## 3 Mechanical Test Results of Cretaceous Red Sandstone
### Analysis of Mechanical Properties of Rock  
Generally, the ground stress of rock in the stratum is relatively complex, usually in the state of three-way loading, and obvious differences also exist due to the influence of temperature during freezing construction. On the basis of the Mohr-Coulomb strength criterion, the peak strength of frozen sandstone under different low temperatures and confining pressures can be obtained from triaxial tests using the values of \(\sigma_{\text{max}}\), Poisson's ratio \(\mu\), and rock's elastic modulus \(E\). The test results are shown in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen Number** & \(\sigma_{\text{max}}\)/MPa & \(E\)/MPa & \(\mu\) \\ \hline W0T25 & 12.242 & 2407.36 & 0.249 \\ W0T-5 & 20.689 & 2973.21 & 0.239 \\ W0T-10 & 27.122 & 3728.39 & 0.230 \\ W0T-15 & 31.812 & 4947.37 & 0.210 \\ W2T25 & 16.827 & 3026.18 & 0.234 \\ W2T-5 & 25.557 & 3802.13 & 0.226 \\ W2T-10 & 33.277 & 4507.64 & 0.216 \\ W2T-15 & 38.981 & 5807.88 & 0.197 \\ W4T25 & 23.611 & 3713.07 & 0.222 \\ W4T-5 & 31.276 & 4478.78 & 0.216 \\ W4T-10 & 38.704 & 5292.02 & 0.204 \\ W4T-15 & 49.109 & 6723.26 & 0.187 \\ W6T25 & 32.784 & 4376.91 & 0.213 \\ W6T-5 & 41.582 & 5439.73 & 0.207 \\ W6T-10 & 51.706 & 6390.49 & 0.196 \\ W6T-15 & 58.642 & 7507.74 & 0.180 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Test results. 3.1 Relationship between the Freezing Temperature/Confining Pressure and the Peak Intensity of Sandstone  
The variation in the sandstone peak intensity as a function of the temperature and confining pressure is depicted in Figure 6. It can be seen in Figure 6a that the peak strength of sandstone was linearly proportional to the confining pressure. At room temperature, as the confining pressure of sandstone rose from 0 MPa to 2, 4, and 6 MPa, the corresponding peak strengths increased by 37.45%, 92.87%, and 167.80%, respectively. When the rock temperature was \(-15\)\({}^{\circ}\)C, as the confining pressure increased from 0 MPa to 2, 4, and 6 MPa, the peak strengths of sandstone rose by 22.54%, 54.37%, and 84.34%, respectively. When the temperatures of sandstone were 25, \(-5\), 10, and \(-15\)\({}^{\circ}\)C, and the applied confining pressure increased from 0 to 6 MPa, the peak strengths increased by 167.80%, 96.33%, 90.64%, and 84.34%, respectively. This indicated that the peak strength of sandstone can be improved by increasing the confining pressure. This is because at a high confining pressure, the lateral deformation of sandstone is constrained, the rock particles are continuously compressed and compacted in the axial direction, and the compacted particles are subjected to axial compression until the rock is sheared. According to Figure 6b, as the confining pressure was constant, the peak strength linearly increased with the decrease in the negative temperature, but the peak strength at room temperature was different from that at negative temperature. This was because under the condition of negative temperature, the bearing capacity of the rock not only depends on the rock particles, but also on the ice-water mixture generated by the temperature reduction in the rock. The water in the rock can be divided into unfrozen water and ice. The ice not only has a certain strength, but also contributes to the cementation between rock particles, greatly improving the bearing capacity of the rock. As depicted in Figure 6b, as the temperature dropped from room temperature to \(-5\)\({}^{\circ}\)C, the peak strengths at the confining pressures of 0, 2, 4, and 6 MPa increased by 40.83%, 34.16%, 24.51%, and 26.84%, respectively. As the confining pressure was 0 MPa, and the temperature dropped from \(-5\) to \(-15\)\({}^{\circ}\)C, the growth rates of rock peak strength were 40.83%, 31.09%, and 17.29%, respectively. When the confining pressure was 6 MPa, the corresponding values were 26.84%, 24.35%, and 13.41%, respectively. This shows that under triaxial compression, the strength of rock decreased with the decrease in the negative temperature.