## 4 Numerical Simulation Analysis
### Distribution of the Mining Stress Field in Front of the Working Face  
Working face mining disrupts the original rock stress equilibrium state and redistributes stresses. Due to the strong influence of mining, stress concentration occurs in the coal pillars in front of the working face. The stress field changes after the stress redistribution. The changes in P1 and P3 in the range of 0-70 m for the front coal pillars were tested to obtain the stress field distribution change law, as shown in Figure 9.  
The original rock stress (\(\gamma\)H) at the site without mining disturbance is 5 MPa. P1, in the coal pillars in front of the working face, sharply increases from less than \(\gamma\)H to a peak  
Figure 8: Numerical simulation model and mechanical parameters of each coal and rock stratum. Figure 9: Distribution of the 0–70 m mining stress field in front of the working face. of 22.35 MPa at 2.5 m, which is 4.47 times the original rock stress; then, it decreases to 2\(\gamma\)H at a faster rate and slowly returns to \(\gamma\)H when the stress magnitude moves further from the working face. P\({}_{3}\) relatively quickly increases from 0 to a peak of 10.42 MPa at 5 m in front, slowly decreases to \(\gamma\)H and, subsequently, stabilizes. Based on the above analysis, the numerical simulation results for the stress field distribution in front of the working surface are consistent with the theoretical analysis. The stress concentration phenomenon caused by mining leads to a nonuniform distribution of P\({}_{1}\) and P\({}_{3}\), and forms a deviatoric stress environment. Moreover, while the stress is concentrated, the directions of P\({}_{1}\) and P\({}_{3}\) will simultaneously be deflected at a certain angle. Protective coal pillars with different widths will mean that the ARC is located under different degrees of deviatoric stress, and the roadway surrounding rock will be damaged in different forms. Shape Characteristics of the Plastic Zone in the Surrounding Rock of the Auxiliary Retracement Channel in the Deviatoric Stress Environment  
With the changes in the width of the protective coal pillars between the two retracement channels, the evolution characteristics of the shape of the plastic zone in the ARC surrounding rocks in different deviatoric stress environments are obtained after the working face has been cut through the MRC. With the increasing width of the protective coal pillars, the plastic zones in the ARC are butterfly shaped, elliptical and circular, as shown in Figure 10.  
For example, Figure 10 shows that the surrounding rock plastic zone is uniformly distributed along the centerline of the rectangular roadway when the retracement channel is not influenced by mining. The plastic zone in the two sides and floor has a maximum depth of 0.5 m, while that of the roof is 1 m.  
When the working face is cut through the MRC, the self-weight stress of the overlying rock is transferred to the protective coal pillars between the two retracement channels. When the protective coal pillars are narrow, the ARC is in a high deviatoric stress environment with stress concentration and deflection, which forms a butterfly-shaped plastic zone. Deflection of the butterfly-shaped plastic zone occurs. The deepest part of plasticity occurs above the roof, the plastic zone at the roof expands to the secondary side, and the plastic zone at the floor expands to the primary side (it is defined as the primary side near the working face, and the far side is the secondary side). Asymmetric, large deformation occurs in the ARC (Figure 10b-d). When the width of the protective coal pillars increases, the ARC is located far from the working face and the mining stress influence is reduced. The concentration degree, deflection angle, and uneven distribution of stresses are weakened, which gradually reduces the range of the plastic zone. As shown in Figure 10e-h, the plastic zone is concentrated in the corner of the rectangular roadway, and the deepest part is in the roof near the secondary side, which has an elliptical shape.