## 2 Analysis of Overburden Load Transfer Mechanical Model  
### Roof Breaking and Load Transfer Process  
The breaking characteristics and caving morphology of the stope roof are closely related to the mining progress and advancing time. The coal seam starts from the open-off cut of the working face. With the advancement of the working face, the complete process of the breaking movement of the stope roof has four stages: initial mining, initial pressure, periodic pressure, and full mining. Corresponding to the complete process of roof breaking movement, there are three kinds of structures in the transfer of overburden load: beam structure, plate structure, and arch structure. The movement of rock strata before the initial mining roof rock mass does not produce cracks is small. With the mining of the working face, the roof of the direct roof and the basic roof strata begins to fall when the goaf reaches a certain distance. However, due to the filling effect of the residual coal in the direct roof and the mining area, the basic roof strata can often form a stable beam plate structure. The so-called beam plate refers to a conversion process from beam to plate structure, not a single structure. Through the research on the characteristics of the roof caving by many scholars at home and abroad, it is found that with the increase of the exposed area of the roof, the failure characteristics are often O-X type, that is, the roof first cracks at the bottom boundary near the coal wall and then cracks appear on the plate surface [31]. The upper rock layer of the main roof transfers the self-weight to the main roof, and the load gathers at the roof boundary and the coal wall and transfers to the deep rock mass. At this time, the load transfer can be regarded as a process of transfer through the beam structure. With the increase of the exposed area of the roof, the beam structure breaks after reaching its ultimate load capacity. After the beam structure of the roof boundary is unstable and broken, the load is difficult to transfer to the deep rock mass through the roof boundary and begins to transfer to the roof center, as shown in Figure 1.  
The beam structure often exists before the first weighting of the main roof. After the first weighting, the beam structure is destroyed, and the roof instability begins to bend, separate, and collapse. The collapse area increases with the increase of the hanging area of the roof advancing the working face, that is, the periodic weighting stage of the main roof. At this time, the way of load transfer and transfer in overlying strata has changed. Because the beam is broken and cannot be used as the carrier of load transfer to deep rock mass, the load begins to gather in the center of the roof and then spreads outward from the center. The transformation from beam structure to plate structure can be approximately regarded as the process from 'line' to'surface'. After the initial weighting, the stress concentration phenomenon begins to appear in the center of the plate and intensifies with the further increase of the exposed area until it breaks and collapses beyond the ultimate load capacity of the plate, and its fracture characteristics are often X-O type. Due to the weak load capacity of the overlying strata, the overlying strata bend and sink as a whole with the sinking and breaking of the basic roof. At this time, the transfer and transfer of load are carried out in the plate structure until the large-scale instability and breaking of the plate structure occurs [32]. Collapse and accumulation form a structural arch supported by the coal body at the open-off cut and the coal wall in front of the stope, as shown in Figures 2 and 3. Figure 1: Initial mining. Figure 2: First weighting. In terms of spatial form, the structural arch formed by the combination of overlying rock blocks in the working face or goaf after full mining is the secondary structure formed by the instability and fracture of the overlying rock plate structure.