Variations in hydraulic properties and collapse deformation of loess under vertical stress

To study the effect of vertical stress on the collapsibility and hydraulic properties of Lanzhou loess during infiltration, a series of constant head permeability tests were conducted on unsaturated remolded loess under different vertical stresses using a one-dimensional soil column permeameter. The tests yielded collapse deformation curves, wetting front, cumulative infiltration volume, volumetric water content time-history curves, and soil–water characteristic curves. The unsaturated permeability coefficient was calculated using the wetting front advancement method, and its relationship with suction and volumetric water content was established. The study results indicated: (1) The wetting front advancement rate gradually slowed down over time, especially under higher vertical stress; the relationship between the wetting front advancement and time can be described by a power function. (2) The infiltration rate experienced three stages with increasing time: rapid infiltration, significant infiltration, and stable infiltration, significantly influenced by vertical stress. The relationship between cumulative infiltration volume and time can be described by the Philip infiltration model. (3) The collapse deformation over time can be divided into three stages, with stress significantly affecting the time intervals and collapse deformation amounts at each stage. (4) The soil–water characteristic curves during the infiltration process shifted upward with increasing stress, which also significantly impacts the air entry value and the absorption rate. The unsaturated permeability coefficient increased with decreasing matric suction and increased with increasing volumetric water content.