The 9th Victor de Mello lecture: reflections and speculations

The paper reviews the published literature on some aspects of fabric and particle behaviour in cohesionless soils. It uses insights from Discrete Element Modelling and Microcomputed Tomography to speculate on reasons for the difference in behaviour observed between moist tamped and sedimented sands at the same global void ratio and stress state. It is suggested that inhomogeneities in the form of macrovoids in moist tamped samples trigger localisation, collapse and the development of a local scale chain reaction. It questions whether a similar sequence applies at the field scale and whether expansive partial drainage influences the rate of propagation of the chain reaction. The paper offers reasons why the use of critical state based on moist tamped samples to assess the stability of tailings, may not be the best approach, and suggests that identifying instability lines for sedimented samples at appropriate void ratios and principal stress directions, may be preferable. Critical state reflects behaviour at large strains where initial and evolving inhomogeneities affect identification of relevant void ratios; instability lines reflect behaviour at small strains where inhomogeneities probably initiate collapse. The paper emphasises the importance of spatial variations in local void ratios in loose material, the importance of anisotropy of collapse potential and of load-controlled rather that strain-controlled shearing.