Stress-Dilatancy Relationship of Zipingpu Gravel under Cyclic Loading in Triaxial Stress States

In this study, cyclic triaxial tests were performed on Zipingpu gravel, and they were followed by a discrete element study that was designed to investigate the stress-dilatancy relationship of gravelly soils under cyclic loading in triaxial stress states. Several conclusions emerged from the results. (1) A nearly linear relationship was found between the stress ratio $\eta =q/p$ and the dilatancy ratio $D^p=d{\epsilon }_v^p/d{\epsilon }_s^p$ under both conventional compression and extension monotonic loading. The slope parameter $\alpha$, which relates $\eta$ and $D^p$, was smaller during triaxial compression; (2) The stress-dilatancy relationship was different during the virgin and cyclic loading. The dilatancy line under cyclic loading was located inside the virgin/monotonic loading dilatancy lines, and the dilatancy relationship was related to the location of the most recent load reversal point; and (3) A nearly parallel linear relationship was found between $\eta$ and $D^p$ in the $d\eta >0$ and $d\eta <0$ paths under cyclic loading, with $\alpha$ smaller under cyclic loading than under virgin loading. Numerical simulations were performed using discrete element software, and the results of these simulations showed that the stress-dilatancy characteristics under two-way cyclic loading were similar to the stress-dilatancy characteristics under one-way cyclic loading. The study also found that a hyperbolic relationship exists between the input plastic work and the particle breakage for Zipingpu gravel, regardless of the initial void ratio or the confining pressure under both monotonic and cyclic loading.