Dynamic Dilatancy in Saturated Sands

New experimental aspects of saturated sands dynamic response are discussed. It is demonstrated that shear straining in saturated sands with any density occurs in several stages. Incremental changes of pore pressure and effective stresses at the shear zones triggering point are determined by initial sand porosity relative to its critical value. Sand porosity before shearing is considered as critical, if triggering of shear zones does not result in porosity alteration, which in undrained conditions is physically equivalent to zero change of mean effective stress and pore pressure. Type of sand response is revealed as incremental changes of pore pressure and mean effective stress at the stage of shear zones triggering, indicating negative or positive soil dilatancy. The notion of critical porosity is not applicable in dynamic loading conditions since in different phases of a single cycle changes of pore pressure and effective stress are observed. Progressive strain accumulation under dynamic loading of saturated sands with any density results in the development of a specific state with zero sum changes of pore pressure and mean effective stress called zero dynamic dilatancy. It is assumed to relate to the development of shear zones with constant thickness where subsequent straining is further located/