On the mechanics of structured sands

To date the effect of structure on the behaviour of natural sands has focused almost exclusively on the component of bonding, and the effect of fabric has been largely overlooked. The paper describes a detailed investigation of the behaviour of two natural sands by means of triaxial testing over a wide range of pressures. One material had bonding as the principal element of its structure and the other fabric. Following on from a paper of Cuccovillo and Coop which examined the influence of the two structural elements on the small-strain stiffness, the current paper develops a new framework for the yielding and large-strain behaviour. It is suggested that structure should be considered as an element of the nature of a sand in addition to properties such as mineralogy, particle shape and grading. The resulting framework is then capable of encompassing the patterns of behaviour seen for both bonding and fabric-dominated sands. While bonding results in a cohesive mode of shearing, it is demonstrated that when fabric dominates, the shearing behaviour remains predominantly frictional, although the rates of dilation and peak strengths may be very much higher than for the reconstituted soil at the same stress-volume state. It is shown that it is not necessarily the position of the state of the soil relative to the critical-state line that distinguishes strain-hardening and strain-softening behaviour, but the proximity to the boundary determined in isotropic compression.