Simplified calculation method for settlement of necking piles accounting for pile-soil interaction in the necking zone

Existing calculation methods often neglect the pile-soil interaction within the necking zone, leading to inaccuracies in predicting the settlement of necking piles. Through transparent soil model tests, the deformation mechanism of the soil around necking piles was investigated. The results indicate that the soil surrounding the necking zone undergoes plastic deformation similar to that beneath the pile end, thereby generating end resistance. Furthermore, the deformation direction of the soil adjacent to the pile shaft aligns with the pile settlement direction, which reduces the relative pile-soil displacement and consequently diminishes the skin friction in that section. Based on these findings, a calculation model for the resistance in the necking zone was established, considering both the end resistance and the reduction in skin friction. Using this model together with the load-transfer method, a simplified approach for predicting the settlement of necking piles was proposed, which incorporates the reduction in cross-sectional stiffness at the necking zone. The effectiveness and accuracy of this method were preliminarily validated through the load-settlement responses obtained from both model tests and one field test on necking piles. The proposed method provides a preliminary theoretical basis for the safety assessment of necking piles under vertical loading.