Modelling effects of recent suction history on small-strain stiffness of unsaturated soil

Recent suction history has been found to affect shear modulus of unsaturated soil at small strains (i.e., from 0.001% to 1%). In this study, a bubble model for unsaturated soil is developed within the framework of kinematic hardening and bounding surface plasticity. An elliptical elastic bubble is defined inside a modified Cam-clay bounding surface. Being a key feature of the proposed model, the size of the elastic bubble is modelled as a function of suction, degree of saturation, and plastic volumetric strain. Translation of the elastic bubble is governed by suction, degree of saturation, and stress increments. Moreover, hardening modulus depends on not only stress and void ratio, but also suction, degree of saturation, and relative position of the elastic bubble and the bounding surface. The proposed model is evaluated using suction-controlled constant-p shear tests on completely decomposed tuff (silt). It is evident that the new model is capable of capturing well the effects of recent suction history on nonlinear stress–strain relation and shear modulus degradation at small strains.

Effects of current suction ratio and recent suction history on small-strain behaviour of an unsaturated soil

Although the small-strain shear modulus of saturated soils is known to be significantly affected by stress history, consisting of the overconsolidation ratio (OCR) and recent stress history, the effects of suction history on the small-strain shear modulus of unsaturated soils have rarely been reported. In this study, the effects of suction history, which refers to current suction ratio (CSR) and recent suction history, on both the very-small-strain shear modulus (G0) and shear modulus reduction curve of an unsaturated soil, are investigated by carrying out constant net mean stress compression triaxial tests with bender elements and local strain measurements. In addition, the effect of suction magnitude on G0 and the shear modulus reduction curve is also investigated. At a given suction, G0, elastic threshold strain (εe), and the rate of shear modulus reduction all increase with CSR. On the other hand, the effect of recent suction history on G0 is not significant. The effect of direction of recent suction path (θ) on the shear modulus reduction curve is not distinct. However, the magnitude of recent suction path (l) affects the shear modulus reduction curve significantly when θ = –90°.

Modelling the influence of stress–strain history on the initial shear stiffness of an unsaturated compacted silt

This is the follow-up paper to Vassallo et al. (2007), which discussed the experimentally observed small-strain behaviour of an unsaturated compacted silt. The influence that suction and, more in general, mean net stress – suction history has on the initial shear stiffness was analysed and ascribed to the accumulation of irreversible volumetric strains. In this study, a model able to predict the observed behaviour is proposed, based on classical unsaturated soil volumetric hardening elastoplastic formulations. Starting from the interpretation of the results relative to simple stress paths, such as preliminary equalization and loading–unloading compression, the results of "complex" stress paths, such as those including drying–wetting cycles, have subsequently been modelled by introducing some additional parameters.Key words: unsaturated, compacted, small strain, stiffness, volumetric behaviour, stress history.