ALBERT

Description: The mechanical behaviour of cemented sands at macro-scale has been widely studied in the past, while there is still a lack of laboratory test data for the micro-mechanical response. Therefore, a series of uniaxial compression and tangential shear tests on artificially cemented sand particles have been conducted using a microscope to observe their behaviour. First, the consistency of cemented particles with different cementing agents is discussed. Three breakage modes are proposed according to the images taken using a microscope camera. A new parameter named ‘local roundness at contact’ is introduced to emphasise the effect of contact morphology on sample strength. Moreover, the effect of bond thickness on the mechanical response of cemented particles has been investigated. Finally, the shear strength parameters of artificially cemented sand samples have been determined for two different sample diameters.

Description: 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.

Description: 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.