Fundamental issues of elastic viscoplastic modeling of the time-dependent stress–strain behavior of geomaterials

In this paper, a number of fundamental concepts are presented and explained. These include (1) differences among an instant compression line, a normal consolidation line, and a true instant compression line; (2) the uniqueness of viscoplastic strain rates with a stress–strain state; (3) whether the creep compression is smaller than the instant compression; (4) the separation of the total strain rates; (5) the relation between elastic–plastic models and elastic viscoplastic (EVP) models, etc. The major conclusions are the following: (1) the elastic compression is the true instant compression; (2) the magnitude of a creep-strain rate at a stress–strain state point is unique, independent of the loading path to reach this point; (3) the true instant (elastic) compression is much smaller than the creep compression; (4) it is more appropriate that strain rates of geomaterials are composed of elastic strain rates and viscoplastic strain rates; (5) the one-dimensional (1D) EVP (1D EVP) is a genuine extension of Maxwell’s linear rheological model for considering the nonlinear behavior of soils; (6) the EVP model is more general than an elastic–plastic model; (7) the nonlinear functions proposed by the author are good for fitting the creep compression and the compression under high stress of most soft soils in 1D straining; and (8) the three-dimensional EVP model is rigorously derived using the 1D EVP model approach and the modified Cam–Clay model, but further improvements of this model are still needed. At the end, a number of areas are presented for further study.Department of Civil and Environmental Engineerin