Modeling Cyclic Behavior of Rockfill Materials in a Framework of Generalized Plasticity

Typical triaxial compression experiments were revisited to investigate the essential mechanical behavior of rockfill materials to be reflected in constitutive modeling, such as the nonlinear dependence of the strength and the dilation on the confining pressure and the accumulation of permanent strains during cyclic loading. The mathematical descriptions of the axial stress-strain behavior during initial loading, unloading, and reloading were formulated, respectively, which enables us to represent the hysteresis loops directly without recourse to complex concepts and parameters. The axial stress-strain model was then incorporated into the constitutive framework of generalized plasticity for the modeling of cyclic behavior of rockfill materials. This task was fulfilled by defining the elastic modulus, the plastic flow direction tensor, the loading direction tensor, and the plastic modulus for different loading conditions. In particular, the plastic flow direction tensor was derived based on a stress-dilatancy equation considering the influence of loading direction, and the representation of the plastic modulus was established in terms of the tangential modulus and the elastic modulus by using the special constitutive equations under axisymmetric stress states. The cyclic model proposed in this study has three distinct features. First, the hysteresis behavior and the accumulation of permanent strains were unified and described under the framework of generalized plasticity. Second, all the loading phases were treated as elastoplastic processes so that no purely elastic regions exist in the principal stress space. Third, the introduction of two aging functions for the consideration of the hardening effect facilitates the controlling of the magnitudes of permanent strains. There are in total 13 parameters in the model, all of which can be determined easily from (cyclic) triaxial compression experiments. To check the capabilities of the model in reproducing the monotonic and cyclic behavior, typical triaxial compression experiments were simulated with the constitutive equation. Satisfactory agreement between the experimental results and the corresponding model predictions lent sufficient creditability to the effectiveness of the proposed model, which further motivates us to extend the model for more complex stress paths and apply the model in practical engineering in the future.