On the plastic hardening of sand

Abstract

Recent research on the micromechanics of sands has suggested that plastic hardening is due to grain fracture. A theory based on a work equation suggests that, for a sand subjected to plastic deformation, suitable soil hardening parameters are the isotropic preconsolidation pressure relating to plastic reduction in voids ratio, the surface area per unit mass of solids, and the plastic work done. The theory also suggests that these parameters are linear functions of one another. An alternative theory based on fractal crushing also suggests that surface area is a suitable hardening parameter. This paper examines the behaviour of silica sand subjected to a range of high-pressure stress path tests in the triaxial apparatus, in order to try to establish suitable parameters for plastic hardening, and whether the recent theories can be supported. For this sand a simple state boundary surface can be established, and it can reasonably be assumed that strains inside the surface are elastic and isotropic. The size of the yield surface is indeed shown to be a linear function of the increase in sample surface area and of the plastic work per unit volume done on the sample, so that for stress paths to points on the state boundary surface the plastic work done per unit volume and increase in surface area are approximately independent of the path taken. However, although particle breakage is necessary for plastic hardening, most of the plastic work done is shown to be dissipated in friction.