Abstract: 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.

Key words: PTC pipe pile, pile cap, composite foundation, static loading test, load sharing ratio , pile-soil stress ratio