UPLIFT CAPACITY OF BELLED AND MULTI-BELLED PILES IN DENSE SAND

This paper evaluates the uplift capacity of belled and multi-belled piles in dense sand. A two-dimensional distinct element (DE) analysis was applied in pullout tests on single piles to investigate the uplift resistance of the piles, the soil behavior around the piles, and the interaction between the soil and the pile surface. It was observed from the DE analysis that the soil mass adjacent to the projections of the belled and multi-belled piles moved vertically, and that the soil movements leaned slightly with the occurrence of relative displacement between the soil and the pile surface. A theoretical solution for predicting the uplift capacity of belled and multi-belled piles was derived from an upper bound limit analysis based on the soil movements in the DE analysis. The solution was able to reproduce the ultimate uplift resistance in the DE analysis using the friction angles in the aggregates and on the pile surface that were evaluated from a simulation of direct and simple shear tests. In addition, a continuity equation that satisfied the relationship between the displacement vector of the soil mass adjacent to the projections and the change in volume around the soil mass was proposed for predicting the uplift capacity of actual piles under axisymmetric conditions. The theoretical solution obtained with the continuity equation was in good agreement with the pullout resistance of the belled and multi-belled piles in centrifuge model tests and full-scale tests conducted in situ.