Microscale Multifractal Characteristics of Fuping Loess in Seasonal Freeze–Thaw Environments: SEM Observations

ABSTRACT

To reveal the degradation patterns and damage mechanisms of loess microstructure under freeze–thaw cycles (FTCs), this study investigates typical loess from the Fuping area at the southeastern edge of the Loess Plateau. Using scanning electron microscopy combined with multifractal analysis, a multiscale quantitative study was conducted on the pore and fracture structures after varying FTCs (0–100 cycles). Results indicate that FTCs significantly alter the spatial distribution characteristics of loess pore networks. Macropores (50–500 μm) and mesopores (10–50 μm) are the most sensitive to FTCs, exhibiting rapid expansion and interconnection, whereas micropores (1–10 μm) maintain structural stability with negligible variation. The generalized fractal dimension D(q) exhibits a “first-rise-then-stabilization” pattern with increasing FTCs, where the volume dimension D0 peaks at approximately 1.85 after 50 FTCs and subsequently stabilizes. The information dimension D1 and correlation dimension D2 exhibit consistent changes, reflecting enhanced pore connectivity and structural reorganization. The spectral width Δα increased from an initial value of 0.545 to a peak of 0.621, then decreased to 0.472 at approximately 100 FTCs, demonstrating a three-stage evolution pattern of “enhancement-homogenization-stabilization.” At the microscopic level, the freeze–thaw action promotes pore expansion and crack initiation through ice crystal growth and frost heave pressure. Subsequent thawing leads to water redistribution and cementation weakening, thereby enhancing pore connectivity and structural homogeneity. The coupled hydro-thermal-mechanical processes during freeze–thaw control the multifractal evolution of loess microstructure. These findings provide a new analytical perspective for structural degradation assessment and stability analysis in cold region loess engineering.