Over the last few decades, the description of the stressstrain-strength behavior of unsaturated soils has been closely linked with efforts to isolate the relevant stress fields governing the soil’s mechanical response. The adoption of matric suction, s = (ua − uw), and the excess of total stress over air pressure, (σ − ua), as the relevant stress state variables, have allowed the modeling of various key features of unsaturated soil behavior via suction-controlled oedometer, triaxial, and direct shear testing (Alonso et al., 1990; Wheeler and Sivakumar, 1992; Fredlund and Rahardjo, 1993).