Plastic strain directions of rockfill materials during loading and creeping

The determination of the incremental plastic strain direction (IPSD) is one of the kernels in elastoplastic constitutive models. It is generally assume that the IPSD depends only on the stress states and is not influenced by the stress increments. A creep test is a special experiment that the stress state is kept constant without increment and the strains are purely plastic. Studying the dependence of the IPSD during creeping upon the stress states and its difference from that during loading may offer valuable reference to constitutive modeling. In this study, large-scale triaxial compression experiments and triaxial creep experiments are conducted on a typical rockfill material used in a pumped storage power station. The relationships between the dilatancy ratio and the stress ratio during both loading and creeping are investigated, respectively. It is found that in both kinds of experiments the dilatancy ratio decreases nonlinearly when the stress ratio increases, and the positive dilatancy ratio during creeping is considerably higher than that during loading under the same stress state. That is to say, the shear volume contraction behavior is much more evident during creeping than that during loading. Therefore, it is irrational to use the same potential function to determine the IPSDs during both loading and creeping. Different stress dilatancy equations or different plastic potential functions are required in establishing constitutive models considering the creep behavior of rockfill materials.