Small-strain stiffness of compacted loess upon wetting, drying and loading: Experiments and model interpretation

Stiffness of soil at very small strains G₀ is mainly affected by void ratio, effective stress and suction. Empirical equations considering those factors have been proposed to estimate G₀. However, for collapsible soil like loess, variations in suction might induce changes in void ratio of soil. The combined effect of these two factors poses challenges in accurately estimating of G₀. This paper first presents an experimental study on the G₀ of collapsible loess under various conditions, including as-compacted states, wetting/drying and K₀ loading. G₀ is estimated from shear wave velocity obtained with bender element technique. The changes of G₀ with respect to void ratio, suction, effective stress, and wetting under K₀ stress conditions are evaluated. Test results reveal that power relationships can be defined between G₀ and void ratio, suction and effective stress, respectively. The changes in G₀ along wetting/drying shows an “S” shape due to the different dominant effects on soil structure, as well as the induced non-uniform volume changes when suction change at different zones. Under K₀ loading, G₀ decreases upon wetting at stresses below the compaction stress, while it increases upon wetting at stresses above the compaction stress, due to the combined effects of densification caused by volume collapse during wetting and softening induced by suction decrease. Finally, a G₀ model considering net stress and suction as independent stress variable is proposed. This model could effectively capture the change of G₀ during wetting, drying and loading, as well as upon wetting under K₀ loading for collapsible loess.