Based on using linear load transfer function to pile-side soil and pile-end soil

and considering influence of sharing load of the pile-side soil on load transfer law of piles

and using theory of mechanics and approximate solution method—district method of differential equation，a set of approximate analytical formulas of interaction for a flexible single pile with variable cross section and ground soil are derived. For rigid pile cap

situation of flexible pile with variable cross-section are compared with situation of flexible pile with constant cross section by an example; the results show that the bearing capacity of the flexible pile and the composite foundation may be raised by changing pile type. The results of the model test are compared with the results. It is shown that the proposed method has good precision.