In developing constitutive equations for materials we have, so far, considered only
relationships between changes of effective stress and changes of strain. This means
that no strains occur at constant load (except at failure). In addition it was assumed
that the relationships between stresses and strains were independent of the rate of
loading or the rate of straining. In soils there are a number of time and rate effects
mainly due to drainage of water and, to a limited extent, due to creep and viscosity in
the soil skeleton.