Enhanced understanding of subgrade soil hydraulic characteristics: Effects of wetting–drying cycles and stress states on subgrade water migration

The wetting–drying cycles and overlying stresses experienced by unsaturated subgrade alter soil pore structure, resulting in variations in its hydraulic characteristics, ultimately affecting subgrade water migration and moisture distribution. However, the combined effects of these two phenomena on key subgrade hydraulic characteristics including the soil–water characteristics and the permeability property remain unclear. This paper aimed to experimentally measure soil–water characteristic curve (SWCC) and saturated permeability of subgrade soil under various overlying stresses and wetting–drying cycles. A customized SWCC measurement system and a modified permeability testing device were developed to conduct the test program on compacted silty clay soil specimens. Subsequently, the influence of overlying stress and wetting–drying cycles on the hydraulic characteristics of the subgrade was analyzed, with a focus on the SWCC and saturation permeability coefficient, and the corresponding fitting model is established. Finally, the water transfer behavior of subgrade considering the effect of wetting–drying cycles and overlying stresses was analyzed using finite element method (FEM). The results indicated that an increased number of wetting–drying cycles and higher overlying stress led to a decrease in the saturated moisture content of subgrade soil, an increase in the air-entry value, and a flatter SWCC. The saturation permeability coefficient increased with the number of wetting–drying cycles, but decreased with increasing overlying stress. Furthermore, a high correlation was observed between the SWCC and permeability coefficient predictive model, considering both the number of wetting–drying cycles and overlying stress. Lastly, when considering the stress state and wetting–drying cycles, significant differences in the evolution of matric suction of the subgrade during rainfall were observed. The overlying stress amplifies the decrease in matric suction of the soil above the subgrade, while wetting–drying cycles reduce this decrease.