Abstract:The true triaxial test results for unsaturated soils indicated that the intermediate principal stress affected the unsaturated soil strength significantly. However, most existing strength formulae of unsaturated soils were based on the Mohr-Coulomb strength criterion and did not consider the intermediate principal stress effect. On the basis of the new twin-shear strength criterion of Mao-Hong Yu and the two stress state variables theory, a new twin-shear true triaxial strength criterion is derived and validated by the rigid and flexible true triaxial tests of unsaturated silty sand available in the literature. It is shown that the new piecewise linear twin-shear true triaxial strength criterion proposed in this study can reflect strength characteristics of unsaturated soils, such as the effect and its range of intermediate principal stress and strength differences among uniaxial tension and uniaxial compression; the new twin-shear strength criterion can be a linear approximation of the extended nonlinear Spatially Mobilized Plane criterion; neither the Mohr-Coulomb strength criterion nor the circumscribed Drucker-Prager criterion has good applicability to true three dimensional stress states and test data of unsaturated soils, in that the predictions of Mohr-Coulomb strength criterion are small, whereas the predictions of circumscribed Drucker-Prager criterion are obviously large.
