## 4 Results and Discussion
### Change the Rule of Mechanical Properties  
The stress-strain curves of selected rock samples are presented in Figure 7. As water content increases, the peak strength of siltstone rock samples consistently decreases. The peak strength of dry rock samples is measured at 10.566 MPa, while the peak strength of samples with 4% water content (natural state) is 9.475 MPa. When the water content rises to 6%, the peak strength further decreases to 8.562 MPa. At 9% water content, the peak strength drops to 7.413 MPa, and it remains at 7.413 MPa when the water content reaches 11% (saturated state). Notably, the peak strength of the samples declines to 4.798 MPa at 11% water content (saturated state), representing a reduction of more than half compared to the peak strength of the dry samples. Specifically, there is a 54.59% decrease in peak  
Figure 6: Water content change rule with soaking time: (**a**) siltstone, (**b**) coarse-grained sandstone (I: rapid growth stage; II: stable growth stage; III: decelerated growth stage; IV: stable stage). strength relative to the dry samples. This observation suggests that water action has inflicted damage on the internal structure of the rock samples, adversely affecting their uniaxial compressive strength. Prior to reaching peak strength, the stress-strain curves of the rock samples exhibit an increase, with higher water content leading to a gradual reduction in the elastic deformation of the samples during the loading process. With the increase in water content, the peak strength of the coarse-grained sandstone samples continued to decline. Specifically, the peak strength of the dry samples was measured at 86.12 MPa, whereas the peak strength decreased to 27.57 MPa when the water content reached 5% (saturation), representing a reduction of 67.99% compared to the dry samples. This substantial decrease indicates a more pronounced sensitivity of coarse-grained sandstone to water, as the rock sample absorbs water at a lower rate, leading to a greater decline in peak strength. Consequently, the damage inflicted by water on the internal structure of the sandstone is more significant. The physical and mechanical parameters of the rock layer at the roof of the 51,102 return airway were derived by collating and summarizing the results of the aforementioned experiments, with detailed information presented in Tables 2 and 3. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Split Tensile Test**} & \multicolumn{3}{c}{**Uniaxial Compression Test**} & \multicolumn{3}{c}{**Shear Test**} \\ \cline{2-9} \multirow{2}{*}{**Water**} & \multirow{2}{*}{**Tensile Strength**} & **Compressive Strength** & **Elasticity** & **Poisson’s** & **Shear** & **Cobhesion** & **The Angle of Internal** \\  & & **(MPa)** & **(MPa)** & **(GPa)** & **Ratio** & **(GPa)** & **(MPa)** & **Friction (\({}^{\circ}\))** \\ \hline
0 & 1.45 & 10.57 & 0.45 & 0.24 & 0.18 & 3.27 & 36 \\
4 & 1.02 & 9.48 & 0.41 & 0.23 & 0.17 & 2.62 & 31 \\
6 & 0.79 & 8.56 & 0.35 & 0.21 & 0.16 & 2.41 & 27 \\
9 & 0.67 & 7.40 & 0.34 & 0.18 & 0.15 & 1.41 & 25 \\
11 (saturated) & 0.43 & 4.80 & 0.26 & 0.17 & 0.11 & 0.36 & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of physical and mechanical parameters of siltstone. Figure 7: Uniaxial compressive stress–strain relationship curves of rock samples with different water content: (**a**) siltstone, (**b**) coarse-grained sandstone. In summary, the axial compressive strength of siltstone decreased from 10.57 MPa in dry conditions to 4.80 MPa at 11% water content (saturated), reflecting a reduction of 54.59%. Similarly, the axial compressive strength of coarse-grained sandstone decreased from 86.12 MPa in dry conditions to 27.57 MPa at 5% water content (saturated), resulting in a decrease of 67.99%.