Fractal Shear Strength Model for Unsaturated Soils Considering the Combined Effects of Matric Suction and Surface Tension

Existing shear strength models for unsaturated soils have paid limited attention to the effect of surface tension. Recent research has demonstrated that this effect cannot be disregarded. This paper examines the influence of surface tension on soil shear strength, with the soil–water characteristic curve serving as an indirect indicator of the soil’s microscale pore size distribution. Utilizing fractal theory, this study explores the characterization methods of matric suction and surface tension on shear surfaces from the perspective of microscale pores during the transition of soil from a saturated to an unsaturated state. A new forecasting model for predicting the shear strength of unsaturated soil has been developed, considering the combined effects of matric suction and surface tension. To validate the effectiveness of the model, shear strength tests were conducted under different matric suction conditions using a global digital systems (GDS) unsaturated triaxial testing system for the two soil samples. Finally, the measured shear strength was compared to the model’s predictions. The results indicate that the R2 values for the prediction model were all above 0.90 under varying dry densities, net confining pressures, and matric suction levels, indicating a strong agreement between predicted and measured values. The predictive model has a good forecasting performance.