## 5 Numerical Simulation of Surrounding Rock Control in Retracement Roadway
### Analysis on Supporting Effect of Surrounding Rock in Retracement Roadway  
The discrete element UDEC (v.5.0) software [29] was used to establish a numerical model measuring 200 m in length and 83 m in width. The upper \(2^{-2}\) coal seam was modeled with a mining height of 4 m and a mining range from 20 m to 180 m. Before mining the \(3^{-1}\) coal, an RR was first excavated at 140 m with a width of 4.5 m and a height of 3 m. The software's built-in \"cable\" and \"support\" support commands were used to construct the bolts and caving-style supports for the RR, respectively. Then, the section from 36 m to 140 m was mined to study the stress distribution and deformation characteristics of the surrounding rock in the RR under different support schemes during the final extraction of the 3301 working face, as depicted in Figure 9.  
Figures 10-12 illustrate the stress distribution and displacement nephograms of the overlying strata when the working face and the RR are interconnected under different support schemes. Upon interconnection, the main roof rotates towards the goaf side, and the coal wall within 2 to 8 m in front of the working face is the peak stress zone. As shown in Figure 10, in Scheme 1, the peak stress on the solid coal side reaches 16 MPa, the RR roof subsides by 0.5 m, and the top cable slips and deforms with the rotation of the roof rock  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**RR Size: 4500 mm \(\times\) 3000 mm**} & \multicolumn{2}{c}{**Scheme 1**} & \multicolumn{1}{c}{**Scheme 2**} & \multicolumn{1}{c}{**Scheme 3**} \\ \cline{3-5}  & & **(Original Support)** & & & \\ \hline \multirow{5}{*}{Roof bolt} & Rebar bolt & \(\Phi 20\times 2200\) mm & \(\Phi 18\times 2200\) mm & \(\Phi 18\times 2200\) mm \\  & & 1000 \(\times\) 900 mm & & \\  & Inter-row spacing & (5 in each row) & 1000 \(\times\) 1100 mm & 1000 \(\times\) 1000 mm \\  & Anchoring force & 125 kN & 125 kN & 125 kN \\  & Tightening torque & 120 N\({}^{\text{m}}\)m & 150 N\({}^{\text{m}}\)m & 150 N\({}^{\text{m}}\)m \\  & Anchor cable specifications & \(\Phi 17.8\times 8000\) mm & \(\Phi 21.6\times 6000\) mm & \(\Phi 21.6\times 6000\) mm \\  & & 3000 \(\times\) 1800 mm & & \\  & Inter-row spacing & (Rectangular) & 3000 \(\times\) 2200 mm & 3000 \(\times\) 2000 mm \\  & Anchoring force & 240 kN & 240 kN & 240 kN \\  & Pre-tightening force & 150 kN & 150 kN & 150 kN \\  & Rebar bolt & \(\Phi 16\times 1800\) mm & \(\Phi 18\times 2400\) mm & \(\Phi 18\times 2400\) mm \\  & & 1000 \(\times\) 1100 mm & 1000 \(\times\) 1100 mm & 1000 \(\times\) 1000 mm \\  & & (3 in each row) & & \\  & Anchoring force & 100 kN & 105 kN & 105 kN \\  & Tightening torque & 100 N\({}^{\text{m}}\)m & 140 N\({}^{\text{m}}\)m & 140 N\({}^{\text{m}}\)m \\ \hline \hline \end{tabular}
\end{table}
Table 2: Design scheme of internal support parameters of RR.  
layer. As shown in Figure 11, in Scheme 2, the peak stress on the solid coal side reaches 25 MPa, and the RR roof subsides by 0.4-0.8 m, resulting in side spalling of 0.3 m. As shown in Figure 12, in Scheme 3, the amount of coal wall spalling on the coal side is small, and the coal wall within 2 to 8 m in front of the working face is the peak stress zone, where the peak stress reaches 14 MPa and the RR roof subsides by 0.2 m. Therefore, Scheme 3 was identified as the most effective internal support scheme for the RR under this condition.  
Figure 10: Scheme 1 shows the stress distribution and displacement change cloud diagram of overlying strata during the end mining of the working face.  
Figure 9: Caving characteristics of overlying strata 40 m away from withdrawal roadway in the working face.