Freeze–thaw effect on the mechanical strength of compacted loess and its engineering inspirations

Compacted loess samples with different dry density (ρ_d) and water content (w) were subjected to freeze–thaw cycles, and subsequent laboratory tests were conducted to reveal their mechanical strength degradation and internal structural evolution. Results indicate that freeze–thaw action significantly reduces the unconfined compressive strength (UCS) of compacted loess, with the UCS reduction ratio (D_UCS) strongly correlated with both w and ρ_d. At low water content, compacted loess presents a cellular structure, where aggregates act as the primary soil skeleton and are highly sensitive to freeze–thaw damage. During the freeze–thaw process, the water within aggregates freezes into ice, destroying this metastable structure and leading to a high D_UCS. As water content increases, a more uniform and stable soil skeleton forms, showing lower freeze–thaw sensitivity and thus a lower D_UCS. At a given water content, the water-filled pore volume remains nearly unchanged, while the total pore volume decreases with increasing dry density. This increases the ice volume ratio in the total pores during freezing, causing more severe structural damage and a higher D_UCS. For loess subgrade in seasonal frozen regions, within allowable ρ_d and w ranges, relatively lower dry density and higher water content are recommended to alleviate freeze–thaw-induced subgrade deterioration.