Abstract: This paper is presented to respond to the phenomenon that swelling and failure are liable to appear within the top of the traditional gravel piles (within depth range of 1 to 3 times the pile diameter). Firstly, the solid (concrete)-bulk (gravel bulk material) tandem composite pile is proposed on the basis of the existing procedures, and the bearing mechanism, construction technology and failure mode of the tandem composite piles are introduced briefly. Secondly, a model for the bulging deformation of tandem composite piles is established to study the mechanism of the bulging deformation of tandem composite piles. Then, based on the modified strain wedge model and the generalized Hooke's law, the bulging deformations and settlements of the solid-bulk tandem composite piles are predicted. Finally, the proposed method is verified by the tests on the swelling deformations and settlements of the traditional gravel piles and reinforced-hoop-gravel piles. The comparative results show that the predicted values are in good agreement with the experimental ones, which indicates that the reference is thus provided, which is of theoretical and engineering application values for the design of the composite foundation of gravel piles.