The use of the effective stress principle led to a general theory for the
strength and volumetric behavior of saturated soils. Presently, all constitutive models
for saturated soils are based on this principle. In 1959, Bishop proposed an equation for
the effective stress of unsaturated soils. However, it was severely criticized because it
could not explain by itself the phenomenon of collapse upon wetting. Moreover, an
analytical expression for the determination of its main parameter χ was not provided
and in addition, its value could not be easily determined in the laboratory. Since then
several equations to determine the value of parameter χ have been proposed. Fifty years
later, it has been acknowledged that Bishop’s effective stress equation can be employed
to simulate the behavior of unsaturated soils when it is complemented with a proper
elastoplastic framework.
Keywords: Air pressure, Collapse, Constitutive model, Effective stress, Effective
stress parameter, Elastoplastic framework, Independent stress variables, Pore
water pressure, Saturated soil, Shear strength, State surface, Suction, Total stress,
Unsaturated soil, Volumetric behavior.
