Capillary-Based Nonisothermal Suction Stress and Nonlinear Shear Strength Criteria for Unsaturated Compacted Soils

The unsaturated geological layers are a common site of many engineering projects; however, it is challenging to predict their water retention capacity and shear strength. Through analysis of soils with dual-porosity structures, a new effective stress equation was then established in this paper that considered the saturated and unsaturated fractions of a bimodal structure soil. The effective stress theory was extended to provide a new shear strength criterion that incorporates capillary water, local characteristics, and the micro–macro transition behavior of unsaturated soils. This paper also introduces a novel model of the soil–water characteristic curve (SWCC) that simultaneously combines the dual-porosity structure theory and the capillarity-adsorption water retention theory of unsaturated soils. In this model, the total amount of water retained in soil pores is explicitly distinguished by capillary and adsorbed water attraction. The adsorbed component of the SWCC displays a peak value at an intermediate suction with a hill-shaped curve, indicating the degree of saturation due to adsorption decreases after an initial increase. The capillary component of SWCC is a typical s-shaped curve, which indicates that more capillary pores are dried along with increasing suction. An extended model of the suction stress characteristic curve was then established under both isothermal and nonisothermal conditions. A good agreement is found when comparing the outcomes of experiments with the present method. It has been proved that the capillary water and particle contact area ratio impact the shear behavior of unsaturated soils. The results demonstrate that the proposed model offers a significantly more accurate prediction across various suction levels and soil types.