Macro-micro analysis of soil failure mechanism in unloading condition

Numerical tests are carried out to research mechanism of hydraulic fracturing. As well as that: reservoir storage leads to bending of the embankment dam because of water pressure, especially upright core wall when reservoir storing rapidly, and then bending leads to vertical stress unload. The macro and micro mechanism of soil fracturing in unloading condition are analyzed by the Distinct Element Method (DEM). The results indicate that: (1) Initial stage of specimen will lead to volume response of dilatancy in the unloading condition. Actually, volume response is expressed as dilatancy consistently with the low level confining pressure and is expressed as dilatancy then compression latterly with the high level confining pressure. It suggests that the unloading condition may be a factor for granular material cracks. Particularly, not only the unloading condition with low level confining' pressure leads to cracks, but also it helps the cracks development. So we argue that: unload condition, such as reservoir storage is a factor for hydraulic fracturing in the embankment dams, especially in low level stress areas. (2)The strength of soil in the unloading condition decreases than the loading condition, which contributes to hydraulic fracturing.