Abstract: Replacement rate is one of the key indicators for designers to determine the composite foundation of CFG piles. After determining the pile length according to the distribution of soil layer, the choice of replacement rate is the basis of vertical structure system decision-making according to additional load for target. At present, we have a lack of common knowledge and understanding of the mechanical properties of composite foundation affected by additional load under different replacement rates. Two centrifuge model tests with the same additional load and reinforcement depth and different replacement rates have been carried out to study the change law of the pile axial forces, pile side frictional resistances, pile-soil stress ratio and foundation settlement of the composite foundation during loading process. As the increase of the replacement rate, the test results show that: (1) The axial force of CFG pile is reduced; and the position of the maximum axial force is up along the pile. (2) The positions that the direction of pile side frictional resistance changes or that has the maximum negative frictional resistance in the upper part of the pile are also up along the pile; and the rate of change of the latter is reduced. The maximum positive frictional resistance in the lower part of the pile is obviously reduced. (3) The pile-soil stress ratio is reduced and its reduced rate along the depth direction in deep soil is decreased. (4) The foundation settlement and the ground surface settlement are effectively decreased; and the surface settlement changes linearly with the load; and the composite modulus of CFG composite foundation is improved.

Key words: replacement rate, centrifuge model test, composite foundation, pile-soil stress ratio, foundation settlement