Uplift Resistance of Enlarged Base Pile Foundations

Pile foundations with enlarged bases are effective either in expansive soils or when piles are liable to attract tensile loading. In this paper, the uplift behaviour of enlarged base piles (referred to as under-reamed piles in the text) in layered strata has been experimentally investigated, using half-space small-scale 1g models, and compared with theoretical predictions developed using upper and lower bound theorems. The failure mechanisms observed were characterised using particle image velocimetry (PIV) techniques. It was observed that an upper stratum of clay overlying sand increased the post-peak uplift capacity when compared to two-layered sand, but had little effect at small strain. The small strain failure mechanisms observed were similar, consisting of a balloon-shaped bulb above the pile base, before extending vertically and radially out from the pile with increased displacement. This extension was seen to be wider in the clay than in the sand. These results show that when considering the design of under-reamed pile foundations, anchoring the base of the pile in a stronger stratum at greater depth is beneficial for increasing uplift capacity. At small strains, the overlying stratum has little consequence on the peak uplift resistance. However, at larger strains it can influence the post-peak uplift capacity, which would be an important consideration for possible situations of large imposed displacements.