Yielding and pre-failure deformation of structured sands

Many natural sands have a structure that is imparted to them during their geological life and that significantly modifies their pre-failure behaviour when compared with that of the reconstituted material. Unlike clays, for which emphosis has been given to both the bonding and fabric components of structure, for sands structure has generally been simply identified with bonding, whereas the effects of fabric have often been overlooked. An understanding of the way in which these components of structure influence the yielding and stiffness of these materials is required to formulate costitutive models that can adequately predict their response. The paper examines the pre-failure behaviour of two structured sands in triaxial compression performed over a wide range of pressures. Since yielding occurred at small strains, laboratoty techniques were developed to produce reliable and accurate determinations of stiffness ranging from 1 microstrain to failure. The influence of structure on the shear stiffness was investigated by comparing the behaviour of intact and reconstituted soils after accounting for differences in state. To explain how bond degradation developed, the study focused in particular on the shear behaviour between yielding and failure. The bond degradation resulted in a progressive transformation of the structured soil into a frictional material, giving rise to changes in the yield stress and shear stiffness that contrasted with the strain-hardening behaviour of the unbonded sands. As bonding degraded, the variation of the shear stiffness with state was seen to depend on which structural feature was predominant