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SUMMARYTests on air-dried Hostun sand were performed on a newly developed triaxial apparatus, which allows static and dynamic measurements at 'Département Génie Civil et Bâtiment' (DGCB) of ENTPE. The prototype was designed to perform tests in order to investigate the viscous effects of sand from small strain (some 10 −5 m/m) up to intermediate strain (some 10 −3 m/m). The accuracy of the measuring system is ensured by the use of local non-contact displacements transducers and by the use of an internal load cell. The viscous behaviour of sand is experimentally studied for loose and dense specimens for confining pressure values from 80 up to 400 kPa and considering different histories of loading. Three types of viscous loading: creep periods; stress relaxation periods; and paths with stepwise changes in the strain rate are specifically analysed. The proposed viscous modelling is issued from a three-component general framework and is able to reproduce the experimental observations. More specifically, the peculiar behaviour of sand observed during a stepwise increase or decrease in the strain rate is detailed and modelled by the viscous evanescent (VE) model, developed at DGCB/ENTPE. Based on the three types of viscous tests, a simple relation is finally proposed for the viscous parameters of the VE model. Comparisons between data and simulations validate the approach.

A constitutive modelling of the elasto-viscoplastic stress-strain behaviour of geomaterials in shear that has been developed within a non-linear three-component model framework is validated by simulating a comprehensive series of drained triaxial compression (TC) and direct shear (DS) tests on a wide variety of granular materials. Illustrative simulations of rate-dependent stress-strain behaviour of geomaterial under typical laboratory test conditions were performed to analyse the structure of the model. The versatility of the proposed model and its applicability to a wide variety of shear loading histories is examined and demonstrated by these simulations. The following results are shown. Commonly with diŠerent basic viscosity types, Isotach, TESRA and P&N, the viscous stress component has a positive component that increases with an increase in the irreversible strain rate, which makes feasible stable and realistic simulations of rate-dependent stress-strain behaviour, including creep deformation, based on the proposed model. With diŠerent unbound granular material types having similar relative densities, the creep strain in TC tests and creep shear displacement in DS tests that develop by sustained loading at a given shear stress level for a given period tends to decrease with an increase in the particle roundness. This trend of behaviours is explained by a decrease in the viscosity type parameter, u, associated with an increase in the particle roundness based on the simulations of these tests.