Contact versus noncontact piled raft foundations

In the last few decades there has been a rapid increase in the number of piled foundations where the piles have been employed as settlement reducers; in some recent projects, the piles have been separated from the raft by a granular layer, which creates a more uniform pressure distribution on the raft bottom and reduces constraint reactions in the soil, foundation, and superstructure. A series of centrifuge model tests has been performed to investigate the load transfer mechanisms between a square rigid raft and a group of instrumented piles jacked in dry dense sand, in direct contact with the raft or separated from the raft by an interposed granular layer. The test results have shown that contact piles act as settlement reducers by diffusing the load applied to their heads to greater and deeper volumes of soil. The insertion of a deformable layer between a raft and pile heads does not ensure displacement compatibility, and the pressure diffused by the granular fill acts partly on the pile heads and partly produces shallow soil settlements, which mobilize negative skin friction on the upper part of the pile shaft. Noncontact piles act mainly as soil reinforcement.