Impact of freeze-thaw cycles on loess microstructure: A comparison of fine-grained and coarse-grained soils

The alterations in the microstructure of loess induced by freezing-thawing (FT) processes were crucial for assessing damage to engineering structures in regions affected by seasonal frost. In this study, a series of indoor tests, including freezing-thawing cycles (FTCs), mercury intrusion porosimetry (MIP) tests, and scanning electron microscopy (SEM) observations, were conducted to compare the development patterns of microstructural characteristics between fine-grained and coarse-grained loess. Prior to any FTC test, all loess specimens were prepared with the same initial void ratio (e), although they exhibited significantly different initial pore structures. The results indicated that fine-grained loess tended to develop more pronounced cracks with an increasing number of FTCs, while the surfaces of coarse-grained loess often displayed ice crystals and unevenness. Additionally, the variation in the content of medium and large pores (>0.4 μm) in fine-grained loess was more pronounced than in coarse-grained loess during the FT process. The evolution of microstructural parameters in loess specimens was associated with both FTCs and loess type. The microstructural characteristics of the loess specimens were reflected in changes to micro-parameters such as relative pore content, peak value (Pm), dominant pore diameter (Dm), fractal dimension (D) and particle size distribution. After undergoing more than ten FTCs, the microstructural characteristics of the different loess types tended to stabilize, yet they still retained some initial structural features. These research findings contribute to a better understanding of the failure mechanisms of FT on loess with varying initial structures, thereby providing a scientific foundation for engineering designs that account for regional differences in loess properties.