A unified constitutive model for instantaneous elastic–plastic and time-dependent creep behaviour of gravelly soils under complex loading

The behaviours of gravelly soils under monotonic, cyclic and creep loading are generally simulated in separated processes with different models, which cannot describe the interactions between the instantaneous elastic–plastic and time-dependent creep behaviours under complex loading. To address the problem, an elastic–plastic–creep model framework for gravelly soils is described to present its basic components and the differences from the elastic–plastic model framework. Within this framework, a unified constitutive model for instantaneous elastic–plastic and time-dependent creep behaviour is proposed based on the widely used generalized plasticity model. In modelling the elastic–plastic behaviour, the plastic modulus in the original model is modified to capture the creep hardening by a unified hardening state parameter for monotonic, cyclic and creep loading. In modelling the creep behaviour, the creep strain rate independent of the time variable is presented to reflect the creep behaviours under the unloading–reloading path and its difference from that of virgin loading. The proposed model can be easily implemented into the computing program as the original model with only minor adjustments. The ability to simulate the time-dependent behaviour under multistage loading/reloading-creep, different strain rates, step changes in strain rate, and stress relaxation are proven with the experiments.