The influence of particle characteristics on the behaviour of coarse grained soils

This paper describes an experimental study examining the influence of the mechanical and geometrical properties of the constituent grains on the overall material response of cohesionless granular materials. Glass ballotini were used as an analogue soil; their relatively simple geometry allowed the influence of particle shape and inter-particle friction to be examined independently. Techniques were developed to control the surface roughness of the ballotini to facilitate a parametric study. The particle shape was also varied by crushing the ballotini. At the micro-scale, the particle characterisation included accurate measurements of inter-particle friction, contact stiffness, particle surface roughness and particle shape. At the macro-scale the sensitivity of overall material response to changes in surface roughness and geometry was characterised using triaxial tests and oedometer tests on smooth spherical ballotini, roughened ballotini and crushed angular ballotini. Compression tests indicated that the initial load deformation response at particle-particle contact points is significantly softer than previously believed. Optical interferometry of particles after single particle-particle shearing tests confirmed that plastic strains occurred at the contact point, which were related to plastic yield. A Hertzian response was only seen at higher contact loads. A clear relationship between the inter-particle friction and the particle surface roughness was found. However, the macro-scale experiments indicated that while the material response may be slightly dependent on the surface roughness and friction, the influence of particle shape is very much more significant.