Constitutive, or stress-strain laws, represent mathematical models that describe the macroscopic material behavior that results from the internal constitution of a material. To establish the constitutive relationship in geomaterial, the plasticity formulation takes the most important part since the behavior of gematerial shows that most deformation occurs in the inelastic range even though the purely elastic deformation exists. The importance of constitutive laws has been enhanced significantly with the great increase in development and application of many modern computer-based technicques such as the finite element, finite difference, and boundary integral equation methods. In any engineering analysis, these constitutive laws play a crucial role providing reliability to the results obtained from the numerical prodedures.

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