Elastic viscoplastic modelling of the time-dependent stress-strain behaviour of soils

This paper presents a new framework for elastic viscoplastic (EVP) constitu tive modelling. In developing the model, a general one-dimensional elastic viscoplastic (1D EVP) relationship is first derived for isotropic stressing conditions using an "equivalent-time" concept. This 1D EVP model is then g eneralized into a three-dimensional EVP model based on Modified Cam-Clay an d viscoplasticity. Fitting functions are proposed for fitting data when mod el parameters are being determined. Using these functions, a specific EVP m odel is developed which describes the time-dependent stress-strain behaviou r of soils under triaxial stress states. This model has been calibrated usi ng data from a densely compacted sand-bentonite mixture. The calibrated mod el is used to compute time-dependent (or strain rate dependent) stress-stra in curves from a multistage shear creep test and a step-changed, constant s train rate undrained triaxial compression test. Predictions from the EVP mo del are in general agreement with measured values. It is demonstrated that the model can simulate accelerating creep when deviator stresses are close to the shear strength envelope in a q creep test. It can also model the beh aviour in unloading-reloading and relaxation. Limitations and possible impr ovements are also indicated.