Asymmetric creep behavior and a mechanism of longitudinal crack development in cold-region subgrades under the sunny-shady slope effect

Longitudinal cracking is a common form of distress that compromises the performance of frozen ground subgrades. Persistent thermal asymmetry between the sunny and shady sides of a subgrade contributes to differential deformation, but the coupling among temperature evolution, ice-water phase change, and creep remains unclear. This study investigates the mechanism using paired unconfined creep tests on silty clay specimens subjected to contrasting initial temperatures and thawing conditions. Constant axial stresses of 30, 60, and 90 kPa are applied. Nuclear magnetic resonance (NMR) quantifies unfrozen water and estimates ice content, while digital image correlation (DIC) records strain localization on the monitored specimen surfaces. The results show that asymmetric thawing produces creep lag between the two specimens. The thawing response of initially frozen specimens comprises instantaneous deformation, initial decelerating creep, accelerated creep induced by phase change, second decelerating creep, and final steady-state creep. The phase change softening point (PCSP) and peak creep rate point (PCRP) mark the onset of accelerated creep and its transition to second decelerating creep, respectively. A lower initial temperature on the shady side prolongs ice-water phase change, delays both characteristic points, and increases the accumulated strain difference. This increase results from delayed pore ice melting on the shady side, which preserves ice cementation and restrains deformation for longer. At 30 and 60 kPa, the response retains five stages, whereas at 90 kPa progressive localization leads to instability. These findings provide a mechanistic basis at the specimen scale for interpreting differential deformation and longitudinal cracking under thermal asymmetry.