## 4 Results
### Dynamic Evolution Mechanism of Surface Cracks  
From Figure 4, it can be seen that the dynamic evolution of crack-I is controlled by block M and block N. The width of crack-I will diminish during the mining process due to the M block's rotational motion, while it will rise due to the vertical sliding motion. Additionally, the N block's sliding down the slope will result in an increase in crack-I. Thus, the mechanism of surface crack activity can be clarified through computing the vertical stability coefficient and slope stability coefficient using Formulas (12) and (19). Table 4 displays the findings computed using Formulas (12) and (19) in accordance with the actual mining circumstances of the 25,214 working face  
Figure 16: Height of the WCFZ with the mining of the \(4^{-2}\) coal seam and \(5^{-2}\) coal seam. Figure 17: Development process of the WCFZ. It can be seen from Table 3 that when \(\alpha>6.4^{\circ}\), \(Q_{\rm A}/\rm T\) is always greater than 0.5, which indicates that the surface blocks will experience vertical sliding, and the collapsed cracks begin to develop during the process of 24,205 working face. In addition, when \(\alpha>42^{\circ}\), \(\eta_{N}\) begins to be less than 1.0, which indicates that with the mining of the 24,205 working face, the sliding cracks induced by the surface blocks sliding along the slope begin to appear. In the 25,214 working face, the conditions for causing collapsed cracks and surface blocks sliding along slopes during mining are \(\alpha>7^{\circ}\) and \(\alpha>30^{\circ}\), respectively. The dynamic evolution process of surface cracks is recreated, and the activity mechanism is clarified, by examining the stability and mobility properties of surface block during coal seam mining. The dynamic evolution process of surface cracks in the \(4^{-2}\) coal seam and \(5^{-2}\) coal seam mining is shown in Figure 17. It is evident from the information in Section 4.4.2 that the surface crack DC1 is a boundary crack, which results from block B's departure along the mining direction of working face under tensile stress. Block B continuously veered in the direction of mining due to the mining of \(4^{-2}\) coal seam and \(5^{-2}\) coal seam, which resulted in an increase in the width of surface crack DC1. Previous research results have shown that the blocks above the goat typically experience deflection cracking and rotation movement with coal seam mining, which is the key to causing the crack width to first expand and then diminish without considering the terrain. The width of crack DC5 developed in the flat area at the top of the slope has experienced two \"increase-decrease\" shifts as a result of the \(4^{-2}\) coal seam and \(5^{-2}\) coal seam mining (Figure 18g-o). The blocks on each side of the valley area's cracks, however, have additional movement characteristics due to coal seam mining under the terrain effect, which causes the cracks to evolve dynamically in diverse ways. For example, the block C experienced deflection cracking, rotation and vertical sliding with the \(4^{-2}\) coal seam mining, resulting in an \"increase-decrease-increase\" in the width of the DC2 crack. Following the mining of the \(5^{-2}\) coal seam, the block C experiences deflection cracking and rotation, causing the DC2 crack's width to fluctuate between increasing and reducing. With the mining of the \(4^{-2}\) and \(5^{-2}\) coal seam, the block D, which regulates the change in DC3 crack width, has encountered two-degree \"deflection cracking-rotating-vertical sliding\". The DC3 crack width changes twice as a result of this repeating motion: \"increase-decrease-increase\" (Figure 18c-o). Block E regulates the activity of the DC4 crack above the slope. The breadth of the DC4 crack increased steadily as a result of the \"deflection cracking-oblique sliding-vertical sliding\" movement that occurred in the E block during the mining of the \(4^{-2}\) coal seam. However, the breadth of the DC4 crack once again changed, first expanding and then decreasing, as a