Large-scale model experimental study on cyclic penetration process and vertical bearing characteristics of open-ended pipe piles

It is extremely important to understand the mechanism of pile-soil interaction during pile installation for accurately predicting its bearing capacity. This paper presents a large-scale (1:10) model test to investigate this interaction specifically for open-ended pile penetrating sandy soil. The study focuses on pile with a diameter of 0.273 m penetrating sandy soil having a relative density of approximately 40%. Advanced monitoring techniques, including fibre Bragg grating (FBG) strain sensors and thin-film pressure sensors, were employed to meticulously analyze changes in soil plug height, strain, axial force, shaft resistance, and pile-soil interfacial stress throughout the cyclic penetration process. Following pile installation, vertical compressive static load tests were conducted with intervals of 10 and 30 days, respectively, to assess the long-term performance of the piles. Subsequently, the ICP-05 method and UWA-05 method, which are based on CPT to predict the vertical bearing capacity of piles, were compared with the bearing capacity measured by static load test. The results indicate the following: Firstly, the soil plug effect of open-ended pipe piles gradually increases during cyclic penetration, and the soil plug is partially occluded at the end of penetration. Secondly, as the depth of penetration increases, an "inverse point" of strain near the pile tip emerges. Specifically, the compressive strain at the final stage of penetration changes to a tensile strain during static stabilization, while the strain at the pile tip remains tensile at both stages. Thirdly, the radial stress of the pile body peaks at the pile tip and gradually tapers off as the h / R (h is the height from the sensor to the pile tip, R is the radius of the pile) ratio increases, indicating a significant h / R effect. Fourthly, the bearing capacity of the pile increased by approximately 12%, from 289 kN after 10 days to 323 kN after 30 days, demonstrating a notable time effect. Finally, both the ICP-05 and UWA-05 methods underestimate the shaft resistance near the pile tip, while their predictions above the pile tip align closely with measured values. Overall, the predicted pile bearing capacity using both methods is conservative, with the UWA-05 method yielding closer results to the measured values.