Abstract: To study the nonlinear constitutive model of unsaturated intact loess under the condition of complex stress, by considering the effects of the intermediate principal stress and suction on the major principal strain under the condition of true triaxial test, a nonlinear constitutive model considering the effects of the intermediate principal stress for unsaturated intact loess is developed. The model contains 12 parameters, which can be determined by the true triaxial tests of unsaturated soil with constant water content. The effect of the intermediate principal stress on the deformation and strength of soil is studied, and the change rules of the model parameters are discussed. When the initial suction is constant, the cohesion of the soil enlarges with the increase of the intermediate principal stress, while the internal friction angle decreases linearly with the increase of the intermediate principal stress. When the suction is less than 162 kPa, the suction friction angle is equal to 21.8°. If the suction is greater than 162 kPa, the suction friction angle is equal to 5.5°. When the initial suction is the same, the bulk modulus of the soil under the condition of different intermediate principal stresses can be taken as a constant. The model is used to predict the variation of suction in the true triaxial isotropic consolidation test with constant moisture content. The theoretical results are found close to the experimental results. However, the prediction of suction of true triaxial shear test still needs to be further explored.

Key words: nonlinear constitutive model, unsaturated soil true tri-axial apparatus, intermediate principal stress, parameter