TRIPLE-SHEAR ELASTO-PLASTIC CONSTITUTIVE MODELS OF UNSATURATED CLAYS IN NORMAL CONSOLIDATION

In the framework of the critical state soil mechanics and adopting the triple-shear unified strength criteria with the methods of the single-stress variable and the double-stress variable for unsaturated soils, the yield stress and the failure stress ratio in the yield function of saturated soils are replaced respectively with the yield stress that can reflect the influence on soil saturation and the triple-shear failure stress ratio. Four triple-shear elastoplastic models of unsaturated clays are established with the methods of the equivalent substitution and the coordinate translation. These models can reflect the soil strength interval effects, the tension and compression strength differences and the cohesions for unsaturated soils under full stress states. The Nanchang unsaturated clays are taken as an example to determine the parameters of the four models, and to verify the models with the conventional triaxial consolidation drainage tests (CD tests) and true triaxial numerical simulations. The CD tests show that all the four models can well reflect the mechanical characteristics of the unsaturated clays. The models of the double-stress variable gave better approximations to the tests than that of the single-stress variable. The models with the equivalent substitution method fit better with the tests than that with the relative coordinate translation method. The true triaxial numerical simulation shows that the shear stresses and the bulk strains obtained by the double-stress variable method are larger than those obtained by the single-stress variable method. With the increase of the intermediate principal stress influence coefficient b, the shear stresses and the bulk strains from the four models will also increase. If other conditions remain unchanged, the shear stress increases with the increase of matrix suctions.