Abstract: Model tests of piled raft are performed, the high quality digital pictures of displacements of soil and piled raft under each vertical load increment are taken during testing process. The axial forces of piles are measured. The detailed displacement fields of soil and piles are obtained by analyzing the digital pictures. Based on this, the failure mode and deformation behavior of the piled raft are studied. The effect of pile spacing and pile length on the settlement beneath pile tip and shaft resistance of pile is analyzed. Test results show that pile spacing is the major factor which has significant effect on the slip at the interface of pile shaft and surrounding soil. The piled foundation with bigger pile spacing produces larger pile – soil slip and pile tip penetration. The central pile under comparatively flexible raft can produce larger pile – soil slip and pile tip penetration than that of edge piles when pile spacing is large enough. The pile tip penetration accounts for most part of the settlement of piled raft with larger pile spacing. This implies that it’s necessary to construct a theory, where the pile tip penetration should be taken into account, to calculate the settlement of piled raft with large pile spacing. By laying a layer of sand cushion, compressible block, or simply a gap between raft and pile head to make it possible for the soil beneath raft to carry load first, the soil beneath the raft can be compacted before piles begin to carry load significantly and thus can reduce the pile-soil slip and pile tip penetration and even change the failure mode.

Key words: piled raft, visual model test, displacement field, shaft resistance