Observed effects of interparticle friction and particle size on shear behavior of granular materials

This paper presents an experimental study on the shear behavior of granular materials, focusing on the effects of interparticle friction and particle size, which are of fundamental importance but are not yet well understood. The experimental program consisted of a large number of direct shear tests on glass beads of varying sizes and interparticle friction conditions, performed under a range of packing densities and normal stress levels. Test data were interpreted in terms of the stress-dilatancy relationship and shear strength parameters. The study indicates that under otherwise similar testing conditions, oil-lubricated glass beads tend to have substantially lower shear strength as compared with water-lubricated, water-flooded, and dry glass beads. It has also been found that at similar particle size uniformity, increasing mean particle size (d50) leads to more dilatant shear response and higher shear strength. A generalized stress-dilatancy relation is proposed, which introduces a variable dilatancy coefficient that reflects on the effects of interparticle friction and particle size. It is shown that classical stress-dilatancy relations can be regarded as special cases of this generalized case, with the dilatancy coefficient being taken as a constant. Further explanations for the observed effects on macroscopic behavior are provided from the micromechanics perspectives. © 2015 American Society of Civil Engineers