A novel elasto-viscoplastic formulation for compression behaviour of clays

This paper presents a novel elasto-viscoplastic formulation for describing the time-dependent compression behaviour of clays. The formulation incorporates a novel flow rule that attributes viscoplastic strain rate to an internal state variable representing the perturbation of the clay particle assembly due to historical straining. The proposed model can describe a wide range of observed time effects under constant rate of strain loading conditions, including both isotache and temporary effects of step changes in strain rate, while allowing creep and relaxation by way of an auto-decay process of internal strain rate. The model facilitates rational estimates of the initial creep rate in the analyses of consolidation for thick clay layers (field scale) and, hence, overcomes drawbacks of existing isotache models. The proposed formulation is also able to represent diverse observations of primary consolidation for clay layers of different thicknesses, providing a unifying framework that can resolve the long-standing paradox in observed clay behaviour (hypotheses A and B). The model is applied to re-interpret the effects of specimen thickness on the consolidation behaviour of Osaka Bay mud in laboratory tests and show that apparent contradictions in the data can be related to rate-dependent properties of the clay.