Influence of Fine Content on the Soil–Water Characteristic Curve of Unsaturated Soils

Unsaturated soils often exist as a mixture of coarse and fine particles in nature. However, the relationship between fine content and the soil–water characteristic curve (SWCC) is not sufficiently understood. In this paper, ten kinds of mixed soils with different percentages of fine contents were tested with the pressure-membrane apparatus, and the SWCC for each fine content was obtained. The results showed that when keeping the fine content between 10 and 60%, the water-holding capacity of the soil gradually decreased as the fine content increased. Otherwise, the water-holding capacity of the soil increased. With the same effect of matrix suction, the volumetric water content increased initially and then decreased with the increase of fine content. With the fine content within the range of 10–60%, a higher fine content resulted in a larger dehydration rate; otherwise, the dehydration rate of the soil decreased. In addition, a new fitting model was established for SWCC. The applicability of the new model was verified by combining new data with the existing experimental data and then discussing the physical meaning of each parameter in the model. Through the research in this paper, the quantitative relationship between fine content and SWCC was clarified, and a new SWCC model was established. The conclusions in this paper are helpful to understand the effect of the proportion of fine particles on the SWCC. The physical parameters of unsaturated soils which mixes fine and coarse particles, such as adsorption coefficient, permeability coefficient, and shear strength, could be obtained.