The viscous properties, or loading-rate eŠects on the stress-strain behaviour, of unbound and bound soils, in particular unbound granular materials, are summarised. The viscous properties were evaluated by stepwise changing the strain rate, ·e, and performing sustained loading during otherwise monotonic loading (ML) at a constant ·e and also by performing ML tests at diŠerent constant values of ·e. Four basic viscosity types, Isotach, Combined, TESRA (or Viscous Evanescent) and Positive & Negative (P & N), which were recently found are described. The Isotach type is the most classical one and, in the case of ML, the current viscous stress component is a function of instantaneous irreversible strain, e ir and its rate, ·e ir . So, the strength during ML at a constant ·e increases with ·e. With the other three types, the viscous stress increment that has developed at a given moment, denoted as Ds v , decays with e ir towards diŠerent residual values during subsequent ML. With the TESRA type, Ds v decays eventually totally and the strength during ML at constant ·e is essentially independent of ·e. With the Combined type, Ds v decays with e ir like the TESRA type, but it does not decay totally. So, the strength during ML at constant ·e increases with ·e like the Isotach type. With the P & N type, found latest, a positive value of Ds v decays towards a negative value. So, the strength during ML at constant ·e decreases with an increase in ·e. The viscosity type tends to change with e ir : e.g., from Isotach toward TESRA and from TESRA toward P & N. A general mathematical expression that can describe these four viscosity types and transitions among them is proposed. Numerical simulations of typical drained triaxial compression tests of geomaterials based on a non-linear three-component model incorporating the general expression of the viscous stress are presented. The viscosity type is controlled by at least, grading characteristics and particle shape.
