Hydro-mechanical behavior of undisturbed collapsible loessial soils under different stress state conditions

• Hydro-mechanical behavior of an undisturbed collapsible loessial soil is studied.
• A conventional Triaxial test device was modified and used for unsaturated testing.
• Isotropic consolidation, wetting and shear loadings are applied to the tested soil.
• The effective stress approach and the suction stress concept are considered.
• A hardening model is presented to predict the yielding behavior of the tested soil.

A conventional triaxial test device was modified to characterize the hydro-mechanical behavior of a loessial soil during isotropic and shear loadings. This device is capable of precise and continuous measurements of water outflow during the application of loading. The tests were performed on “undisturbed” cylindrical specimens, which were taken from loessial deposits in Gorgan, a city in the northeast of Iran. Experimental measurements indicate that the hydro-mechanical behavior of loess is highly affected by the extent of applied mean net stress and the level of suction. During both isotropic and shearing stages of loading, the tested specimens may exhibit collapse, abrupt decrease in volume or sudden positive volumetric strain, upon wetting or applied loading. However, the magnitude and extent of collapse are different depending on the applied state of the stress and the hydro-mechanical loading path. The results of the experiments reveal that the peak shear strength of the soil increases, as the applied mean net stress during isotropic loading or the applied matric suction increases. The shearing test results are also used to investigate the efficiency of suction stress in describing the state of stress for unsaturated loessial soils. The outcome indicated a unique critical state line for unsaturated specimens under different stress paths and loading conditions. Furthermore, considering the effective stress concept, a hardening constitutive law is presented in this study to demonstrate the hardening/softening behavior of the collapsible loessial soils.