In order to improve the design and calculation of single piles under combined vertical and lateral loads when the pile deformation is comparatively great

considering the P-? effect (with the meaning of vertical load can magnify the deformation induced by lateral load) caused by vertical load and yield of soil

dead weight of the pile

and skin friction of the pile side

assuming that the coefficient of subgrade reaction increases linearly with depth

the power series solutions of the deformation and internal force for the pile shaft under ground surface are obtained. Combining the pre-existing power series solutions for the pile shaft above ground

the relevant computer program is developed by Fortran language. The calculation results indicate that the displacements at the pile head and the ground surface

and the maximum bending moment increase when the vertical and lateral loads

and the pile length above ground increase; and decrease when the yield displacement of soil increases. The vertical load may cause the instability problem for the pile. The boundary condition at the pile head plays an important role in pile responses. The computed and the model test results are in good agreement

meaning that the obtained solutions and program are dependable.