Predicting uplift resistance of deep piles with enlarged bases

A theoretical method is presented for predicting the uplift resistance of deep piles with enlarged bases. The piles are deeply embedded in soil, so that an axisymmetric logarithmic-spiral (or 'peach-shaped') slip surface in the soil around the pile is buried underground throughout the uplifting process. The uplift resistance is considered to be equal to the sum of the pile shaft and base uplift resistance as well as the pile weight. The pile shaft and base uplift resistance were derived from the unit skin frictional resistance model and the peachshaped slip surface model in the soil around the base. The equations associated with the proposed theoretical method are analytically formulated using the classical limit equilibrium method and the axisymmetric pile uplift model. Three base uplift resistance capacity factors, N_c, N_q and N_γ, were obtained, corresponding to the effects of cohesion, surcharge and self-weight respectively. Charts of these three factors are presented at the serviceability failure limit (defined as pile upward displacement equal to 10% of the pile shaft diameter). The proposed theoretical method is then validated with results from full-scale field tests. It is shown that the proposed theoretical method can effectively estimate the uplift resistance capacity at the serviceability failure limit for deep piles with enlarged bases in various soils.