Abstract: Based on the triple-shear failure criterion of materials and the mechanical properties of unsaturated soils, the triple-shear failure criteria of the single stress variable and the double stress variable for unsaturated soils are put forward, and their characteristics are analyzed. The results show that the existing failure criteria for unsaturated soils can be interpreted approximately with the new criteria by changing the influence coefficient b of the principal stress. For loci on the π plane, the failure criteria proposed in this paper cover all the convex regions from the inner boundary of the single-shear failure criterion to the outer boundary of the triple-shear failure criterion. Therefore, the new criteria are suitable for unsaturated soils under various complex stress states and can reflect the inequality in uniaxial tension and compression of the unsaturated soils. In addition, several true triaxial test results reported by other researchers are adopted to verify the proposed criteria. For the unsaturated clayey sands, the true triaxial test results were found in good agreement with the values predicted by the triple-shear failure criteria of the single stress variable and the double stress variable when the influence coefficient of the principal stress b=0.6, and the predicted values given by the double stress variable criterion are more consistent than that of the single stress variable criterion. For the unsaturated loess, true triaxial test values are also in good agreement with the values predicted by the triple-shear failure criteria when the influence coefficient of principal stress b=0.2, and the difference is smaller between the two kinds of triple-failure failure criteria than that of the clayey sands.

Key words: unsaturated soils, triple-shear failure criteria, single stress variable, double stress variable, experimental verifications