Time Dependent Behaviour of Naturally and Artificially Structured Clays

Abstract

Recent studies on the time dependent behaviour of geotechnical structures have highlighted the need for careful consideration of a number of natural soil characteristics such as anisotropy, structure and strain rate dependency. Increased emphasis of the accurate prediction of structural deformations has placed further importance on correctly interpreting these soil characteristics. Yet, it appears that despite detailed ground investigations and laboratory sties, inadequate predictions of ground movements are still encountered. This has resulted in the need for a careful revision of the general time-dependent properties of structured materials. This research aimed to find a framework for understanding the various time-dependent phenomena. High quality samples from Ballina, N.S.W., Australia and Hyde Park, London, United Kingdom, and one artificially cemented material, a gypsum--kaolin mixture, were tested in their intact, remoulded and reconstituted states using oedometer and advanced triaxial apparatus. The structure and nature of the clays from the different origins were investigated microscopically and correlated with its large and small strain mechanical response. The research program investigated effects of soil structuration, strain-rate and stress-rate on the measured geotechnical properties. The evolution of plastic strains along defined stress paths were also studied in order to investigate creep behaviour in stress space. The time-dependent behaviour of structured and structureless soils was found to be consistent and were predictable under the same framework. The capabilities of a number of existing time-dependent models were investigated in capturing the viscous behaviour observed. It was found that while a number of models were able to capture some of the time effects, there is a need for (i) the inclusion of rate effects within the standard yield surface, (ii) modelling of stress ratio degradation following peak strength and (iii) a new approach to modelling the plastic strain component during creep.