A thermomechanical analysis of a family of soil models

Techniques of thermomechanics, based upon the use of internal variables, are used to develop a systematic procedure for deriving elastic/plastic models of soils and granular materials. Fundamental thermodynamic state variables are defined and used to formulate free energy and dissipation potentials. These are used to determine yield conditions and flow rules. It is demonstrated that it is necessary to distinguish between plastic work and plastic energy dissipation. It is suggested that the stored energy associated with the plastic deformations is due to the fact that only a proportion of the intergrain contacts are actually plastic in a plastically deforming continuum element. The stored plastic energy of the continuum model arises from locked-in elastic energy on the micro scale. Some well-known existing critical-state models are re-examined and some of their shortcomings are highlighted. New models are proposed that overcome some of these objections. These models are able to predict non-associated flow rules, contractive behaviour and pre-peak failure for ‘loose’ soils, and aspects of static liquefaction, and can predict the position of the failure, phase change, instability and ultimate state lines. In some extreme cases the yield surfaces are found to contain concave segments, and dilatant behaviour can occur below the critical (characteristic) state line.