Abstract: Undisturbed Q3 loess is a typical unsaturated soil and has obvious structure which is closely related with its mechanical and deformation properties. The elastic-plastic constitutive model for undisturbed Q3 loess would consider structured evolution in the loading and collapsing process, which would meet the inherent characteristics of the mechanical deformation of undisturbed loess. Assuming that the yielding response of undisturbed loess is the coupling of remolded loess and structure of undisturbed loess, a new elastic-plastic constitutive model of undisturbed Q3 loess considering the influence of suction and structure is established based on meso-structured evolution in this paper. The new elastic-plastic constitutive model has two aspects including soil skeleton deformation and water changes, loading and loading-collapsing process are described, respectively. For soil skeleton deformation, the structured evolution equations obtained by CT triaxial tests of undisturbed Q3 loess in the process of loading and loading-collapsing are introduced into Barcelona unsaturated soil elastic-plastic modified model, respectively. The generalized soil-water characteristic curve is used to describe the aspect of water content considering influence of net mean stress and deviator stress. All of 22 material parameters of the new proposed elastic-plastic constitutive model can be determined by the unsaturated soil tests. The validity of elastic-plastic constitutive model is verified preliminarily by comparison of computation results and test data in this paper. The establishment of elastic-plastic constitutive model deepens the understanding of mechanical characteristics of unsaturated undisturbed Q3 loess, which will provide the gateway to reasonably analyze collapsible deformation coupling problems of loess foundation.

Key words: Q3 loess, structure, collapsible deformation, elastic-plastic constitutive model