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.