Continuously unique anisotropic critical state hyperplasticity.

This paper presents the theoretical development and methodological motivation of a single surface anisotropic hyperplasticity model. The model extends the isotropic family of models developed by Coombs and Crouch by: (i) introducing anisotropic shearing into the yield surface, (ii) relating two of the material constants to a single physical quantity and (iii) using a more physically realistic pressure sensitive elastic free energy function. This model overcomes the difficulty of determining the constants of the isotropic two-parameter surface by analytically relating them to a single experimentally measurable physical quantity, namely the normalised hydrostatic position of the Critical State. This provides a model with a Critical State surface that is constant throughout the loading process, invariant of the level of anisotropy inherent in the yield envelope. The model is compared with experimental data from triaxial tests on Lower Cromer Till, contrasted against the SANIclay model and the recent model of Yang et al. (2015) as well as being compared with rarely considered experimental data from hollow cylinder tests on London Clay