A critical state plasticity model for granular soils

A new constitutive model for granular soils is introduced. The primary objective of this study was to develop a relatively simple constitutive framework that could describe important characteristics of granular soil behavior with a limited number of soil parameters. The goal was for the model to be useful as a conceptual tool in teaching and in practice, and also be suitable for incorporation into numerical models to improve predictions of granular soil behavior. The proposed constitutive model is based on classical plasticity theory, critical state soil mechanics, and the concept of a state parameter. A key feature of the model is that shear resistance is generated from both frictional and dilational components. The model is capable of describing the constitutive response of granular soils loaded monotonically in shear for both drained and undrained conditions with one consistent set of soil parameters, seven of which are the same for both drained and undrained conditions. An axisymmetric formulation is developed to simulate standard triaxial compression tests from which all soil parameters are obtained directly. The stress-strain behavior estimated by the model compares favorably to triaxial test data provided by Mr. Mike Jefferies of Golder Associates. Model predictions are also illustrated over a range of initial void ratio, mean effective confining stress, and axisymmetric stress paths. The proposed model is shown to simulate strain hardening, strain softening, liquefaction, and phase transformation in a consistent manner