An elastic-viscous-plastic model for overconsolidated clays

The influences of time on clays are discussed first, and the concept of the instant normal compression line is proposed by ana- lyzing the existing theories and experimental results. Based on the creep law, the relationship between the aging time and the overconsolidation parameter is built. With the reloading equation of the UH model （unified hardening model for overconsoli- dated clays） used to calculate the instant compression deformation, a one-dimensional stress-strain-time relationship is pro- posed. Furthermore, the evolution of this relationship is analyzed, and the characteristic rate that is a function of the overcon- solidation parameter is defined. Then a three-dimensional elastic-viscous-plastic constitutive model is suggested by incorpo- rating equivalent time into the current yield function of the UH model. The new model can describe not only creep, rate effect and other viscous phenomena, but also shear dilatancy, strain softening and other behaviors of overconsolidated clays. Besides, compared with the modified Cam-clay model it requires only one additional parameter （the coefficient of secondary compres- sion） to consider the creep law. Finally, because the proposed model can be changed into the UH model under instantaneous loading, the elastic-plastic and elastic-viscous-plastic frameworks are unified.