Estimation of belled pile uplift resistance in dense sand based on centrifugal experiments

Abstract

Belled piles are expected to resist the uplift motion of buildings caused by rocking during earthquakes. In supertall buildings with pile foundations, uplift forces are assumed to increase owing to the increase in the spans and heights of such buildings. In order to understand the uplift resistance mechanism of a belled pile, the authors conducted two series of experiments in a centrifuge. One is the ground deformation visualization experiments, and the other is the measurement experiments. During the visualization experiments, ground behavior in the belled pile uplift was visualized using Digital Image Correlation (DIC). From the ground behavior visualized by DIC, it was revealed that the ground deformed area expands as the belled pile is uplifted, which is related to the uplift resistance, and that the uplift resistance is proportional to the belled angles. Because the friction between the observation window and the physical model (both ground and pile) is not negligible in visualization experiments, a full-cylinder soil box and a full-section model pile were employed in the following series of measurement experiments to quantitatively investigate the uplift resistance of the belled pile. Centrifugal experiments were performed using the same procedure and conditions as the visualization ones to obtain the load-displacement relationships of the belled piles with different belled angles and diameter ratios. The load-displacement relationships obtained from the measurement experiments were then analyzed and normalized in an integrated form. From the obtained experimental results, the maximum uplift resistance can be formulated by normalizing uplift stress acting on the bell vertically at the pile end, and the normalized uplift resistance is proportional to the belled angle and can be expressed by an exponential form of the diameter ratio. The formulation proposed above enables estimation of the uplift bearing capacity of a belled pile by considering both the belled angle and diameter ratio effects.