Experiment, modelling, and simulation of rate-dependent stress–strain behaviour of unbound geomaterials

Various trends in rate-dependent stress–strain behaviour caused by the viscous properties of a wide variety of unbound geomaterials that were observed under drained triaxial compression, plane strain compression, direct shear, and one-dimensional compression (i.e., oedometer testing) are summarized referring to those of bound geomaterials and others. The following findings are shown. As the interparticle contact points become more stable and the stability of the particles against rotation increases, the viscous properties become closer to the traditional type, namely, Isotach, whose stress–strain behaviour is determined by the instantaneous irreversible strain rate and whose strength during monotonic loading (ML) at a constant strain rate increases with an increase in the strain rate. A couple of non-traditional (i.e., non-Isotach) types of properties were found with granular materials (GMs) that exhibit noticeable creep deformation, stress relaxation, and changes in stress upon changes in the strain rate. Unbound poorly graded sub-angular to angular GMs exhibit the so-called TESRA type of properties (also called the viscous evanescent type), for which the stress–strain behaviour during ML at a constant strain rate is essentially independent of the strain rate. Unbound poorly graded sub-round to round GMs display the so-called Positive and Negative (P&N) type of properties, for which the strength during ML at a constant strain rate decreases with an increase in the strain rate. The combined type displays intermediate behaviour combining the Isotach and TESRA types of properties. These types of viscous properties are quantitatively characterized by a couple of functions and parameters, and they were incorporated into the non-linear three-component (NTC) model. All the observed trends in rate-dependent stress–strain behaviour are well simulated by the NTC model.