Since non-associated flow rule cannot be derived theoretically from Drucker’s postulate in classical plasticity

the basic principles of thermo-mechanics in continua are applied to establish the elastic/plastic clay model. According to the theory of rate-independent plasticity

multiple yield criteria are constructed from the known multiple rate independent dissipation functions and it is proved that the number of yielding functions is equal to the number of independent dissipation functions. The influence of dissipation functions on yielding criterions and flow rule is studied. A new set of functions for clay are considered taking into account associated flow rule and non-associated flow rule

Gibbs free energy and multiple dissipation functions and experimental data. The empirically derived function from the present study will include the double yielding function plastic model provided by Yin Zong-ze. These new clay models can provide better physical meaning as well as including non-associated flow rule into the formulation. The parameters of the new model are determined by laboratory tests on clay. It is demonstrated that the new clay models can simulate the constitutive relation of clay with multiple yielding functions by comparing the theoretical stress-strain curve with the experiment stress-strain results.