# Stability and Force Chain Characteristics of \"Inclined Step Cutting Body\" in Stope
## 4 Discussion  
Based on the 22,021 working face, the numerical calculation model is established by PFC, and the overburden caving form and force chain evolution law in the process of the working face advancing are analyzed. It should be noted that the research results of this paper are most suitable for mining in shallow thick coal seam. The main discussion is as follows:  
1. Under the condition of shallow buried thick coal seam mining, there is trapezoidal damage above the goat. When the overlying rock falls periodically, periodic cracks appear on the surface. Due to the shallow buried depth, after the end of the mining face, the surface cracks are connected with the working face to form a penetrating mining crack, and the surface subsidence value will change due to the step damage. 2. A large number of scholars have carried out a lot of research on the type of overburden structure, formation conditions and motion characteristics, forming the \"masnorry beam\" [25], \"step rock beam\" [5] theory and analyzing its structural stability. Different from the existing results, this paper considers that the block formed by the fracture of overlying strata in a mined-out area under the condition of shallow buried thick  
Figure 8: Curve track and height relation diagram of the force chain arch. (**a**) Curve track of force chain arch. (**b**) Arch height relationship of force chain. coal seam is an inclined block, rather than a normal block. Therefore, according to the collapse morphology, it is found that the overlying rock will form an \"inclined step cutting body\" structure in the process of coal seam mining. Through the mechanical analysis of key blocks, it is concluded that the main factors affecting the stability of the structure are rock block size and rotation angle. Because the rock block index of a shallow coal seam is generally more than 1.0, the structure is prone to sliding instability. At the same time, the condition equation for solving the support force is provided, and the calculation results are in line with the actual support force in the mining process of the working face. 3. Particle matter is a complex system with internal organic connections formed by the interaction of many discrete particles. The force chain is a linear and relatively stable structure linked between particles in contact. In References [33; 34], the force chain and its influence on particle flow motion were discussed. Different from the existing research, this paper links the force chain to the overburden movement, which provides a new idea for explaining the surface subsidence damage. In the mining stage of the working face, there is a force chain network structure in the overlying rock, and the force chain morphology undergoes a dynamic process of force chain arch formation--force chain arch development--force chain arch failure. When the force chain arch develops to the surface due to the failure of continuous development, fracture instability will occur. At this time, the working face will collapse periodically, and there is step sinking at the surface. In Reference [17], the authors fitted the equation of arch height and coal seam mining distance, but it was not applied to practice and the influence of abutment pressure on the span of the force chain arch was not considered. According to the evolution law of force chains, the force chain arch curve equation is modified and applied to another working face in the same mining area. The theoretical calculation results are consistent with the actual situation, which proves that the failure of the force chain arch is the root cause of the surface step subsidence. The highlights of this study:  
1. In this study, for mining under the condition of a shallow buried thick coal seam, the structure of an \"inclined step shear body\" was proposed for the first time, and its correctness was verified by numerical simulation.