# Stability and Force Chain Characteristics of \"Inclined Step Cutting Body\" in Stope
## 3 Results
### Distribution Characteristics of Force Chain  
#### 3.2.1 Analysis of Force Chain Evolution Law  
Force chain is an important concept about granular materials, and is usually a linear-like chain structure formed by the continuous contact of several or dozens of particles. Relevant studies have shown that the bending deformation of the force chain is the internal cause of the deformation of granular material. In the process of numerical simulation, with the increase of external load, the force chain will deform and cause the overburden to break. Therefore, the force chain is the main form of load transfer between particles [34]. Figure 6 is the force chain evolution process in coal seam mining. In the early stage of mining for the working face, the original balance state of the force chain was broken, and a horizontal force chain arch structure was formed above the goaf. When the coal seam is mined 40 m, the arch height is about 18 m, and the force chain arch takes the weight of the overlying strata in the goaf and makes the surrounding goat rebalance. The front and rear arch feet of the force chain are located on the side of the coal wall of the unmined coal seam, and the force chain distribution in the unmined area of the working face is in an irregular state, as shown in Figure 6.  
With the increasing range of the mined-out area, the span and height of the force chain arch increase. When the coal seam is mined 80 m, the working face collapses for the first time, and the strata are separated. The force chain shape originally located at the low level gradually develops from the arch state to the horizontal state, and a new force chain arch appears in the strata at a higher level, with an arch height of about 34 m.  
Figure 5: Numerical simulation of subsidence curve. When the coal seam is mined 160 m, the force chain arch develops to the surface, the height cannot be increased, and the span continues to increase. Due to exceeding its own bearing limit, the force chain arch cannot bear the load of the overlying strata, and fracture instability occurs. At this time, the working face is periodically collapsed, and the collapsed rock mass continues to fill the goal. Due to the fracture of the force chain arch, a large number of cracks appear in the overlying strata and connect with the surface loose layer, and step subsidence begins to appear at the surface. As the working face continues to advance, the arch force chain structure gradually disappears due to instability, and the force chain continues to develop forward. When the working face is pushed to 200 m and 248 m, the second and third periodic collapses occur respectively. At this time, at a lower position above the goal, due to the destruction of the force chain arch, the force chain distribution is transformed into a flat state. In front of the goat, the force chain is distributed in a vertical state. In summary, the form of the force chain affected by mining experienced the dynamic process of force chain arch formation--force chain arch development--force chain arch failure. The force chain arch is the main force system to carry the weight of the overlying strata. When it develops to the surface, fracture instability will appear because it cannot continue to develop. At this time, the working face shows periodic collapse, the collapsed rock mass continues to fill the goal, and step subsidence appears at the surface. In order to further analyze the stress distribution of overlying strata in the mining process of shallow thick coal seam, the support pressure change above the coal seam is monitored by arranging the measurement circle, as shown in Figure 7. When the coal seam is mined, the increased area of abutment pressure is located in the rear of the unmined coal body and the front of the working face, and the increased area of tensile stress is located directly above the working face. With the continuous advancement of the working face, the maximum abutment pressure values in the front and rear of the goat gradually increase.