Mechanical modelling of drained creep triaxial tests on loose sand

The problem of mechanical instability of loose sand specimens during drained load controlled triaxial compression tests (creep tests) is discussed. In order to model mathematically such a phenomenon, a strain hardening elasto-visco-plastic constitutive model is extended so as to incorporate frictional strain-rate softening. The model is defined by a non-associated flow rule and by a Drucker–Prager-type yield surface which is equipped with two state variables, r and a. Variable r accounts for shear strain-induced anisotropy, while variable a accounts for volumetric strain hardening. The dynamic non-linearities of the strain response are discussed and theoretical predictions seem to reproduce experimental results quite well.