Sand Compaction Pile Method (SCP Method) has been often adopted to improve the bearing capacity, stability and consolidation settlement of soft ground. The fixed type improvement,compacted sand piles reach the stiff layer, is preferable for the improvement purpose, but the floating type improvement,compacted sand piles do not reach the stiff layer but penetrate partially in the soft soil layer, is recently adopted for several reasons. The failure pattern and deformation of the floating type SCP improved ground subjected to caisson and backfill loading were investigated by centrifugal model tests. The effect of depth of the improved area was studied. Moreover, numerical calculations based on the failure patterns observed in the model tests were also carried out to evaluate the stability of the improved ground.
　The study indicates that the floating type SCP improved ground can fail with slip circle failure which passes underneath the improved area, on the other hand while the fixed type SCP ground can fail with large lateral deformation at the ground surface. The numerical analysis shows that the failure pattern is dependent upon the size of the improved area especially improved depth.