  
(1) Activation characteristics of type I-I mining fissures  
When the key strata are broken into a hinged rock beam structure, the key block C is in a stable state, and the lower broken caving rock blocks are compacted. The key block A has no obvious change, and the key block B turns counterclockwise, articulating with key blocks A and C, playing a bearing role in the upper rock layer. During the process of pushing the working surface to the key block A, the key block A has the same movement characteristics as the key block B, and the key block B rotates clockwise at the F\({}_{1}\) point until it is in the state of the key block C. When the key block B rotates counterclockwise, it carries the overlying rock and soil layer to produce similar coaxial rotation and subsidence, and the up and down type I-I mining fissures between the key blocks A and B are generated and expanded to form a fissure, as shown in Figure 6, displaying the mining fissure morphology. When the key block B rotates clockwise, the mining fissure between the key blocks A and B gradually closes, and the mining fissure between the key blocks B and C that tends to close is reactivated and expanded, with the level generated during the continuous advancement of the working face closing again under the action of extrusion stress. (2) Activation characteristics of type I-II mining fissures  
Mining of shallow coal seams is easy in terms of forming a stepped rock beam structure, and under the action of this structure, type I-II mining fissures are produced in the rock and soil layer. The broken block of the stepped rock beam structure formed by the key strata is divided into four parts, namely, D, E, F, and G. The key block G is in a stable state and only forms horizontal extrusion stress with the key block F. The key blocks E and F are the support blocks of the stepped rock beam. When the key blocks D and E form a relative cutting displacement, the rock and soil layer above them will form a synchronous migration phenomenon and form II mining fissures under the action of shear stress. At the same time, the surface produces step subsidence. Since the rock mass below the key block D has collapsed to form a free movement space, the key block D is prompted to rotate counterclockwise around the F\({}_{1}\) point and carry the rock and soil layer above it to rotate. Tensile stress occurs between rock and soil layers, and when the ultimate tensile strength is reached, type I mining fissures develop along a certain angle on the surface. With the mining of the working face, the key block D breaks to form a migration feature similar to that of the key block E, and the evolution process of mining fissures is almost the same. At this time, the key block E turns clockwise and moves downward, the mining fissure between the key blocks D and E gradually closes, with the mining fissure between the key blocks E and F expanding. When the stepped rock beam structure formed by the key block E is not unstable, the surface mining fissures still evolve cyclically based on the step drop. From the above analysis, it can be seen that under the action of mining rotation on the key blocks, the mining fissures located in the advancing direction of the working face experience five dynamic activation processes, namely, generation, expansion, closure, regeneration, and reclosure. (3) Analysis of mining fissure activation time  
The evolution process of mining fissures is periodic. In order to better study the relationship between mining fissures and the advancing distance of the working face, the research was carried out by combining the development position of mining fissures and the pushing distance of the working face during the field measurement and similar simulation tests.