Abstract
A generalized three-dimensional constitutive model for isotropic cohesive soils, based on the concept of the bounding surface in stress space, is developed within the framework of coupled elastoplasticity-viscoplasticity and critical state soil mechanics. The prominent feature of the bounding surface concept is the fact that inelastic deformations can occur for stress points within the bounding surface. The consideration of viscoplastic characteristics sets the current model apart from previous bounding surface formulations for soils, and introduces rate and time effects. The coupling between plasticity and viscoplasticity for stress states within the bounding surface differentiates the present work from the classical formulation of pure viscoplasticity (no coupling), or from formulations involving plasticity and viscoplasticity with a yield surface (coupling only for states on the yield surface). General incremental constitutive relations applicable to cohesive soils are developed and suitably specialized for isotropic cohesive soils. A minimum number of parameters are introduced and a procedure for their determination is outlined. The predictive capabilities of the model are treated in a companion paper.
