## 4 Results and Analysis
### Analysis
The theoretical calculation results of rock fragmentation and expansion show that when the sand-mud ratio is 6:4 and the sand layer coefficient exceeds 66.7%, the maximum development height of water-conducting fractures produced by 5 m and 3 m mining heights is very small, only within 0.4 m; when the sand-mud ratio is 8:2 and the sand layer coefficient exceeds 83.3%, the maximum development height of water-conducting fractures produced by 5 m and 3 m mining heights is very small, only within 0.4 m; when the sand-mud ratio is 8:2 and the sand layer coefficient exceeds 83.3%, the maximum development height of water-conducting fractures produced by 5 m and 3 m mining heights is very small, only within 0.2 m. The results of the two methods are highly consistent, which shows that: First, under the same buried depth, the increase in the number of sandstone layers weakens the anti-interference ability of the overlying rock mass to the small mining height, so that when the sand layer coefficient exceeds 70-80%, the maximum development height of the water-flowing fracture caused by the small mining height is equivalent to the large mining height. Secondly, the overburden structure of coal seam in the study area should meet the conditions of \"sand-mud ratio 6:4 and sand layer coefficient less than 70%\" and \"sand-mud ratio 8:2 and sand layer coefficient less than 80%\" so as to achieve the purpose of water-preserved coal mining through the method of height-limited mining. 3. Under the condition of a fixed sand-mud ratio, the fracture-mining ratio will increase with the increase in the sand layer coefficient, and the increase in mining height will significantly reduce the increase in the fracture-mining ratio. Both the numerical simulation test results and the theoretical calculation results of rock fragmentation and expansion show that when the \"sand-mud ratio is 6:4 + mining height is 5 m or 3 m\", the fracture-mining ratio increases uniformly with the increase in sand layer coefficient. When the \"sand-mud ratio is 8:2 + mining height is 5 m\", the fracture-mining ratio increases with the increase in sand layer coefficient, and the increase is not obvious. When the \"sand-mud ratio is 8:2 + mining height is 3 m\", the fracture-  
4.2.2 The Influence of Sand-Mud Ratio on the Maximum Development Height of Water-Conducting Fracture  
According to Table 4, when the mining height is 5 m and 3 m, the corresponding relationship curves between the sand-mud ratio of coal seam overburden and the maximum development height and fracture-mining ratio of water-conducting fracture based on numerical simulation tests (N-SMR-6:4 and N-SMR-8:2) and the maximum development height and fracture-mining ratio of water-conducting fracture based on the theory of rock fragmentation and expansion (T-SMR-6:4 and T-SMR-8:2) are drawn, as shown in Figures 7 and 8. The following can be seen from Table 4 and Figures 7 and 8:  
Figure 8: Columnar diagram of the corresponding relationship between sand–mud ratio and fracture-mining ratio. (**a**) The mining height is 5 m. (**b**) The mining height is 3 m.  
Figure 7: The corresponding relationship curve between sand–mud ratio and the maximum development height of water-conducting fracture. (**a**) The mining height is 5 m. (**b**) The mining height is 3 m.  
Under the condition of a fixed sand layer coefficient, the maximum development height of a water-conducting fracture increases with the increase in the sand-mud ratio. That is to say, under the same buried depth, the greater the total thickness of sandstone in the coal seam overburden is, the greater the maximum development height of the water-conducting fracture.