GENERALIZED CREEP AND STRESS-RELAXATION MODEL FOR CLAYS

Creep and stress relaxation are two processes where phenomenological models are most useful in predicting soil behavior. However, they are often treated as two separate phenomena although physical considerations suggest that one process could be responsible for bringing about the other. This paper aims at unifying existing phenomenological models for creep and stress relaxation. A unique stress-strain-time function for cohesive soils is postulated and used to construct a rate-constitutive equation. The delayed model is generalized in terms of the first two stress invariants to describe the volumetric and deviatoric responses of soils, herein interpreted as induced by stress relaxation. For the volumetric model, the inverted secondary compression law is used; for the deviatoric model, the inverted Singh-Mitchell creep equation is assumed. To further generalize the model, the characteristic time scales are replaced by so-called age variables computed from the stress distances of a state point to its volumetric and deviatoric images. The performance of the composite model is then assessed by comparing the model predictions with reported creep and stress relaxation behavior of bay muds under isotropic and triaxial stress conditions.