Abstract: The negative skin friction of composite foundation with rigid pile has a significant effect on the pile-soil interaction under embankment load. Based on the results of shear tests that ultimate shear displacement is less affected by stress levels but greatly influenced by specimen sizes, an ideal elastic-plastic load transfer model considering the variation of stress-dependent shear stiffness of pile-soil interface is developed. According to the three-section distribution pattern of pile-soil interaction, i.e., the upper plastic zone of negative friction, the central elastic zone of coordinate deformation and the lower plastic zone of friction bearing, a distribution model for skin friction on pile-soil interface describing the nonlinear elastic-zone and inhomogeneous plastic-zone is established. To investigate the equivalent reinforcement of single pile with equal area in rigid pile composite ground under uniformly distributed embankment load, the formulas of the depth of the neutral plane and stress concentration ratio are derived according to the force equilibriums differential equations of a simplified analytical unit cell and the compression deformation compatibility equations of pile-soil-cushion,. the effect of the embankment load and the flexibility coefficient of cushion on the depth of the neutral plane and the stress concentration ratio are analyzed. The analysis indicates that the depth of the neutral plane descends with the increasing of the flexibility coefficient of cushion, and then ascends with the increase of embankment load. The stress concentration ratio decreases with the increasing of the flexibility coefficient of cushion, and decreases afterwards with the increase of embankment load. Hereby, the technical ideas including the pile arrangement and the cushion design using the maximum stress concentration ratio are provided.

Key words: rigid pile composite ground, embankment load, load transfer model, stress concentration ratio, depth of the neutral plane