## 4 Numerical Simulation
### Stress Analysis of Roadway-Surrounding Rock  
Figure 6 displays that the stress in the surrounding rock shows significant asymmetric distribution after the excavation of the roadway. It is interesting in Figure 6a that the horizontal stress is significantly released on the ribs of the roadway. Herein, the stress contour of the surrounding rock presents a semi-circular distribution. In this case, the high-level stress centralized region occurred at the bottom corner of the left and right ribs of the roadway, with the maximum stress value of 34.6 MPa, and the stress concentration coefficient is 1.4. As shown in Figure 6b, the different rock layers exhibit significant stress discontinuity because of dislocation due to the relatively fractured lithology of the coal rock mass. The vertical stress values that measured from the roof and floor of the roadway are relatively smaller than that of the ribs. Note that the high-stress contour on the right rib extends relatively wide to the deep rock mass. The high-stress centralized region is situated within the rock layer 1m apart from the right rib of the roadway, with a maximum stress value of 19.7 MPa. In this case, the large stress concentration coefficient of 2.49 results in the instability and damage of surrounding rock. The roadway is featured with a significant asymmetry, as the maximum principal stress distributed around the surrounding rock is evaluated. The peak values of the maximum principal stress in both ribs are located within the rock layers at a distance of 4 m from the  
Figure 5: FLAC3D numerical calculation model and model calculation diagram. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Rock**} & **Density** & **Tensile Strength** & **Bulk Modulus** & **Shear Modulus** & **Cohesion** & **Internal Friction** \\  & **(kg\(\cdot\)m\({}^{-3}\))** & **(MPa)** & **(GPa)** & **(GPa)** & **(MPa)** & **Angle (\({}^{\circ}\))** \\ \hline sandstone & 2370 & 4.54 & 1.42 & 0.88 & 3.50 & 39 \\ mudstone & 2110 & 1.65 & 1.36 & 0.80 & 3.53 & 25.8 \\ coal & 1315 & 0.98 & 0.96 & 0.60 & 1.58 & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical parameters of roadway-surrounding rock. left and 5 m from the right sides, respectively. The influence range is large, with a stress value of 13.1 MPa, mainly dominated by compressive stress. It is in Figure 6d that the distribution of shear stress in the surrounding rock exhibits obvious stress discontinuity, mainly attributed to the occurrence features of coal and rock. The maximum shear stress areas mainly dominated in tension are observed from the bottom corners of the ribs, with a maximum shear stress value of 13.7 MPa. The stress change curve of the surrounding rock of the roadway is seen in Figure 7.  
As seen in Figure 7a, there is a significant tensile stress at the right rib of 0.7 MPa. However, the significant compressive stress on the left rib is small. Particularly, the horizontal compressive stress on both ribs gradually increases, with the increasing depth of the surrounding rock. This situation will last and become stable at a depth of 6 m.  
It is apparent in Figure 7b that the significant tensile stress was observed at the floor of the roadway, with a stress value of 0.54 MPa. However, there is a significant compressive stress at the surrounding rock of the roof, with a smaller stress value. As the surrounding rock's depth increases, the vertical compressive stress on the roof and floor gradually increases. Even though the growth trend of the stress on the roof is greater than that on the floor, it will finally be stable at a distance of 5.5 m from the roof and floor. As shown in Figure 7c, an obvious shear stress occurred at the bottom corners of the ribs. The shear stress value at the right corner is greater than that at the left corner, with a shear stress value of 15.1 MPa.