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To characterize the role of specimen disturbance and structure in the hydromechanical behavior of collapsible soils, two sets of wetting-induced collapse and suction-controlled triaxial tests were conducted on intact and reconstituted specimens of a loessial soil taken from a loess deposit in Gorgan, a city in the northeastern Golestan province of Iran. The testing approach used an advanced triaxial testing device that was specifically modified to control pressures applied to a soil specimen and to monitor and measure the amount of changes in volume and water content of the soil specimens during testing using highly sensitive digital sensors with an accuracy of 60.01 cm³. Results of the wetting-induced collapse tests show that the collapse phenomenon in intact loessial specimens was mostly a stepwise reduction in volume rather than a continuous decrease with a constant rate or a sudden drop in volume as water entered the voids. This behavior is believed to be attributed to the nonhomogenous distribution of macropores or micropores in intact specimens, which results in the presence of void spaces with different degrees of collapse potential within the soil matrix. A comparison of the experimental measurements during wetting-induced collapse also indicated that soil structure has negligible effects on the behavior of collapsible soils at high values of mean net stress (i.e., 200 kPa or higher) but significantly influences the behavior under low mean net stresses (i.e., 50 kPa or lower). The shear strength values of reconstituted specimens were higher than those of intact specimens when sheared at the same applied mean net stresses and matric suctions. Observations from this study suggest that the use of a critical-state line generated based on reconstituted specimens is not suitable for the characterization of the behavior of natural loessial soils. Finally, the disturbed-state concept (DSC) was used to characterize the stress-strain behavior of the intact and reconstituted specimens of collapsible soils during shear. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]