A PROCEDURE TO DETERMINE NONASSOCIATED CONSTITUTIVE-EQUATIONS FOR GEOMATERIALS

A procedure for determining a phenomenological elastic/viscoplastic nonassociated constitutive equation for geomaterials is presented. For this purpose, triaxial test data obtained with either a ''true'' or a classical triaxial device are necessary. The constitutive equation is aimed at describing such geomaterial properties as creep, irreversible compressibility or dilatancy, work-hardening, damage, and failure. Long-term failure can also be described with this model. According to the procedure, first the elastic parameters are determined from unloading tests (which follow short-term creep tests), then the yield function is determined, and finally the viscoplastic potential. No a priori assumption is made concerning the form of the yield function or of the viscoplastic potential; their expressions are obtained from the data by using the procedure suggested here. Examples for sand and rock salt are given. Comparisons of the model predictions with the experimental data are discussed.