Hydro-mechanical coupling of unsaturated red clay considering temperature and time effects

To systematically investigate the hydro-mechanical behavior of unsaturated red clay used as subgrade fill along the Sichuan-Tibet Railway, a series of temperature- and strain-rate-controlled triaxial shear tests were conducted. The coupled effects of temperature, matric suction, and strain rate on shear strength, volumetric deformation, and microstructural evolution were examined. Scanning electron microscopy (SEM) was employed to characterize fabric changes before and after shearing. The results indicate that increasing temperature significantly alters particle arrangement and pore structure, leading to enhanced interparticle interaction and pronounced changes in both strength and volumetric response. The mechanical behavior of unsaturated red clay exhibits strong rate dependency, which is jointly regulated by temperature and matric suction. With decreasing strain rate, the strengthening effect induced by temperature is gradually weakened, whereas higher temperatures and strain rates markedly increase the initial tangent stiffness, resulting in an enhanced initial structural resistance. Moreover, at elevated temperatures (50 °C and 80 °C), the axial strain-volumetric strain relationship exhibits a distinct two-stage contraction pattern during the early shearing stage, characterized by a rapid contraction followed by a slower contraction phase, which is less evident at ambient temperature. The stress ratio is also governed by the coupled effects of temperature, suction, and strain rate, with stronger thermo-suction interaction observed at lower strain rates. These findings provide new insights into the coupled thermo-hydro-mechanical behavior of unsaturated red clay and offer valuable guidance for deformation control and long-term performance assessment of subgrade systems under complex environmental conditions.