Abstract: A widely accepted two parameters theory of unsaturated soil mechanics by Fredlund, using net stress and suction as stress-state variables, cannot consider the effect of suction history and saturation on stress-strain behaviour and strength of unsaturated soils. In this paper，the triaxial compression test results show that even if the same path of net stress and suction, stress-strain behaviour and strength are not the same between the samples experienced different suction history. While other conditions are the same, the samples experienced drying-wetting cycle have higher relationship of stress ratio-strain, higher strength, smaller volumetric deformation than the sample not experienced drying-wetting cycle. The samples experienced drying-wetting cycle have low saturation but high relationship of stress ratio-strain and strength. It is because larger suction experienced previously is equivalent to larger effective pressure experienced, that makes the sample to become overconsolidated state. Study of effect of different suction histories on unsaturated soil behaviour is needed further for providing the basis of experimental data. With the data, the elastoplastic constitutive model coupling hydraulic and mechanical behaviour could express behaviour of unsaturated soils more accurately and quantitatively.

Key words: unsaturated soil, stress-state variables, suction history, stress-strain behaviour