Microstructure evolution and permeability of fine-grained loess subjected to freeze-thaw cycles

This study focused on the stability analysis of loess slopes within seasonal frozen regions, revealing how clay type and content govern permeability evolution through freeze-thaw cycles (FTCs). By performing mercury intrusion porosimetry and saturated permeability coefficient (K_sat) measurements in two representative fine-grained loess samples with the same void ratio from Changchun (CC) and Xianyang (XY), China, novel findings emerged: (1) Under the coupled effects of initial water content (ω) and FTCs, the two loess types exhibited distinct evolutionary pathways. When ω = 21%, CC loess transitioned smaller pores (0.04–0.4 μm) to larger pores (>0.4 μm) notably after five FTCs, whereas XY loess exhibited a continuous increment of pores >0.4 μm during FTCs. Elevating ω to 24% suppressed the larger-pore transformation in CC loess, but the behavior of XY loess was less relevant to the change in ω. (2) Regional divergence characterizes permeability evolution: CC loess consistently exhibited lower K_sat than XY loess under identical FTCs, by approximately two orders of magnitude. Notably, CC loess required five FTCs to achieve approximate stabilization of permeability, whereas XY loess stabilized after only one cycle. (3) Excluding high-ω (24%) CC loess, the initial ω effects on Kₛₐₜ diminished with prolonged FTCs (beyond 10 cycles). Mechanistically, FTC-induced ice crystallization drove pore expansion [correlation between pores (0.4–4 μm) and K_sat, whereas particle rearrangement reduced hydraulic heterogeneity. These findings establish a mechanistic framework for optimizing linear infrastructure (e.g., railways and pipelines) in cold regions, advocating site-specific FTC susceptibility mapping to mitigate homogenization-induced instability risks.