Mechanical degradation characterization in remolded moraine soils and microstructure evolution under freeze–thaw cycles

Remolded moraine soils are highly susceptible to degradation caused by freeze–thaw (F-T) cycles, posing significant risks to the stability of slopes in hilly regions. This study investigates the mechanical degradation and microstructural changes induced by F-T cycles through a comprehensive experimental program, including F-T cycle tests, triaxial shear tests, unconfined compression tests, and micro-computed tomography (micro-CT) scanning of moraine soil from the southeast of the Xizang Autonomous Region, China. The results reveal three stages of mechanical parameter degradation: rapid decline, moderate reduction, and plateau. Cohesion, initial average elastic modulus, and strength decrease significantly under low confining pressure (40%–50% reduction), while the decrease is less pronounced under higher pressures (10%–30%). Micro-CT scanning shows a clear evolution in the microstructure, with the porosity increasing from 14.25% to 18.69% and the pore-crack size rising from 69.83 to 90.44 µm. These microstructural changes are identified as the primary mechanism behind macroscopic degradation. A new gray model was developed to predict mechanical parameter evolution, showing high accuracy for predicting the strength of moraine soils in later F-T cycles (e.g., 15th, 20th, and beyond). The new gray model provides valuable insights for the design of moraine slopes and infrastructure subjected to F-T conditions.