Microstructural transformation of compacted loess: a study on initial void ratio and cryogenic temperature effects

Understanding the microstructural evolution of compacted loess subjected to freeze–thaw cycles (FTCs) is crucial for evaluating its engineering durability. This study employed mercury intrusion porosimetry (MIP) to investigate microstructural changes in loess during FTCs, with particular emphasis on the effects of initial void ratio and freezing temperature. Fine-grained loess demonstrated an “expansion–densification” behavior predominantly influenced by mesopores, whereas coarse-grained loess exhibited progressive degradation driven by macropores. Notably, coarse-grained loess experienced severe damage, characterized by a damage variable S exceeding 0.4 and macropore content surpassing 80% at elevated void ratios (e = 0.78). Within the temperature range of − 5 °C to − 15 °C, temperature was found to regulate moisture migration and phase transitions: fine-grained loess showed high sensitivity, with significant mesopore refinement, while coarse-grained loess displayed limited sensitivity, primarily undergoing in situ ice growth. Based on the observed damage patterns, a critical threshold void ratio of approximately e_cr = 0.65 was identified for coarse-grained loess; maintaining the post-construction void ratio below this threshold (e.g., e ≤ 0.60) is essential to mitigate pore coarsening and severe frost heave. These quantitative thresholds provide a scientific foundation for transitioning from empirical design approaches to microstructure-based regulation, thereby offering practical guidance for the engineering of durable loess structures in cold regions.