## 3 Materials and Methods  
### Experimental Scheme  
Considering the characteristics of this specific type of sandstone, the experimental plan was designed from both micro- and macro-perspectives, as shown in Figure 5.  
To reveal the intrinsic structural changes in the sandstone under water-rock interactions, a series of microscopic experiments, including X-ray diffraction (XRD) and scanning electron microscopy (SEM), were designed for this study. These experiments aimed to investigate changes in the microstructure and mineral composition of the water-saturated sandstone. XRD analysis was employed to identify the mineral composition and crystal structure of the sandstone, clarifying the types and concentrations of the internal minerals. SEM was used to observe the microfractures, pore structure, and the effects of water on these structures at the surface of the sandstone. To simulate the actual stress state of the surrounding rock in a complex stress environment and investigate the deformation and failure behavior of the sandstone, this study designed a true triaxial loading experimental plan. Through multi-path stress loading, the deformation and failure characteristics of the surrounding rock under water-saturated conditions were systematically investigated [37]. Based on in situ geostress measurements from the No. 6 mining district of the Renlou Coal Mine, the experiment included three moisture content states (dry, medium saturation at 30%, and fully saturated) and four true triaxial loading-unloading paths, as shown in Figure 6. The specific loading-unloading paths and experimental procedures are outlined below. 1. Loading-Unloading Path I and Procedure: The minimum horizontal stress was maintained constant, while the maximum horizontal stress was unloaded, and vertical stress was applied. The specific steps were as follows: 1 Axial stress \(\sigma_{1}\) was applied in the z-direction, and horizontal stresses \(\sigma_{2}\) and \(\sigma_{3}\) were applied in the x- and y-directions simultaneously to the design values (\(\sigma_{2}>\sigma_{1}>\sigma_{3}\)), and this initial stress  
Figure 5: Design of experimental scheme. level was maintained for 180 s. 2 Horizontal stress \(\sigma_{3}\) was kept constant, horizontal stress \(\sigma_{2}\) was unloaded, and axial stress \(\sigma_{1}\) was applied until the specimen failed. The loading and unloading rates were set at 0.1 MPa/s. The specific loading-unloading path is illustrated in Figure 6a. 2. Loading-Unloading Path II and Procedure: The maximum horizontal stress was maintained constant, while the minimum horizontal stress was unloaded, and vertical stress was applied. The specific steps were as follows: 1 Axial stress \(\sigma_{1}\) was applied in the z-direction, and horizontal stresses \(\sigma_{2}\) and \(\sigma_{3}\) were applied in the x- and y-directions simultaneously to the design values (\(\sigma_{2}>\sigma_{1}>\sigma_{3}\)), and this initial stress level was maintained for 180 s. 2 Horizontal stress \(\sigma_{2}\) was kept constant, horizontal stress \(\sigma_{3}\) was unloaded, and axial stress \(\sigma_{1}\) was applied until the specimen failed. The loading rate was set at 0.1 MPa/s, and the unloading rate was set at 0.05 MPa/s. The specific loading-unloading path is illustrated in Figure 6b. 3. Loading-Unloading Path III and Procedure: Both the maximum and minimum horizontal stresses (\(\sigma_{2}\) and \(\sigma_{3}\)) were unloaded while applying the vertical stress (\(\sigma_{1}\)). The specific steps were as follows: (i) Axial stress \(\sigma_{1}\) was applied in the z-direction, and horizontal stresses \(\sigma_{2}\) and \(\sigma_{3}\) were applied in the x- and y-directions simultaneously to the design values (\(\sigma_{2}>\sigma_{1}>\sigma_{3}\)), and this initial stress level was maintained for 180 s. (ii) Both horizontal stresses \(\sigma_{2}\) and \(\sigma_{3}\) were unloaded without change, and axial stress \(\sigma_{1}\) was applied until the specimen failed.