THE TRIPLE-SHEAR UNIFIED CONSTITUTIVE MODEL FOR UNSATURATED CLAYS BASED ON THE GENERALIZED EFFECTIVE STRESS CONCEPT

Combining the triple-shear strength criterion with the generalized effective stress principle of unsaturated soils, the triple-shear strength criterion based on generalized effective stress for unsaturated soils was proposed. The new failure stress ratio was derived by using the equivalent substitution method and coordinate translation method, respectively. The new failure stress ratio was substituted into the modified Cambridge model for unsaturated soils to obtain the new yield function. Compared with the original modified Cambridge model whose failure stress ratio is constant, the new yield function can better reflect the total stress state, intermediate principal stress effect and tension-compression unequal effect of soils. A unified triple-shear elastoplastic constitutive model for unsaturated soils was established based on the generalized effective stress variable method under the framework of elasto-plastic theory. Taking Jiangxi remolded unsaturated clay in normal consolidation as the experimental research object, the paper carried out indoor geotechnical test, soil-water characteristic curve test, compression rebound test and conventional triaxial consolidation drainage test of unsaturated soil. The results of the constitutive model were compared with the results of unsaturated soil triaxial consolidation and drainage test. The results show that the numerical simulation results are in good agreement with the experimental results, which verifies the correctness of the constitutive model. When the axial strain is small, the simulation results of the equivalent substitution method and the coordinate translation method are relatively close. With the increase of the axial strain until the deviant stress reaches the steady state, the calculation results of the equal substitution method are greater than those of the coordinate translation method, and are closer to the experimental values. The prediction results of true triaxial calculation show that the initial compacting degree, net confining pressure and matric suction are the same under consolidation drainage condition, and the shear stress and bulk strain increase with the influence coefficient of intermediate principal stress. When the value of b is the same and the axial strain is small, the calculation results of the equivalent substitution method and the coordinate translation method are relatively close. With the increase of the axial strain, the difference between the two methods becomes larger.