## 4 Test Results and Analysis
### Softening Coefficient Test  
Table 4 shows the main softening characteristics of the mudstone and sandstone, which are obtained by the UCT of the dried and saturated samples. The softening coefficient of mudstone is as low as 0.33, which indicates that the moisture content has a significant effect on the strength of mudstone. The softening coefficient of sandstone is 0.73, which reveals that the moisture content also has a great effect on the strength of sandstone. In addition, the stress-strain curve also shows that the Young's modulus of the saturated group is significantly lower than that of the dried group for mudstone, while this characteristic is not obvious for sandstone.  
Figure 9 shows the typical complete stress-strain curves and failure photos of the mudstone and sandstone samples. For mudstone, the curve of the dried group rises faster in the pre-peak area, and the ultimate strength is greater, but the brittleness is higher than that of the saturated group. The failure modes are all shear failures. For sandstone, the rising rate of the curve in the pre-peak area of the dried group and saturated group is close, and the brittleness is higher than that of mudstone. The ultimate strength of the dried group is higher than that of the saturated group. The failure mode of the dried group is a shear failure with a crisp sound and rock block collapse. However, the failure mode of the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen** & **Lithology** & **UCS (MPa)** & **Group** & \
\begin{tabular}{c} **Average** \\ **Value (MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Softening** \\ **Coefficient** \\ \end{tabular} \\ \hline
1–14–D–1 & Mudstone & 54.12 & dried & & \\
1–14–D–2 & Mudstone & 69.05 & dried & & \\
1–14–D–3 & Mudstone & 17.90 &
\begin{tabular}{c} cracks before test \\ (excluded) \\ \end{tabular} & & \\ \hline
1–14–E–1 & Mudstone & 17.64 & saturated & & \\
1–14–E–2 & Mudstone & 18.59 & The specimen has & & \\
1–14–E–3 & Mudstone & 23.11 &
\begin{tabular}{c} cracks before \\ test(excluded) \\ \end{tabular} & 20.38 & & \\ \hline
4–4–D–1 & Sandstone & 87.40 & dried & 89.08 & \\
4–4–D–2 & Sandstone & 90.76 & dried & & \\ \hline
4–4–E–1 & Sandstone & 69.46 &
\begin{tabular}{c} Saturated \\ Saturated \\ \end{tabular} & 64.63 & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: The softening coefficients of mudstone and sandstone samples.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(m_{i}\) & \(\mathbf{\kappa}\) & **GSI** & \(m_{b}\) & \(s\) & **UCS (MPa)** & **Young’s Modulus (GPa)** \\ \hline  & & 40 & 1.567 & 0.0013 & 0.481 & 0.42 \\  & & 42 & 1.683 & 0.0016 & 0.544 & 0.48 \\
13.36 & 0.5 & 44 & 1.808 & 0.0020 & 0.614 & 0.55 \\  & & 46 & 1.942 & 0.0025 & 0.692 & 0.63 \\  & & 48 & 2.086 & 0.0031 & 0.779 & 0.72 \\  & & 50 & 2.240 & 0.0039 & 0.875 & 0.81 \\ \hline \hline \end{tabular} \saturated group shows shear and splitting failures. It should be noted that, in the softening coefficient test, the strength of the mudstone samples in the saturated state is higher than the average value in the natural state shown in Section 4.1. On the one hand, the samples are taken from 1-14 boreholes, and the UCS in the natural state is higher than the average value. On the other hand, it also shows that the water content in the surrounding rock of the project area with shallow cover depth is close to the saturated state due to coring in the rainy season.  
In general, the strength of the surrounding rock of Guobo station is highly sensitive to water. In the construction and operation management of tunnels, the prevention and control of the risk caused by precipitation and groundwater should be carefully considered.