A CONSTITUTIVE MODEL FOR UNSATURATED NATURAL SOILS BASED ON DISTURBING VARIABLE

Based on the homogenization theory of composite materials, a constitutive model for unsaturated natural soils considering the coupling of hydraulic hysteresis and stress-strain behaviour is proposed. The main idea of the homogenization theory is to consider the deforming unsaturated natural soil as a mixture of two parts in relative intact (RISP) and fully adjusted states (FASP), whose mechanical properties are the same as those of natural and reconstituted soils, respectively. As the deformation progresses, RISP will gradually transform into FASP. Therefore, the deformation characteristics of them should be described by different models respectively. If elastic and elasto-plastic constitutive relations are applied, the total constitutive relation for unsaturated natural soils could be obtained according to the homogenization theory. The hydraulic hysteresis behaviours of RISP and FASP are described by the bounding surface model of Wei. The evolution equation of disturbing variable D, describing the degradation law of unsaturated natural soil structure characteristics, is proposed. There are sixteen parameters in the present model, in which two structural parameters attenuation index  and peak factor  are introduced to describe the influence of structure on the mechanical behaviour of soil, and the influences of them are investigated. By making comparisons of predictions with experimental data of unsaturated natural expansive soil and by conducting the parametric study, it is demonstrated that the new model captures the essential behaviours of the unsaturated natural soils, especially the strain-softening behaviour. Besides, the degradation law of soil structure can be well described by the proposed evolution equation of disturbing variable.