Abstract: Owing to the special composition and micro-fabric, the loess exhibits particular engineering mechanical behaviors, such as water sensitivity, structural strength, moistening deformation, etc. The Bishop’ effective stress and degree of saturation are taken as the direct driven variables of the model to consider the direct effects of degree of saturation on the structural strength and deformation behavior of intact loess. The structure of the intact unsaturated loess is divided into the intrinsic structure and degree of saturation dependent cemented structure. The evolution equations are developed for modeling the change of intrinsic structure with strain and the development of compressive behavior with degree of saturation. The load-collapse yield function is developed in the degree of saturation-Bishop effective stress space to represent the change of the structural yield behavior with the degree of saturation. By coupling the void ratio dependent soil-water characteristic curve via the volumetric deformation, a hydro-mechanical coupling elastoplastic constitutive model is developed for unsaturated intact loess. The model possesses 12 parameters, all of which can be determined by the unsaturated triaxial compression test and shear test. The model is validated by comparing the available results of loess in Xi’an, Iran, and Lanzhou from triaxial compression tests and shear tests under different hydraulic and stress paths. The results show that the proposed model is capable of predicting the structural yielding, moistening deformation, hydro-mechanical coupling behavior, and the effects of loading and wetting sequence on the deformation and hydraulic behaviours of the loess. The proposed model takes the degree of saturation instead of the suction as the direct stress variable, which can represent the direct relation between the hydro-mechanical behavior and the water content of the intact loess. The present study provides a new framework for developing constitutive models for unsaturated intact loess.

Key words: intact loess, unsaturated characteristic, structural evolution, moistening deformation, hydro-mechanical coupling, elastoplastic model