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In present research, 17 centrifuge tests have been conducted to study the effect of various parameters such as the number of piles, the distance between piles, gradation and thickness of the granular layer on the load-settlement behavior of a pile raft system. The results showed the importance of granular layer to reduce the settlement of non-connected pile raft system when the roles of piles are to reduce the settlement. In other words when the piles have major contribution on the bearing capacity of pile raft system, presence of a granular layer may increase the settlement.