Evaluation of hydromechanical shear behavior of unsaturated soils is still a challenging issue. The time and cost needed for conducting precise experimental investigation on shear behavior of unsaturated soils have encouraged several investigators to develop analytical, empirical, or semi-empirical models for predicting the shear behavior of unsaturated soils. However, most of the previously proposed models are for specimens subjected to the isotropic state of stress, without considering the effect of initial shear stress. In this study, a hydromechanical constitutive model is proposed for unsaturated collapsible soils during shearing, with consideration of the effect of the initial shear stress. The model implements an effective stress-based disturbed state concept (DSC) to predict the stress-strain behavior of the soil. Accordingly, material/state variables were defined for both the start of the shearing stage and the critical state of the soil. A series of laboratory tests was performed using a fully automated unsaturated triaxial device to verify the proposed model. The experimental program included 23 suction-controlled unsaturated triaxial shear tests on reconstituted specimens of Gorgan clayey loess wetted to different levels of suctions under both isotropic and anisotropic stress states. The results show excellent agreement between the prediction by the proposed model and the experimental results. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

A fully-automated unsaturated triaxial device is developed at the Advanced Soil Mechanic Laboratory of the Sharif University of Technology (SUT) to investigate the hydromechanical behavior of unsaturated soils under any complicated path. The main improvements of the developed device are (1) the ability to continuously measure accurately different stress/ strain variables during long duration of unsaturated tests. This is of great importance in capturing the sudden deformations of the specimen, under wetting (2) providing a user-friendly controller software enabling the definition of any desired stress/strain variable and stress/strain paths under either stress-controlled or strain-controlled conditions. Finally, to evaluate the performance of the developed apparatus, two wetting tests were carried out to study the effect of initial shear stress on the hydromechanical behavior of unsaturated reconstituted specimens of Gorgan loess. In these wetting tests, the specimens were wetted under isotropic and anisotropic stress states by stepwise reduction of suction to approach to the saturated condition. The obtained results showed the excellent performance of the developed device to accurately follow the defined stress path along with the continuous measurement of stress/strain variables. The tested collapsible soil specimen that was wetted under the effect of initial shear stress exhibited a larger volume decrease accompanied by a larger degree of saturation compared to those of the specimen wetted under the isotropic stress state with the same initial mean net stress.