Elastic/plastic models for soils and sands

Some recent advances in the constitutive modelling of soils, sands and other granular materials are reviewed. These new ideas are based on the use of the concepts of the modern theory of the thermomechanics of continua. Most existing engineering theories of soil behaviour are in the nature of “recipes”, providing rules by which yield loci, flow and hardening rules and failure lines may be constructed to provide models which predict the response and failure of granular materials in a limited set of laboratory experiments. These models rarely have any firm physical basis. The thermomechanical approach, on the other hand, enables us to construct models, which both satisfy the fundamental laws of thermodynamics and have definite physical interpretations in terms of energy storage and dissipation. Of particular importance is the distinction between the rate of plastic work and plastic dissipation, the intimate connection between dissipation and the yield loci and flow rule, and between plastic dilation and induced anisotropy. These ideas lead naturally to the concepts of critical state soil mechanics. In addition the thermomechanical approach offers the real possibility of relating the observed and simulated behaviour of granular materials on the micro- and meso-scales, to the observed behaviour at the continuum level. Some initial steps in this direction are outlined at the end of this paper.