Rheology of an artificial smectitic clay

The extremely long time in which clay surrounding canisters with radioactive waste must serve makes it necessary to consider long-term strain. It may have a sufficient shear strength for providing short-term stability but accumulated creep strain may well lead to failure if the microstructural coherence is lost. This can happen to non-expanding clays in which the number of “slip” units is more or less constant or reduced by bulk strain. For smectites, the risk of creep failure is believed to be insignificant because shearing of the originally dense clay aggregates break them up and produces an increasing number of slip units. This microstructural performance is described by a creep model based on stochastical mechanics, implying a spectrum of interparticle bond strengths that represent a first-order variation in barrier heights, and a second represented by the interaction of differently sized aggregates of discrete particles and a transient increase in the frequency of slip units. Low shear stresses cause strong attenuation of the creep rate, while for intermediate stresses the creep rate is of log time type. High shear stresses yield an increasing number of slip units that makes the clay perform as a purely viscous medium. Experiments made by the use of a new ring shear device support the theoretical creep model.