Abstract：The performance of single-type-pile composite foundation and the concept of combined-pile composite foundation are analyzed. The vertical working performance of piles and soil of three-element and four-element composite foundation is compared and analyzed based on data form pressure cells，manometers and sensors under load distribution plate in loading tests. The influence factors of the performance are discussed. The results show that the four-element composite foundation is similar to the three-element composite foundation in the variation of the stress on the top of piles and soil with loading，but different from the three-element composite foundation in the variation of the load sharing ratio and the stress ratio with load，which is determined by the stress growth rate of piles and soil. Data from plate loading tests shows that the settlement of combined-pile composite foundation varies gradually with pressure. With the pressure increasing，the soil between inside piles reaches plasticity prior to piles，and piles bear the major part of load increment when the settlement reaches certain value. The combined-pile composite foundation should be designed according to settlement controlling. The pressure corresponding to the turning point or the point with maximal curvature in the curve of the inter-pile soil or secondary pile¢s load sharing ratio versus load obtained by plate loading test can be regarded as the characteristic value of bearing capacity of combined-pile composite foundation. When the plate area is not equal to the actual treatment range，the characteristic value of bearing capacity may be determined by the ratio of test value and calculation value with two kinds of size. It is pointed out that the performance of combined-pile composite foundation is influenced by the bond strength，length and replacement ratio of piles，the thickness and material of cushion，and the time.
