Abstract: At present, the pile bearing capacity analysis method of single load acting separately and then effect superimposing is adopted in the code, which ignores the coupling between vertical and horizontal loads and does not conform to the real stress situation. In order to investigate the bearing mechanism of foundation pile under combined load, based on the beam theory of elastic foundation, the expression of resistance moment caused by friction was derived considering the friction enhancement effect and displacement increment under load, and the displacement-internal force control equation of foundation pile was established. Use Mathematica software to analyze the existing cases and verify the correctness of the theory. The results show that under the condition of small deformation, considering the friction enhancement effect and displacement increment, the calculation accuracy of pile deformation and internal force is very good, and the accuracy is greatly affected by the change of calculation parameters. The results show that the deformation and internal force calculated by considering the friction enhancement effect and displacement increment have good accuracy when the pile is in small deformation under combined load. With the increase of the free section of the pile, the relative error of the calculated bending moment is still less than 10%. The research results provide a new idea for the calculation of bearing capacity of foundation piles, and enrich the analysis theory of bearing capacity of foundation piles under combined loads.

Key words: friction enhancement effect, displacement increment, combined load, bearing capacity of foundation pile, resisting moment