Abstract: The mixing ratio of sand influences the swelling characteristics of bentonite-sand mixtures. As for the pure bentonite and bentonite-sand mixtures with low sand mixing ratio, the relation between montmorillonite void ratio e_m and vertical stress σ_v is a unique line in their logarithmic coordinates. However, for the mixtures with high sand mixing ratio, sand skeleton will not be formed when the specimen is wetted under a small vertical stress, and the linear relation between e_m and σ_v is still satisfied. When the stress is larger than ‘deviation starting stress’, the sand skeleton is formed and e_m value deviates from the line. According to the concept of sand skeleton void ratio, the ‘deviation starting stress’ when the sand skeleton is formed can be determined for the mixtures with different sand mixing ratios, and the range of sand mixing ratio in which the sand skeleton may be formed can also be obtained. Before sand skeleton comes into being, sand particles are surrounded by montmorillonite, hence the vertical stress is mainly burdened by montmorillonite, and the swelling quantity at full saturation including swelling deformation and swelling pressure is determined by the montmorillonite content per unit volume. After the sand skeleton is formed, the vertical stress is burdened by the montmorillonite and sand skeleton. A method is proposed to determine the load sharing ratio burdened by the two parts, and subsequently, the swelling can be obtained after the sand skeleton is formed. The proposed method is testified by the swelling test results, and it can predict the swelling variations due to wetting for the mixtures with high sand mixing ratios.