UNIFIED 3D CRITICAL-STATE BOUNDING-SURFACE PLASTICITY MODEL FOR SOILSINCORPORATING CONTINUOUS PLASTIC LOADING UNDER CYCLIC PATHS .1. CONSTITUTIVE RELATIONS

The unified three-dimensional (3D) critical state bounding-surface pla sticity model gUTS enables clays, silts and sands to be treated within a single framework. Furthermore, loose and dense states of a particul ar soil subjected to a wide range of confinements are viewed as a sing le material defined by the same set of constants. The model is able to handle both monotonic and complex cyclic paths including those involv ing a rotation of the principal stress directions. The model incorpora tes the following features: combined use of radial and deviatoric mapp ing rules and the use of an apparent normal consolidation line for san ds; use of a non-associated flow rule where the ratio of the rates of volumetric plastic strain to deviatoric plastic strain is a function o nly of the ratio of deviatoric to mean effective stresses and the Lode angle; adoption of a bi-linear critical state line projected onto the plane of the void ratio versus logarithm of mean effective stress; in clusion of a sub-elliptic, or super-elliptic, segment in the plastic d ilatancy surface for stress ratios less than critical; use of elliptic segments in the deviatoric planes; movement of the projection centre in the deviatoric mapping region and incorporation of a plastic stiffe ning effect for cyclic paths which repeatedly load in the same deviato ric direction.