The preliminary study on the effect of pile-soil cooperative work is made at first. Then the condition for formation of rigid pile composite ground and the design method for different pile spacings of composite pile foundation is discussed. When piles are friction ones and end-bearing friction ones

the rigid pile composite ground without cushion is safer and more economical than that with cushion. If the total safety level and settlement of composite pile foundation can be satisfied

design may be carried out in definite stress ratio of pile and soil; and the designed stress ratio of pile and soil is unnecessarily equal to the practical one. Some existing problems in design method of pile foundation with large spacing between piles are discussed

and some proposals of improvement are put forward

too.