A new unified failure criterion for unsaturated soils

True triaxial failure criterion and three-dimensional stress state prediction are significant to implement the unsaturated soil mechanics into engineering applications. Based on the theory of two stress state variables and the characteristics of suction strength, a twin-shear model suitable for unsaturated soils was proposed; then, a new piecewise linear unified failure criterion (UFC) of unsaturated soils was derived and validated by the rigid and flexible true triaxial tests of unsaturated silty sands available in the literature. Special cases, limit loci on a deviatoric plane and limit surfaces in the stress space for the new UFC of unsaturated soils were discussed. It is found that the limit loci on a deviatoric plane for the new UFC cover all the convex regions; the new UFC not only encompasses the UFC of saturated soils and the Mohr–Coulomb criterion of unsaturated soils, but also introduces many additional new failure criteria for unsaturated soils; all the true triaxial test results of unsaturated silty sands are in the range of limit loci on the deviatoric plane for the new UFC, and more the test results agree well with these predictions under the UFC parameter b of 1/2; the circumscribed Drucker–Prager criterion overestimates real strength of unsaturated soils and the extended nonlinear Spatially Mobilized Plane criterion could be linearly approximated by the new UFC of unsaturated soils when b = 1/2.