## 2 Stability Analysis of Ultra-Large Span Open-Off Cutting Areas
### Numerical Analysis of Support Parameter Design  
Monitoring of displacements, stress states, and plastic deformation zones at 30 m and 60 m positions in the roadway for coal mining revealed the following simulated results:  
(1) Displacement field analysis of surrounding rock  
As shown in Figure 7, numerical simulation results indicate that compared to support design Scheme 2, both the bottom heave in the floor and floor heave in Scheme 1 remain within 40 mm. However, the peak subsidence of the roof in Scheme 2 reaches 81.3 mm, while in Scheme 1, the peak subsidence of the roof is 67.9 mm. Under the conditions \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Lithology** & **Density/(kg\(\cdot\)m\({}^{-3}\))** & **K/GPa** & **C/MPa** & **\%/(\({}^{\circ}\))** \\ \hline Coarse sandstone & 2390 & 8.3 & 4.0 & 30 \\ Coal & 1390 & 4.2 & 1.0 & 28 \\ Mudstone & 2250 & 5.0 & 2.0 & 30 \\ Sandy mudstone & 1990 & 6.0 & 2.5 & 30 \\ Fine sandstone & 2550 & 10.4 & 5.4 & 33 \\ Medium-grained sandstone & 2050 & 9.4 & 4.4 & 31 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Rock mechanical parameters. Figure 6: Numerical model diagram. of Scheme 1, compared to Scheme 2, the sidewall displacements are reduced by 16.6%, and the roof subsidence is reduced by 16.3%. Therefore, Scheme 1 exhibits superior support effectiveness in controlling the roof layer displacement in the roadway compared to Scheme 2. (2) Stress field analysis of surrounding rock  
When comparing the stress fields of surrounding rock for the two support design schemes, it is observed that under Scheme 1, the load-bearing capacity of the surrounding rock is increased, resulting in a reduction in peak compressive stress by 3 MPa compared to Scheme 2. The stress concentration areas on both sidewalls are reduced for both schemes. Both schemes exhibit a bimodal stress relief area on the roof, with peak tensile stress concentrated in the middle of the roof. In Scheme 1, the peak tensile stress is reduced by 1 MPa compared to Scheme 2. The stress relief range in the sidewall is reduced by 5% in Scheme 1 compared to Scheme 2, and the high-stress concentration range is reduced by 21%. Therefore, Scheme 1 exhibits a significant effect in controlling the degree of stress concentration in the surrounding rock of the roadway compared to Scheme 2. (3) Plastic zone analysis of surrounding rock  
Through numerical simulation calculations, the distribution of plastic zones in the surrounding rock of the roadway is obtained for both support design schemes, as shown in Figure 8.  
Figure 8: _Cont._  
Figure 7: Displacement field of surrounding rock: (**a**) rock displacement in the surrounding area at 30 m and 60 m for Scheme 1; (**b**) rock displacement in the surrounding area at 30 m and 60 m for Scheme 2. From Figure 9, it is evident that under support design Scheme 1, the distribution range of the plastic zone in the surrounding rock is significantly reduced compared to Scheme 2. In Scheme 2, there is a tension-shear failure zone near the roadway, whereas in Scheme 1, shear failure is observed around the tunnel, with no tension-shear failure. The depth of floor failure in Scheme 2 is greater than that in Scheme 1, with a relatively larger range of deformation failure. Numerical simulation demonstrates that under Scheme 1, the support enhances the stability of the roadway, significantly reducing plastic failure in the top corners of the tunnel and resulting in a 26% decrease in the plastic failure area. Through a numerical simulation and analysis using FLAC3D, a comparison of displacement variations, stress states, and plastic failure zones of the surrounding rock in the roadway was conducted. It is evident that the support effectiveness of Support Scheme 1 surpasses that of Scheme 2 obtained through analogous methods. Scheme 1 effectively maintains the stability of the surrounding rock in ultra-large span roadway excavation, achieving the required support effect.