Abstract: An elastic-plastic load-transfer model (t-z model), which can consider the Poisson's effect, is proposed. The model can reflect both the difference in compressive bearing capacity between the piles of different materials embedded in the same soil and the difference between the compressive friction and the tensile friction for the same pile embedded in the same soil. In addition, the model can also take into account the hysteretic characteristics of t-z curve under the cyclic load. A Matlab program, which can simulate the pile responses under both the compressive load and the tensile load, is developed based on the displacement coordination algorithm. The applicability of the model is demonstrated by comparing the measured results and the calculated results with the program. Finally, the influence of the elastic modulus of pile material on the ratio of the ultimate tensile friction to the ultimate compressive friction is studied. It is shown that the ratio decreases with the decrease of elastic modulus. The ratio is between 0.6 and 0.8 for the concrete pile, which is very close to the value proposed by the standard.

Key words: pile foundation, vertical load, Poisson's effect, elastic-plasticity, boundary surface