The state of a material maybe defined in terms of the current state of
stress and a set of internal variables or ``history parameters'' which
embody the past loading history. Although the name internal variables are
associated with inelasticity in general, the name plastic internal variables
(PIV) is used in the current discussion to emphasize
their association with plasticity in particular. The PIV are usually taken
to be scalars, 
, such as the plastic work or components of second
order strain tensors. Since the constitutive relations refer to the
deformation of the soil skeleton, the current state of stress must be
represented by the effective stresses. According to the effective stress
concept, an effective stress increment, 
, is defined
by the relation between the total stress increment, 
, and the pore water pressure u as
where 
represents the identity tensor(Kronecker's delta). In the
following, stresses are considered to be effective, and compressive stresses
and strains are assumed positive.