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Non-Coaxial Theories of Plasticity for Granular Materials

This Keynote Paper is concerned with the fundamental mechanics of granular materials with a particular reference to non-coaxial behaviour. The non-coaxial behaviour refers to the non-coincidence of principal stress directions and corresponding principal plastic strain rate directions during plastic deformation. At present, the majority of soil models used in geotechnical practice are those of a coaxial type and are not able to model the important non-coaxial behaviour. Failure to account for non-coaxial soil behaviour would lead to an unsafe design in geotechnical practice. This paper presents experimental and micromechanical evidence of the non-coaxial behaviour and its constitutive modelling using plasticity theory. In particular the paper proposes a new micromechanical fabric-based plasticity theory for modelling the non-coaxial behaviour of granular materials.