Abstract：Based on digital image correlation method，a deformation measurement procedure(DIC) has been worked out to study the pile-soil-cap nonlinear interaction mechanism. A series of images are captured by digital camera in the model test，and the images are analyzed by the DIC procedure. Bearing capacity and displacement field of soil beneath cap are obtained during the analysis. The failure pattern of pile foundation is deduced. The test results show that it is viable to analyze the displacement around pile by DIC procedure，and a full displacement field is obtained from image analysis. The deformation behavior of the group piles with 6b spacing is similar to that of single raft foundation. The soil lateral deformation of group piles with 6b spacing is larger than that of 3b spacing，and the pile barrier effect on soil lateral deformation decreases with the increase of pile spacing. The compression region is mainly centralized beneath the pile tip to the 3b spacing foundation，which presents a destruction pattern of block deep foundation. As the increase of pile spacing，the compression region transfers from pile tip to the soil among piles. Group piles with 6b spacing are destroyed by the lateral extrusion of the soil around piles. It is shown that validate the theory of composite pile foundation is validated and 6b spacing is the dividing value between the normally pile foundation and the composite pile foundation.
