Viscoelastic–Viscoplastic Characterization of Unbound Granular Material

The mechanical behavior of unbound granular materials had proven to be extremely challenging. In the pavement field, as well as in other geotechnical disciplines, these materials are usually treated as time-independent. The objective of this work is to explore the applicability and potential for a viscoelastic-viscoplastic constitutive theory to characterize unbound granular materials. Such a theory contains as special cases the commonly applied elastic and elasto-plastic behavior types, it is capable of modeling time-dependence, and can be further compounded to simulate more sophisticated effects. Laboratory investigation is presented, in which a compacted granular specimen was exposed to a sequence of unidirectional creep and recovery cycles while under constant confinement conditions. Such a testing protocol offers an almost “automatic” separation of the behavior into resilient (viscoelastic) and permanent (viscoplastic) components. Described in detail are experimental issues related to specimen fabrication and instrumentation, and also test data processing for size reduction and resolution improvement. The measurements demonstrate that the material creeps under load and exhibits partial, time-dependent recovery while unloaded. A one-dimensional viscoelastic-viscoplastic constitutive theory is applied as a first attempt to reproduce the results. It is found that the model performs very well, simulating the observed data trends and magnitudes; it is therefore deemed potentially generalizable to more advanced conditions.