A bounding surface viscoplasticity model for time-dependent behavior of soils including primary and tertiary creep

A viscoplastic model is presented for creep and strain rate behavior of soils with particular reference to capturing drained, undrained, primary, and tertiary creep. The model is formulated in the context of the bounding surface plasticity using the consistency viscoplastic framework and the critical state theory. The formulation proposed enables capturing the accumulation of viscoplastic strains upon loading and unloading as well as creep rupture observed in overconsolidated clay. The time-dependency of the soil response is accounted for by defining the size of the bounding surface as a function of viscoplastic volumetric strain and strain rate. The model meets the consistency condition and allows for a smooth transition from rate-dependent viscoplasticity to rate-independent plasticity. Simulation results and comparisons with experimental test data are presented for several drained and undrained creep tests, constant strain rate tests, and stress relaxation tests, to demonstrate the application of the constitutive model.