## 2 Engineering Background and Numerical Simulation Scheme
### Step-by-Step Store Structural Arrangement Method  
The main mining method for the large stopes north of the Dongguashan 60 line involves controlling the thickness of each blast in the stope direction, with the amount of explosive used for each blast kept below 5 tons. During the step-by-step extraction process, the volume of the mined-out area gradually increases until the entire stope is extracted, \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Type** & **Density \(\rho\)** & **Cohesion \(\varsigma\)** & **Internal Friction** & **Tensile Strength** & **Bulk** & **Shear** \\  & **(g/cm\({}^{3}\))** & **(MPa)** & **Angle \(\varphi\) (\({}^{\circ}\))** & **(MPa)** & **Modulus/GPa** & **Modulus/GPa** \\ \hline Surrounding Rock & 2.71 & 2.61 & 34.5 & 2.82 & 9.64 & 4.25 \\ Copper Ore Body & 3.97 & 3.89 & 41.1 & 3.62 & 13.2 & 6.72 \\ Fill Material (1:6) & 1.83 & 0.546 & 35.70 & 0.40 & 0.32 & 0.24 \\ Fill Material (1:10) & 1.74 & 0.189 & 27.27 & 0.10 & 0.08 & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical and mechanical parameters for numerical simulation of rock and fill material in Dongguashan. Figure 1: Surrounding rock model and 60 line stope model. followed by filling. As shown in Figure 2a, the extraction sequence for the first step of the 60 line slope advances gradually from the 62 line side to the 60 line. However, this extraction method has led to severe collapses in the mined-out areas such as 60-24\(\#\), 60-8\(\#\), and 60-12\(\#\), which affect the extraction of the adjacent second-step slopes. Although this extraction method is the primary approach for the Dongguashan deposit, it does not meet the current production demands for some tall and fragmented rock mass slopes, where the collapse of mined-out areas is significant. Considering the extraction sequence within the stope and structural factors, three optimization plans have been developed. Plan One: Gradually Extracting from Both Sides of the Stope Towards the Center  
This plan involves gradually extracting from the ends of the 60 and 62 lines towards the center, with a specified thickness for each extraction. After the blasting is completed, the ore is removed, followed by the next blasting cycle, ultimately leaving a certain thickness of mineral pillar in the center of the stope for overall extraction. This plan requires the construction of access tunnels in the two-step mineral pillars to facilitate entry and exit for the staggered blasting operations. Since the central area of the stope is the most susceptible to damage, extracting it last reduces the exposure time in this region, which is beneficial for maintaining the stability of the ore body. However, the mineral pillar at the final extraction stage has stope areas on both sides, necessitating assurances of the pillar's safety thickness to ensure the safety of blasting operations. Plan Two: Extraction-Filling-Extraction  
In response to the significant height of the stope north of the 60 line, this plan proposes to first extract half of the stope, followed by filling, and finally extracting the remaining ore body. This approach can reduce the exposure area and time of the stope, thereby minimizing the risk of collapse in the central area of the ore body. However, due to the extensive engineering requirements for sealing the stope and the need for at least two filling operations, this plan increases production pressure on the mine, making it unsuitable for large-scale implementation. Figure 2: Schematic diagram of the extraction methods for the step one stope. (**a**) Extraction from one side of the stope to the other; (**b**) extraction from both sides to the middle; (**c**) extraction and filling in the stepwise extraction plan; and (**d**) extraction plan with reduced dimensions in the middle of the stope.