Abstract: The model tests on single pile foundation and group piles foundation were designed to investigate the interaction behavior of pile-raft-soil under the ultimate load.The test results indicated that the behavior of group piles with spacing of 4d was like that of the block deep foundation under the ultimate load.As for pile groups with large spacing,the ultimate state of the piles was earlier to reach than that of the soil under the raft,and the following load was taken by the soil under raft.Thus,the concept of plastically bearing piles were validated.The proportion of load carried by piles and rafts changed during the tests.As for group piles with 6d spacing,when the piles were in the ultimate state,the proportion became invariable.The difference of the percentage of load carried by the rafts at the ultimate load and the design load of piles would be considered by using the ultimate bearing capacity in pile-raft foundation design.The pile barrier effect on soil lateral deformation decreased with the increasing pile spacing.The deformation behavior of the rafts in group piles was similar to that of raft foundation,and the soil stress under the pile tip was affected by load carried by the rafts.Spacing of 6d was the dividing value between the normally piled raft foundation and the composite pile foundation.