## 3 Analysis of Test Results
### Uniaxial Compression Test Results
#### 3.3.2 Failure Modes  
By performing uniaxial compression tests on sandstone samples subjected to different soaking times, the variations in mechanical parameters such as peak strength, deformation modulus, and Poisson's ratio can be analyzed. Additionally, following the tests, the macroscopic morphology of the samples reveals different failure modes due to water-induced weakening, as shown in Figure 8.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{\begin{tabular}{c} **Soaking** \\ **Time/d** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Peak Stress/MPa** \\ **Tested** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Average** \\ **Tested** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Peak Strain** \\ **Average** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Elastic Modulus/MPa** \\ **Tested** \\ \end{tabular} } \\ \cline{3-4}  & & & 17.1 & & 0.1031 & & 934.2 & 926.8 & 931.4 \\ A0-2 & & & 16.9 & & 0.1035 & & 594.2 & \\ A1-1 & & & 12.6 & & 0.1087 & & 588.6 & 591.4 \\ A2-3 & & & 10.5 & & 0.1356 & & 440.8 & \\ A1-5 & & & 8.5 & & 0.1406 & & 439.8 & 440.3 \\ A2-5 & & & 8.4 & & 0.1539 & & 318.5 & \\ A1-2 & & & 7.4 & & 0.1423 & & 314.1 & \\ A2-6 & & & 3.9 & & 0.1754 & & 211.4 & \\ A1-6 & & & 4.3 & & 0.1742 & & 208.6 & \\ A2-4 & & & 3.6 & & 0.1922 & & 112.1 & \\ A1-7 & & & 2.4 & & 0.1818 & & 104.3 & 108.2 \\ A2-7 & & & & & & & \\ \hline \end{tabular}
\end{table}
Table 1: Sandstone sample numbers and mechanical characteristics at different soaking times.  
Figure 7: Variational patterns of physical and mechanical indicators at different immersion times: (**a**) stress–strain curves; (**b**) compressive strength fitting curve; (**c**) elastic modulus fitting curve; (**d**) variational trend in Poisson’s ratio.  
Examining sandstone samples at different soaking times reveals changes in their failure modes: in the dry state, nearly all cracks are tensile, indicating brittle failure. After 7 days of soaking, shear cracks began to expand, suggesting that water interacted with the rock. This is evident from the SEM images in Figure 6b, which show water concentration in quartz and clay minerals. After 30 days of soaking, the shear cracks became more prevalent than tensile cracks. The corresponding XRD results reveal a significant expansion of the clay minerals between 7 days and 30 days. This can be attributed to the complete infiltration of water into the clay minerals, causing them to soften. This increases the ductility of the sandstone, leading to the development of shear cracks. After 60 days of soaking, almost all cracks in the samples were shear cracks. The XRD results in Figure 5c,d indicate that the expansion rate of clay minerals is highest between 30 days and 60 days, while it is almost negligible between 60 days and 90 days. The SEM results also indicate that after 60 days of soaking, large, interconnected cracks appeared within the clay minerals, indicating very weak bonding between the clay minerals and the quartz framework. Therefore, under prolonged soaking conditions, the ductility of the samples nearly reached its peak, resulting in this failure mode.