Soil-Water Retention and Shear Strength Behavior of Cambisols under Varying Dry Density and Dry-Wet Cycles

Residual soils in the study area are classified as Cambisols (developed from phyllite), and are widespread in humid subtropical regions where seasonal dry-wet cycles strongly influence soil hydraulic and mechanical behavior. This study investigates their coupled soil-water retention and shear strength responses under varying dry densities and cyclic wetting and drying. Soil-water retention curves (SWRCs) were measured using pressure plate and equilibrium vapor methods, while shear strength was assessed by direct shear tests across 5–40% gravimetric water content. Mercury intrusion porosimetry was employed to evaluate pore-scale changes. Results show that increasing dry density improved the soil-water retention capacity and shear resistance. Early dry-wet cycles reduced the air-entry value and water retention in the funicular regime, with curves tending to stabilize after ≈3 cycles. Shear strength showed a moisture-dependent pattern, cohesion increased then decreased with rising gravimetric water content, whereas the internal friction angle slightly declined from 5 to 30% gravimetric water content. Importantly, repeated dry-wet actions produced a dual mechanism: structural degradation dominated at lower densities, while density-related densification effects became more pronounced at higher densities. A persistent dual-peak pore structure (≈0.1–1 and 10–30 µm) shifted toward larger pores with cycling, reflecting this competitive interplay. A suction stress model was developed by integrating an improved SWRC that explicitly incorporates dry-density effects via initial void ratio and cyclic wetting and drying effects through a degradation function of saturated volumetric water content. This approach successfully reproduced the measured cohesion evolution with good agreement. These results clarify how density and climatic cycling co-regulate suction, pore structure, and strength in Cambisols, contributing to a soil-physics understanding of hydro-mechanical coupling in humid regions.