Undrained and drained (?) stress-strain response

The deformation response of saturated soils to a total stress increment at the ambient void ratio and effective stress state is shown to be dependent on the direction of the effective strain increment. It is argued that in a given field problem, most soil elements neither deform fully drained nor undrained, but do so partially drained. The degree of partial drainage controls the direction of the effective stress increment and hence the deformation response. Experimental data are presented which demonstrate how shear stiffness changes with the direction of effective stress increment as a function of the ambient effective stress state.Key words: stress-strain, undrained, drained, partially drained, stress path, stiffness.