Micro pore structure evolution of a loess-paleosol sequence and its control on macro hydraulic properties

The geo-hydrological and hydro-mechanical properties of loess–paleosol sequences on the Chinese Loess Plateau are critical to groundwater migration, stratigraphic stability, geohazard mitigation, and deep geotechnical engineering. This study investigated samples from a 1–30 m loess–paleosol sequence in the central Chinese Loess Plateau. Oedometer and permeability tests were conducted to characterize macroscopic hydraulic behavior, while scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were used to examine microstructural features. Multivariate statistical analyses were further employed to quantify the effects of microstructural attributes on macroscopic hydraulic responses. The results show that saturated hydraulic conductivity and collapse coefficient generally decrease with burial depth but fluctuate markedly across stratigraphic interfaces. The loess contains abundant elongated overhead pores and exhibits structural anisotropy, whereas paleosol pores are smoother and distributed nearly isotropically. Owing to the high clay content, macropores account for only a small proportion of the pore space. Increased proportions of mesopores and elongated pores provide seepage pathways and constitute the primary compaction space during collapse. After collapse, loess pores undergo blunting and spheroidization, whereas paleosol pores are compressed and flattened into fissure-like structures. Overall, the hydro-mechanical properties of the loess–paleosol sequence are jointly controlled by pore-size distribution and pore geometry. Microstructural indices, including void ratio and mesopore/macropore proportions, contribute 30%–40% to the observed responses. Specifically, void ratio contributes 8.91% to permeability and 18.6% to collapsibility, confirming its importance in regulating stratigraphic permeability and evaluating collapse potential. These findings improve the quantitative understanding of hydro-mechanical and microstructural evolution in deep loess–paleosol sequences and provide theoretical support for geotechnical engineering and geohazard mitigation.