## 2 Current Status of Support Instability in the Working Face of SDCS
### Evolution Law and Distribution Characteristics of Top-Coal Stress  
By arranging the monitoring plane on the upper surface of the coal seam, stresses at each monitoring point are recorded. By extracting the front abutment pressure of the working face under different propulsion distances, the maximum value of the front abutment pressure is identified. To intuitively reveal the variation law of the front abutment pressure peak with the working face advancement, a 3D point cloud diagram (as shown in Figure 11) is plotted. In the diagram, blue dots denote stress values, while red dots indicate the peak positions of the front abutment pressure.  
When the working face advances to 20 m, 40 m, and 180 m, respectively, the peak position of the front abutment pressure is 27 m, 43.75 m, and 45 m, respectively, along the inclined direction of the working face. With the increase in the advancing distance of the working face, the peak position of the front support pressure gradually moves from the lower part of the working face to the middle part and finally remains stable.  
When the advancing distance is between 20 and 100 m, the peak value of the front abutment pressure increases at the fastest rate. The value increases from 10.31 MPa to 15.74 MPa, with an average increase of 1.36 MPa in the peak value for every 20 m of advancement. When the advancing distance is between 100 and 300 m, the growth rate slows down. The value increases from 15.74 MPa to 17.61 MPa, with an average increase of 0.19 MPa in the peak value for every 20 m of advancement. After the working face has advanced 300 m, both the peak value and the position remain stable.  
When the working face advances to 300 m, the mining-induced stress ahead of the working face stabilizes. Principal stress data at the upper boundary of the top coal ahead of the working face is extracted within a range of 100 m in length and 60 m in width. The distribution law of the maximum and minimum principal stresses in the top coal at the 300 m advancement point is plotted, as illustrated in Figure 12.  
Figure 11: Evolution process of front abutment pressure peak of top coal.  
The principal stress within the top coal along the working face exhibits a distinct distribution pattern characterized by an initial increase, followed by a decrease, and ultimately stabilizing.  
The maximum principal stress distribution in the top coal along the inclined direction of the working face can be categorized into two distinct zones: the mining-induced stress influence zone and the stress stabilization zone. The region preceding the blue cross-section represents the mining-induced stress influence zone, where the maximum principal stress initially rises and subsequently declines along the inclined direction. Conversely, the area following the blue cross-section constitutes the stress stabilization zone, wherein the maximum principal stress demonstrates a fluctuating pattern of increase, decrease, increase, and then decrease along the inclined direction.  
The minimum principal stress distribution in the top coal along the inclined direction of the working face can be categorized into two distinct zones: the mining-induced stress influence zone and the stress stabilization zone. The region preceding the blue cross-section in the figure corresponds to the mining-induced stress influence zone, where the minimum principal stress initially increases and then decreases along the inclined direction. In contrast, the area following the blue cross-section represents the stress stabilization zone, where the minimum principal stress decreases gradually along the inclined direction.  
Figure 12: Distribution law of principal stress in top coal. (**a**) Distribution law of maximum principal stress in top coal. (**b**) Distribution law of minor principal stress in top coal.