The concept of stored plastic work or frozen elastic energy in soil mechanics

This paper is part of a series that aims to explore systematically the applications of the modern theory of thermomechanics to the constitutive modelling of soils and other geomaterials. Although the division of the applied work into recoverable elastic energy and irrecoverable plastic energy is straightforward at the level of single grains, this is not true at the continuum level. Because of the highly heterogeneous nature of the stress and deformation fields at the micro level, some of the micro-level elastic energy is 'frozen', and consequently some of the applied plastic work is stored and is recoverable. The concept is explained at the micro level, at the meso level of the force chains, and at the continuum level. It is also explained in terms of a schematic spring-slider model. These ideas have very important ramifications for constitutive modelling at the continuum level. In particular it is shown that there are two causes for dilation in a soil, one due to Reynolds' dilatancy, the other due to the recovery of the frozen energy (or stored plastic work). This has significant consequences for the concept of critical states. Implications for future research are indicated.