Investigation on the mechanical behavior and collapsible mechanism of undisturbed and remolded loess from a dam foundation site

Loess terrains are prone to seepage-triggered collapsible failures that threaten infrastructure safety, yet quantitative correlations between compaction degree, cyclic infiltration, macroscopic mechanical behavior and microstructural evolution in loess remain poorly quantified. This study investigates undisturbed loess from multiple burial depths and remolded loess with graded compaction degrees, integrating compression tests, multi-cycle permeability measurements, collapsibility assessments and microstructural characterization. Results show that compressibility and permeability decrease with increasing burial depth, and loess layers have markedly higher compressibility than palaeosol layers. For remolded loess, 100% compaction reduces the initial void ratio to 59% and initial permeability coefficient to 12.3% of the values at 80% compaction; cyclic seepage stabilizes permeability through fine particle clogging, with more pronounced fluctuations in shallow and lightly compacted specimens. Collapsibility increases with vertical pressure, peaks at intermediate depth in undisturbed loess, and declines by 98.4% across the compaction gradient, and is virtually eliminated at 100% compaction. A cubic polynomial model accurately captures the nonlinear void ratio–compressibility relationship, with R 2 values above 0.92 for most remolded groups and over 0.96 for undisturbed loess layers. Microstructurally, collapsible deformation is dominated by the collapse of medium and large pores, which convert into small pores upon wetting.This study provides quantitative experimental evidence for engineering design and hazard mitigation in loess regions.