Progressive Response and Dynamic Behavior of Loose Gravel–Sand Mixtures

Previous studies investigating dynamic responses of gravelly soil were limited to high-strain conditions, in which a high level of pore-water pressure is developed, leading to a significant reduction in shear strength and subsequent liquefaction. This paper presents a series of dynamic centrifuge modeling tests performed on loose gravel–sand mixtures to evaluate progressive response under various shear strains. The centrifuge models simulated a uniform soil profile of gravel–sand mixtures with gravel contents of 20%, 40%, 65%, 80%, and 100% that were subjected to incrementally increasing shaking amplitudes from 0.01 to 0.40 g. Due to the influence of composition on the void ratio of the specimens, the results were analyzed in terms of their dominant behaviors (i.e., sandlike, gravellike, or transition soil). Although the soils had comparable initial relative densities, the sandlike soils had the lowest void ratio, and the void ratio increased when the gravel content was greater than 65%. Resonant column testing results indicated that the soils had comparable dynamic properties because of their loose condition. The results showed that dynamic shaking generates comparable shear strains ranging from 0.03% to 3.8% in all models, but the accumulation of pore pressure leads to upward flow in sandlike soils, whereas transient pore-pressure behavior leads to oscillatory flow in gravellike soils. Differences in the stress–strain response and the effects of the number of shaking cycles were observed in different soil mixtures depending upon the level of excess pore pressure. At low shaking amplitude and low excess pore pressure, stiffness degradation was observed while the stress–strain loop was symmetric. At high shaking amplitude and high excess pore pressure, significant stiffness degradation was observed followed by shear-induced dilation resulting in an asymmetrical stress–strain loop. This study clarifies the differences in the dynamic responses and behaviors of sandlike, gravellike, and transition soil over a wide range of strains.