## 5 Results and Discussion
### Method for Determining the Reasonable Spacing of Two Retracement Channels  
At the end of the mining period, after the working face has been cut through the MRC, the self-weight stress of the overlying rock is mainly transferred to the protective coal pillar between the two retracement channels, so the spacing of the two retracement channels becomes critical to protect the stability of the ARC. Determining the reasonable spacing of the two retracement channels optimizes the channel layout under the premise of ARC stability, minimizes the size of the protective coal pillars, and improves the productivity of coal mining enterprises, while reducing the waste of coal resources.  
The key to reasonable spacing is to ensure the stability of the surrounding rock in the ARC during the retracement of the equipment. Based on the above analysis, Figure 11 illustrates the ARC with different protective coal pillar widths in the stress environment, the plastic zone shape characteristics, and the surrounding rock stability state.  
Mining makes P1 and P3 nonuniformly distributed in magnitude and direction in front of the working face, which results in ultimate stress equilibrium, high deviatoric stress, decreasing deviatoric stress, and low deviatoric stress environments. With an increase in the double retracement channel spacing, the ARC is in different stress environments during the end of the mining period, which produces different shape characteristics of the plastic zone.  
The ARC is in the ultimate stress equilibrium environment when the coal pillars are set extremely narrow (0-5 m). Because of the stress field migration, the protective coal pillars have experienced the action of multiple stress fields, which have entirely undergone plastic deformation and lost their bearing capacity. The surrounding rock in the ARC is severely non-symmetrically deformed and extremely unstable. When narrow coal pillars  
Figure 11: Reasonable spacing determination method.  
(5\(\sim\)20 m) are set, the ARC is in a high deviatoric stress environment, the plastic zone has a butterfly shape, and the surrounding rock is asymmetrically deformed with poor stability and challenging support control. When moderate-width coal pillars (2\(\sim\)40 m) are set, the ARC is in the decreasing deviatoric stress environment. The shape of the plastic zone is elliptical and more uniformly distributed, which can control the stability of the surrounding rock through the support of anchor bolts and anchor cables. When wide coal pillars (40\(\sim\)70 m) are set, the ARC is less influenced by mining and is in a low deviatoric stress environment. The shape of the plastic zone is circular, which has stable surrounding rock, but the coal pillars cannot be recovered, and additional resources are wasted.  
The width of the protective coal pillars that correspond to the different shape characteristics of the previously mentioned plastic zones is not an invariant quantity. In general, greater ground stress and weaker mechanical strength of the rock correspond to wider protective coal pillars and greater stability in the ARC.  
The above analysis shows that the reasonable spacing of the two retracement channels is 20\(\sim\)40 m; however, this range is not invariant and can be adjusted according to actual environmental changes. The key to reasonable spacing is to ensure that the protective coal pillars are sufficiently wide to mean that the ARC is in a decreasing deviatoric stress environment, and the plastic zone should exhibit a stable elliptical shape.