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Abstract True triaxial strength criteria and three-dimensional stress state predictions for unsaturated soils are important problems in engineering applications. Based on the two stress state variables of unsaturated soils and the characteristics of suction strength，a new twin-shear element model suitable for unsaturated soils was proposed and then a piecewise linear unified strength theory(UST) for unsaturated soils was derived. The model was validated with the data from literature about the rigid and flexible true triaxial tests on unsaturated silty sand. Some special cases and the limit line on the deviatoric plane and the limit plane in the principal stress space for the UST were analyzed. It was shown that the limit loci on a deviatoric plane for the UST covered all the convex regions. The UST of unsaturated soils not only included the UST of saturated soils and the Mohr-Coulomb criterion of unsaturated soils，but also included many new strength criteria. All the true triaxial test data of unsaturated silty sand were in the range of limit loci on a deviatoric plane for the UST of unsaturated soils and agreed well with the predictions when the parameter of the unified strength theory b = 1/2. The circumscribed Drucker-Prager criterion did not reflect the true strength characteristics of unsaturated soils and the nonlinear spatially mobilized plane(SMP) criterion was linearly approximated with the UST of unsaturated soils when b = 1/2.
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2015, Vol. 34 
