## 2 Test Method Aging Capacity of Bottom Structure  
### Bottom Structure Mold Design  
The design and fabrication of the bottom structure mold are critical for accurately simulating the mechanical behavior of the actual bottom structure under various loading conditions. The mold must replicate the essential features of the bottom structure while adhering to the principles of geometric similarity and material properties. The inner frame of the bottom structure model was designed to include critical components such as the roadway and the ore accumulation troughs. The layout of the inner frame is illustrated in Figure 1, which includes three vein lanes with a length of 0.8 m. The height of the actual bottom structure is 16 m, and the corresponding height of the model is 0.16 m, adhering to a geometric similarity ratio of 1:100. The angle between the vein roadway and the ore extraction approach is set at 55 degrees, which is a critical parameter for accurately simulating the stress paths and load distributions in the bottom structure. The inner frame consists of three vein tunnels and nine ore accumulation troughs, scaled down according to the 1:100 ratio. The dimensions of the ore accumulation troughs and the roadway are carefully scaled to ensure that the model accurately represents the spatial relationships and interactions between these components. This design allows for the precise monitoring of stress and deformation characteristics during the experimental tests.  
The fabrication process of the large-scale models involves several critical steps to ensure the accuracy and reliability of the experimental results, shown in Figure 2. The slurry is prepared according to a predetermined ratio, and the external frame and internal mold are assembled. The outer wall of the inner frame and the inner wall of the outer frame are coated with salad oil to facilitate easy demolding. The slurry is then poured into the gap between the outer frame and the inner frame. After 24 h, the polyore troughs are carefully extracted from the top of the exposed position. Subsequently, the model is maintained by regular watering to ensure adequate hydration and curing. Once the model has achieved sufficient hardness, the demolding process for the remaining part of the inner frame begins. To ensure that the model accurately represents the mechanical behavior of the actual bottom structure, the geometric similarity ratio of 1:100 was strictly followed. This ratio ensures that the scaled-down model maintains the same stress distribution and deformation characteristics as the full-scale structure. Additionally, the material properties of the slurry were carefully selected to match the mechanical properties of the actual bottom structure. The formula for the cement mortar is cement/18-mesh sand/water = 5:3:2. The slurry was formulated to have a similar compressive strength, elastic modulus, and failure characteristics as the rock mass in the mine environment. To monitor the mechanical response of the bottom structure during the tests, strain gauges were strategically placed at critical locations on the model. These locations include the arch top, edge sides, and the area near the ore outlet. The strain gauges were attached to the model before the pouring process to ensure accurate and real-time monitoring of deformation characteristics. This instrumentation allows for the detailed analysis of stress and strain distributions under various loading conditions, providing valuable insights into the failure mechanisms of the bottom structure.  
Figure 1: Bottom structure model: (**a**) 3D diagram and (**b**) top view.