Freeze–Thaw Impact on Sandy Clay in Artificial Frozen Walls: An Investigation of Shear Strength and Pore-Size Distribution

Artificial ground freezing construction is an effective and widely adopted method for reinforcement in constructing subway tunnels. Regarding sandy clay in artificial frozen walls, changes in its microstructure can affect macroscopic properties during the freezing and thawing processes. An enlargement in the pore space can cause a reduction in compactness due to the frost heave, resulting in a degradation of mechanical characteristics of sandy clay. This study attempted to quantify the effects of freeze–thaw (F–T) damage with different negative temperatures on the shear strength and pore structure of natural soil. Accordingly, four cooling temperatures were applied to the soil samples, which were tested using a triaxial shear instrument and a nuclear magnetic resonance (NMR) imaging scanner. Moreover, by utilizing pore-size distributions and pseudo-color images, changes in pore structures were evaluated both quantitatively and qualitatively, and the mechanism of mechanical properties was thoroughly elucidated according to NMR observations. The results demonstrated that the F–T process affected the cohesive force more than the internal friction angle. Triaxial shear indexes consecutively declined as the cooling temperature decreased, whereas the attenuation amplitude was stale when the temperature was below −20°C. The F–T process was shown to have an expansive effect on pore characteristics (e.g., an increase in porosity, pore diameter, and proportion of macropores). In addition, a strong negative linear correlation was observed between the total porosity and shear strength of sandy clay. This study can improve the understanding of the frost heave mechanism of soil in artificial walls, which may be beneficial for providing insight into designs and protections in the construction of subway tunnels.