Abstract: The interactions between liquid phase and solid matrix in unsaturated clays can be divided into capillary effects and physicochemical effects according to their formation mechanism. The physicochemical effects (that is, the adsorption effects) become significant at low saturation degrees. However, current strength models for unsaturated soils almost are established on the basis of capillary mechanism; and the contribution of adsorption effect to soil strength is neglected. Furthermore, the existing strength models for unsaturated soils apply only to compressive-shear soils but not to the tensile and tensile-shear soils. A binary-medium compressive-shear strength model is firstly proposed, involving the capillary and adsorption effects. The unsaturated clay is assumed to consist of two ideal parts, i.e. the ideal capillarity part and the ideal adsorption part, and then the corresponding ideal strength formulas are established. Furthermore, a participation function is used to reflect the degrees of capillary effects and adsorption effects. A tensile-shear strength model of unsaturated clays is secondly developed considering the various combinations of tensile-shear in unsaturated soils based on the compressive-shear strength model; and then the tensile-shear coupling strength formulas are obtained. Finally, predictions are performed on the compressive-shear strength and tensile-shear strength. Comparison between predicted results and existing test results shows that the proposed model can well describe the strength of unsaturated clays.

Key words: unsaturated clay, capillary effect, adsorptive effect, compressive shear strength, combinations of tension and shear, tensile-shear coupling strength