Abstract: Organic soils are widely distributed in cold regions, where their engineering properties are significantly influenced by F-T (freeze-thaw) cycles, which directly influence the long-term stability of subgrade in cold areas. While existing research has predominantly focused on conventional soils, the macro-meso response mechanisms of cement-and-fly ash stabilized organic soils under F-T cycles remain unclear. This study investigates the macro-mesoscopic damage evolution of stabilized organic soils subjected to F-T cycles through triaxial shear tests and computed tomography (CT) scanning. The results indicate that a mix proportion of 20% cement and 5% fly ash yields the optimal strength enhancement. When the organic content reaches or exceeds 15%, the failure mode of stabilized organic soils transitions from brittle to ductile. The strength deteriorates in a power-law function with increasing F-T cycles, while porosity increases logarithmically, accompanied by a tendency for pore morphology to become more elongated and narrow. A constitutive model incorporating both the number of F-T cycles and a mesoscopic damage factor modified by fractal dimension is developed. This model quantitatively correlates macro- and mesoscopic damage, elucidating the underlying damage mechanisms induced by F-T actions. The findings provide critical parameters and a theoretical basis for the design and durability assessment of stabilized organic soil subgrade in cold regions.

Key words: strength, mesostructure, damage, freeze-thaw cycles, stabilized organic soil