Fabric characterisation in transitional soils

A “transitional” mode of soil behaviour implies that dense and loose samples do not converge towards the same volumes within the strains and stresses applied by simple oedometer and triaxial tests. As this behaviour involves soils with different gradings and mineralogies (e.g. gap graded, well graded and/or mixed mineralogies), identifying the factors responsible is difficult. Nevertheless, it has been previously speculated that strong forms of fabric that are difficult to break down as strains and stresses are applied, might be the common cause. This paper aims at investigating some elements of fabric at the microscale of transitional soils. A gap graded and two well graded mixtures with large amounts of non-plastic fines were investigated by oedometer and triaxial tests. As it would be difficult to identify experimentally many commonly used elements of fabric in these soils, e.g. the contact network, mercury intrusion porosimetry (MIP) was used as a first step to characterise the evolution of pore size distributions (PSDs) of dense and loose samples undergoing the same stress paths, using the PSDs as a proxy of fabric. Multi-directional bender element testing was performed to confirm the isotropy of the elastic stiffness, from which it might be inferred that the fabric is also isotropic. Statistical parameters of the PSDs were calculated, the changes of which were related to the evolution of macroscale void ratios. The robust fabrics causing lack of convergence were characterised by a complex evolution of the PSDs, the initial differences of which could not be erased during conventional testing. This work also provided a simple method to examine the fabric of particularly well graded or gap graded materials, for which other techniques, such as CT or SEM, could not reveal the multi- scale nature of the fabric