  We develop a non-associated flow rule (NAFR) based elasto-viscoplastic (EVP)
model for isotropic clays. For the model formulation, we introduce the critical
state soil mechanics theory (CSSMT), the bounding surface theory and Perzyna's
overstress theory. The NAFR based EVP model comprises three surfaces: the
potential surface, the reference surface and the loading surface. Additionally,
in the model formulation, assuming the potential surface and the reference
surface are identical, we obtain the associated flow rule-based EVP model. Both
EVP models require seven parameters and five of them are identical to the
Modified Cam Clay model. The other two parameters are the surface shape
parameter and the secondary compression index. Moreover, we introduce the shape
parameter in the model formulation to control the surface shape and to account
for the overconsolidation state of clay. Additionally, we incorporate the
secondary compression index to introduce the viscosity of clay. Also, we
validate the EVP model performances for the Shanghai clay,the San Francisco Bay
Mud (SFBM) clay and the Kaolin clay. Furthermore, we use the EVP models to
predict the long-term field monitoring measurement of the Nerang Broadbeach
roadway embankment in Australia. From the comparison of model predictions, we
find that the non-associated flow rule EVP model captures well a wide range of
experimental results and field monitoring embankment data. Furthermore, we also
observe that the natural clay exhibits the flow rule effect more compared to
the reconstituted clay.
