Effect of Confining Stress on Soil Water Retention Curve and its Impact on the Shear Strength of Unsaturated Soils

Abstract

To predict the shear strength of unsaturated soils, several equations have been proposed in the literature. The majority of these equations have the air entry value as the controlling parameter. In this study, an experimental program is planned to investigate the effect of confining stress on the air entry and air expulsion values, in the drying and wetting process, respectively. To evaluate the influence of stress state on these parameters, the soil water retention curve (SWRC) of the soil is determined under two different applied net stresses. For this purpose, a novel miniature pressure plate apparatus was designed to determine the SWRCs under different net stresses. The soil specimens are sheared in a suction‐controlled triaxial apparatus, and the shear stress of the soil is measured. The measurements are performed under net confining stresses identical to the net stresses for which the SWRCs are obtained. Furthermore, the shear strength behavior of unsaturated soil samples are investigated in drying and wetting branches, and the values of effective internal friction angle and cohesion are determined for each path. Comparison of the obtained values to those of saturated samples revealed a rather significant difference. Finally, the shear strength values obtained from these measurements are compared with the predicted shear strength from some of the existing equations. In addition, variations of the χ parameter during drying and wetting processes and its dependence on the air entry/expulsion value were investigated and compared with reported equations. Moreover, a relationship is proposed for the effective stress parameter that can better capture the effect of net stress and, consequently, results in better predictions for the considered soil type.