Under the background of climate warming in the Qinghai-Tibetan Plateau (QTP), frequent freeze-thaw cycling (FTC) brings about great geological disasters such as subgrade failure, landslides, and mudslides, which is closely related to the strength reduction caused by the structural damage of soils. In this study, to explore the association between macro shear strength and microstructure evolution of soils subjected to FTC, the red clay distributed widely in the QTP was chosen and used to conduct a series of triaxial shear and nuclear magnetic resonance (NMR) tests in the range of 1 to 7 FTCs. Triaxial shear test results reveal that the shear strength reduction of specimens mainly occurs within five FTCs, and the trend of peak deviator stress with increasing FTCs can be described in three stages: rapid descent (FTCs less than three), slow descent (FTCs between three and five), and stabilization (FTCs greater than five). NMR tests show that the T2 spectrum curves exhibit a distinct bimodal distribution characteristic, corresponding to macropores and micropores. Part of the micropores gradually develop into macropores with increasing FTCs, especially within five FTCs. The increase in macropores proportion leads to a loose soil structure, which is consistent with the deterioration of the shear strength of specimens. Finally, based on the experimental results and classical Mohr-Coulomb theory, a new shear strength model with structural damage for red clay has been proposed by introducing a damage factor expressed by T2 spectral area.

A new composite material of polyurethane-bonded rubber particle-sand mixture (PolyBRuS) is presented here. A series of cyclic triaxial tests were carried out the hysteresis curves under different confining pressures, temperatures, and freeze-thaw cycles, and then the morphological characteristics and evolution law of hysteresis curves under low-temperature conditions were analyzed. The results indicate that the dynamic strain amplitude, temperature, and number of freeze-thaw cycles have a great influence on the hysteresis curve, while the confining pressure has little influence on the hysteresis curve. While the temperature (T) is -15 degrees C and the number of freeze-thaw cycles (N) is 25, the long-axis slope K, i.e., the elastic properties and stiffness of the PolyBRuS material, decreases with increasing dynamic strain amplitude, tends to increase with higher confinement pressures. While N = 25 and the confining pressures (sigma(3)) is 25 kPa, the distance between the center point of the adjacent hysteresis curve d, i.e., fine microscopic damage of the PolyBRuS material, grows with increasing dynamic strain amplitude, rises with lower temperature. While T = -10 degrees C and sigma(3) = 25 kPa, hysteresis curve area S, i.e., energy dissipation of the PolyBRuS material, increases non-linearly in relation to the dynamic strain, and reduces with the enlarge of the number of freeze-thaw cycles, however, this reduction is negligible. Further research should focus on the quantitative analysis of the morphological characteristics and evolution law of hysteresis curves.