Using numerical simulation by PFC2D (particle flow code in two dimensions)

the group piles effect of pile tip resistance of under-reamed piles on uplift loading is studied. The increase of tip resistance with uplift displacement and the variations of soil displacement around pile tip are observed in the simulation. The differences between single pile (pier) and group piles (piers) in uplift loading are shown and the development of tip resistance with uplift displacement for group piers in different spaces is compared. It is shown that the uplift characteristics are similar for single pile (pier) and group piles (piers) when the normalized uplift displacement s/D is smaller than 0.1. With the development of uplift displacement

the tip resistance of single pile (pier) is larger than that in group piles (piers) and the interaction of soil particles around tip emerges. When the normalized uplift displacement s/D is smaller than 0.5

the tip resistance of central pile (pier) is larger than that of side pile (pier) in the group piles (piers). When the normalized uplift displacement s/D is larger than 0.5

the tip resistance of side pile is larger than that of center pile in the group piles and that is reverse in the group piers

which embodied the effects of side restriction of pile shaft on the mobilization of tip resistance. The characteristics of group piers had obvious difference with single pier only in larger displacement with the increase of pier distance.