Effect of Soil Structure and Disturbance on Hydromechanical Behavior of Collapsible Loessial Soils

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 ±0.01 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.