Abstract: a new ground improvement method is proposed to treat thick soft soil, which involves the use of concrete-cored sand-gravel piles with combined vacuum and surcharge preloading. In this method, the prefabricated inner piles are used to increase the bearing capacity of soft soil, and the outer sand piles are used to shorten drainage path, transmit vacuum pressures and accelerate consolidation of soft soil. According to the consolidation characteristics of the composite foundation improved by concrete-cored sand-gravel piles under combined vacuum and surcharge preloading, the analytical solutions of average degree of consolidation are derived by calculating the consolidation effects of the composite foundation under surcharge or vacuum preloading separately and then superposing them together. The effects of area replacement ratio of composite pile to soft soil, dimensions and permeability of the smear zone, length-diameter ratio of the concrete-cored sand-gravel column, and area ratio of the inner pile to the composite pile on the consolidation behaviors of the composite foundation are investigated by a parametric analysis based on the proposed solutions. The results show that the consolidation rate of the composite foundation increases with the increase of the area replacement ratio and the area ratio of the inner pile to the composite pile, and decreases with an increase of the diameter ratio of smear zone to composite pile, the permeability ratio of undisturbed zone to smear zone, the length-diameter ratio of the composite pile.

Key words: concrete-cored sand-gravel piles, combined vacuum and surcharge preloading, consolidation, analytical solution, soft soil