## 3 Mechanical Test Results of Cretaceous Red Sandstone
### Analysis of Mechanical Properties of Rock
This was because with the decrease in the temperature, the content of unfrozen water in the rock decreased gradually, but the water content in the rock was fixed, and the water that can be frozen in the rock gradually decreased, leading to a decrease in the growth rate of rock peak strength. Figure 6: Sandstone peak intensity and the relationship between confining pressure and temperature. (**a**) Relationship between peak strength and confining pressure. (**b**) Relationship between peak strength and temperature. 3.2 Relationship between the Freezing Temperature/Confining Pressure and the Elastic Modulus of Sandstone  
Elastic modulus reflects the ability of rock to withstand elastic deformation. The elastic modulus is essentially the bonding strength between micro atoms or molecules. Any change in the temperature and confining pressure obviously affects the elastic modulus. The variations in the elastic modulus of sandstone as a function of the temperature and confining pressure are depicted in Figure 7. The elastic modulus of rock was linearly proportional to the confining pressure. As the confining pressure of rock increased from 0 to 6 MPa, the elastic modulus measured at 25, \(-\)5, \(-\)10, and \(-\)15 \({}^{\circ}\)C increased by 123.04%, 108.19%, 64.89%, and 45.88, respectively. This was because as the confining pressure increased, the lateral constraint increased, and the lateral strain of rock decreased, which undoubtedly had a restraining effect on the destruction of rock and improved its resistance to deformation. It can be seen in Figure 7a that with the decrease in the temperature, the variation trend of the elastic modulus was similar to that of the peak strength, i.e., the elastic modulus of rock increased as the temperature decreased. As the test temperature of rock decreased from room temperature to negative temperature, the elastic modulus corresponding to the confining pressures of 0, 2, 4, and 6 MPa increased by 23.51%, 25.64%, 10.88%, and 15.28%, respectively. When the test temperature dropped from \(-\)5 to \(-\)15 \({}^{\circ}\)C, the elastic modulus of rock under different confining pressures increased to a higher extent, and the corresponding growth rates from low to high confining pressures were 73.09%, 52.75%, 72.21%, and 39.82%, respectively. The above results are mainly attributed to the fact that a part of pore water in the rock under negative temperature froze into ice, which had a certain strength, and its cementation improved the ability of rock mass to resist deformation. Therefore, the elastic modulus of rock was not only determined by the rock skeleton but also by the ice in the pores. 3.3 Relationship between the Freezing Temperature/Confining Pressure and Poisson's Ratio of Sandstone  
The variations in the Poisson's ratio of sandstone as a function of the confining pressure and temperature are depicted in Figure 8. It is clear that the Poisson's ratio of rock quadratically decreased with the rise in the confining pressure. This was because the lateral deformation was restrained with the rise in the confining pressure. According to the formula of Poisson's ratio, the lateral strain decreases, and it is more influenced by  
Figure 7: Sandstone elastic modulus and the relationship between confining pressure and temperature. (**a**) Relationship between elastic modulus and confining pressure. (**b**) Relationship between elastic modulus and temperature. the confining pressure than the axial strain. Therefore, the Poisson's ratio decreases in a quadratic manner with the in the confining pressure [25, 26]. As displayed in Figure 8b, the Poisson's ratio of rock decreased as the temperature was reduced at the same confining pressure, which was quite different from the relation between temperature and the rock's elastic modulus.