## 3 Results and Discussion
### Results of Calculations of Grouting Subsidence Reduction for Different Grouting Opportunities  
This study considers conducting full-range grouting treatment within 0 to 3 years after multi-seam mining. Figure 10 shows the calculation results for the cloud maps of the residual subsidence of the goat for different grouting opportunity conditions. The residual subsidence contour indicates grouting is carried out closer to the mining stopping on the working face, resulting in more residual subsidence in the goat of the multi-coal seam, effectively controlling surface subsidence. When grouting is carried out after mining stops for two years, there is little difference in the subsidence contours of the goat in the multi-coal seam. In addition, the impact of grouting opportunities on the grouting treatment effect of the goat in the multi-coal seam is weakened, since the residual subsidence capacity of the goat in the multi-coal seam gradually weakened after the completion of coal seam excavation. Therefore, when the residual subsidence capacity is large, grouting at a time point closer to the cessation of mining can produce better grouting and subsidence reduction effects. In summary, the later the grouting opportunity is, the greater the residual subsidence in the goat of the multi-coal seam that has already occurred before grouting, resulting in a small amount of grouting subsidence reduction and poor grouting subsidence reduction effect. The relationship between the subsidence reduction ratio R at the center of the subgrade and the opportunity for grouting is plotted in Figure 11. From the graph, it can be concluded that immediately after the coal seam is mined, grouting has the best effect on reducing subsidence with a subsidence reduction ratio of 67.2% at the center of the subgrade. The longer the distance from stopping mining, the subsidence reduction ratio at the center of the subgrade gradually decreases after grouting treatment, and the subsidence reduction rate curve shows an exponential relationship. When grouting is carried out after mining is stopped for 2 years with the extension of grouting opportunity, the rate of decrease in the center subsidence rate of the subgrade gradually decreases, and the curve tends to be flat. Figure 9: Subgrade incline value and incline reduction rate for different grouting range conditions. In summary, due to the completion of the excavation of the working face, the surface subsidence rate gradually slows, and the residual deformation capacity gradually decreases. When the residual deformation capacity is large then grouting is performed closer to the completion of the excavation of the working face, which can produce better grouting and subsidence reduction effects. Figure 10: Residual subsidence nephogram for different grouting opportunity conditions. Figure 12 shows the variation of the center subsidence reduction ratio of the subgrade for different grouting opportunities and coal seam spacing conditions. From the figure, it can be seen that with the extension of grouting opportunity and the decrease in OB/IB, the subsidence reduction ratio at the center of the subgrade decreases exponentially. When \(1\leq\text{OB/IB}\leq 2\) and grouting is carried out 2 years after excavation, the subsidence reduction ratio of the subgrade center decreases rapidly, and the grouting treatment effect is poor. This is because increasing the spacing between coal seams can lead to increased deformation of the surface and goat, and even with grouting treatment, it is difficult to completely prevent the problem of surface subsidence. With the extension of grouting opportunity, there is a significant amount of residual deformation in the goat, resulting in a decrease in the contact area between grouting materials and fractured rock masses and cracks in the goat, which influences the grouting effect. Figure 11: Subgrade center subsidence reduction rate for different grouting opportunity conditions.