## 3 Numerical Model and Calculation Scheme
### PFC Simulation Results Analysis
#### 3.3.3 Overburden Stress Field Evolution Analysis  
In the particle dynamics simulation, the force chain is the force frame of the whole overburden, bearing the external force load in the vertical and horizontal directions. The failure of the force chain indicates that the internal balance of the particles is broken, and the external force promotes the redistribution of the force chain until the new force chain is formed stably. A strong and quasi-linear shape, passing a larger share of the force, is called a strong chain. Deformation is weak and bear force is small, which is called the weak force chain, as shown in Figures 21 and 22. When the coal seam is not mined, the rock layer is in a stable stress balance state, the force chain is in a crisscross network state, the contact form is closed, and the strong and weak force chains jointly bear the load of the overlying rock layer. After the coal seam mining, the stress balance is broken, and the external force promotes the redistribution of the force chain. The strong chain changes with the advancing direction of the working face. The main direction of the force chain indicates the direction and path of load transfer. Figure 21: Regional force chain. As can be seen from the Figure 23, when the coal seam mining after the original stress balance is broken, the overlying strata stress begins to adjust the distribution. The equilibrium state of the particles in the model was broken and began to move. The direction of the contact force chain between the particles began to change and gradually concentrated from the intricate to a certain direction. When the stope advances 10 m, the immediate roof above the mined-out area loses the support of coal, and the load cannot be transmitted directly downward and begins to be transmitted downward along both ends of the roof. Due to the short advance distance of the stope, only a few small fractured rock strata are generated inside the immediate roof. The displacement is small, and the structure is stable. The load on the beam above the stope is small, and the bending degree is small. At this time, the load is mainly concentrated on both sides of the coal wall. The weak force chain around the strong chain transfers the load to the surrounding area of the stope. The vertical stress peak of the overlying strata in the stope is about 22 MPa. When the working face advances 20 m, 30 m, and 40 m, the strong chain zone at the coal wall of the immediate roof collapse begins to expand around, and the load is mainly concentrated on the basic roof and the coal walls on both sides. The basic roof is thick, and the structure is stable. The main object of the load is to bear the load. At this time, many small cracks appear in the 'beam' of the basic roof, and the cracks gradually develop and expand with the advancing distance of the working face. At this time, the maximum overburden vertical stress peak is about 34 Mpa. When the working face advances 50 m, with the increase of the advancing distance of the working face, the space of the goal gradually increases, the exposed area of the roof and the load also increase, the internal cracks of the beam develop maturely and break, the load gradually transfers from the 'beam' to the center of the plate surface, the strong chain gradually evolves into the horizontal form between the plates and the separation phenomenon appears in the main roof. At this time, the vertical stress peak of the overburden rock is about 38 Mpa. When the working face advances to 60 m due to the periodic collapse of the main roof, the strong chain between the plates begins to weaken, and the strong chain gradually transfers to the higher position rock formation to form a new strong chain arch. The arch height increases slightly with the further increase of the span of the force chain arch and the development of the cracks in the main roof through the whole rock formation, resulting in the fracture and collapse of the main roof. At this time, the roof is simply supported on three sides and fixed on one side.