TRIPLE-SHEAR BOUNDING SURFACE MODELS FOR UNSATURATED CLAYS IN NORMAL CONSOLIDATION

The triple-shear unified failure criterion for unsaturated clays is proposed by combining the triple-shear unified strength theory, the single stress variable theory and the double stress variable theory, so that the two triple-shear failure stress ratios which can reflect the effects of full stress states acting on the soils are deduced with the methods of equivalent substitution and coordinate translation. The triple-shear failure stress ratios are then interfered with the yield function of the modified Cambridge model for unsaturated clays to determine the initial bounding surface, and the subsequent bounding surfaces are formulated by the radial mapping method, and the moduli of plasticity on the loading surface are obtained by employing the interpolation function. Based on that, the four triple-shear bounding surface models in elasto-plasticity for unsaturated clays are established according to the theories of single stress variable and double stress variable together with the methods of equivalent substitution and coordinate translation respectively, and are verified with monotonic and cyclic compression tests on several clays. The results show that all the four models can well reflect the mechanical characteristics of the selected unsaturated clays. Comparatively, models of the double stress variables can give closer approximations to the tests than models of the single stress variable, and models with the equivalent substitution method can also give better fits to the tests than models with the relative coordinate translation method. Results of the true triaxial numerical simulations on unsaturated clays show that under monotonic loading, the strength of the unsaturated clays increases with the influence coefficient of the intermediate principal stress under definite stress conditions. Simulations also show that under cyclic loading, the shear strength of the unsaturated clays increases with the minimum principal stress or intermediate principal stress, and a greater amplitude of load vibration leads to a larger axial strain accordingly. The simulated result comparisons among the four constitutive models under true triaxial stress state show that the shear strength of unsaturated clays calculated with the double stress variables model is relatively larger than that with the single stress variable model under the monotonic loading, and the plastic deformation with the single stress variable model is relatively larger under cyclic loading.