Stress-Deformation Analysis for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 669

Figure 13.4 Testing soil structure constitutive surface of illite clay for uniqueness (after Barden
et al., 1969a).

Figure 13.5 Testing degree-of-saturation constitutive surface of illite clay for uniqueness (after
Barden et al., 1969a).

describing volume change behavior (Aitchison and
Woodburn, 1969; Brackley, 1971; Aitchison and Martin,
1973). The use of σ − u a and u a − u w as stress state
variables for an unsaturated soil was further advocated by
Fredlund and Morgenstern (1977). A stress analysis based
on multiphase continuum mechanics showed that any two

of three independent stress variables (i.e., σ − u a , u a − u w ,
and σ − u w ) could be used to describe the stress state.
Experiments to test uniqueness of constitutive surfaces
provided further evidence as to why σ − u a and u a − u w
have been successfully used to describe the volume change
characteristics of an unsaturated soil.