Abstract: This paper is aiming to find a simple method for the settlement calculation of flexible pile composite foundation under embankment load. Firstly, the side frictional distribution of pile is simplified into a piecewise linear model based on the existing research results. Then, according to the relationship between the length of pile and the critical length of pile, combined with the coordination conditions of stress and compression deformation at the interface of pile-soil-cushion, settlement calculation formulas of flexible pile composite foundation reinforcement area are deduced by using the unit element method, and the settlement of underlying layer of composite foundation is calculated by layer summation method. Finally, the proposed approach is used to predict the settlement of an engineering example, and it is found that the predictions are in accordance with measurements, demonstrating that the approach is available in reflecting the working behavior of the flexible pile composite foundation under embankment load better. Further analysis shows that within the critical length of pile, the bearing capacity of soil between piles in composite foundation is maximized by the interaction of piles and soil. In addition, due to the drag effect of the negative frictional resistance, the axial force of pile at the position of the neutral plane reaches the maximum. Therefore, the concept of critical pile length and neutral plane should be highly valued and applied in engineering design.

Key words: composite foundation, settlement analysis, flexible pile, embankment load, critical length of pile