A series of field full scale tests on single PTC (prestressed thin-wall concrete) pile with cap and composite foundation with twin PTC piles with cap have been carried out to cater for the application of a new sparse piles composite foundation in Suzhou-Shanghai Expressway. Based on the data obtained from the tests

the interaction mechanism which is among the pile

the soil under the cap and the soil out of the area of the cap are studied; the load-bearing characteristics of the pile and the soil are elaborated one by one

and the effect that the soil lagging the pile in the development of load-bearing are analyzed

as well as the effect that the soil reducing the load-bearing capacity of the pile. The results indicate that there are two stages when settlement occurred

at the first stage the settlement is controlled by the pile and the next is by the soil. Finally

the changing rules of pile-soil load sharing ratio and stress ratio at the different settlement stages are discussed; the method of designing the dimension of the cap and proposal of eliding the cushion of the composite foundation are presented.