Bearing Characteristics of Large-Diameter Steel Tube Composite Piles

Literature Overview and Engineering Context

The paper by Cui Yunliang, Wang Haifeng, Wang Xin, Wei Gang, and Zhou Feng, published in Journal of Underground Space and Engineering (2020, Vol. 16, No. 5, pp. 1467–1475), investigates the bearing characteristics of large-diameter steel tube composite piles using the Yushan Bridge pile foundation project as a case study. This research was supported by the Ministry of Transport Key Science and Technology Program (2018-MS1-004), the Zhejiang Provincial Department of Transportation Research Program (2018021), the National Natural Science Foundation (51508507), and the Zhejiang Provincial Natural Science Foundation (LGF20E080007).

Large-diameter steel tube composite piles combine the advantages of steel tube confinement, high load-bearing capacity, and construction efficiency with the cost-effectiveness of concrete core materials. They are increasingly used in bridge foundations, offshore platforms, and heavy industrial structures where high axial and lateral loads must be resisted in challenging soil conditions.

Self-Balancing Test Methodology and Results

The study selected pile No. 53 from the Yushan Bridge project and conducted a self-balancing pile test, which is a modern alternative to conventional static load tests. The self-balancing method involves installing a load cell at a predetermined depth within the pile, allowing the pile to be loaded from both above and below the load cell simultaneously. This eliminates the need for large reaction masses or anchor piles, making it particularly suitable for large-diameter piles where conventional testing is impractical.

The test results were converted to equivalent conventional static load test results using established conversion procedures. The Q-s curve (load-settlement curve) obtained from the conversion was smooth and free of abrupt changes, indicating reliable test data. The single pile ultimate bearing capacity was determined to be 71,293.75 kN, which is a substantial capacity suitable for major bridge foundation applications.

Numerical Modeling and Strain Analysis

ABAQUS finite element software was used to develop a realistic numerical model of the pile and surrounding soil. The model was calibrated to reproduce the measured load-settlement response, and the validated model was then used to analyze the distribution of vertical steel tube strain along the pile depth under axial loading.

The vertical strain distribution exhibited a distinctive "plate-bottom" shape, with strain values varying along the pile length in a manner that reflects the interaction between the steel tube, concrete core, and surrounding soil. The steel tube strain is not uniform along the pile depth, indicating that the load transfer mechanism between the pile and soil is complex and depth-dependent.

The numerical model was also used to parametrically study the effect of steel tube wall thickness on pile bearing capacity under both axial and lateral loading conditions. The results showed that:

1. The steel tube provides effective confinement to the core concrete, enhancing the axial load-bearing capacity.
2. The steel tube significantly increases the bending stiffness of the pile, improving lateral load resistance.
3. Thicker steel tubes result in higher lateral ultimate bearing capacity.
4. The maximum bending moment in the pile occurs at approximately 31 m depth, with the moment distribution increasing from the pile head to this depth and then decreasing.
5. The shear force distribution shows a gradual decrease from the pile head downward, with a significant reduction beginning at the steel tube bottom position, and negative shear values appearing below this point.

Effect of Steel Tube Thickness on Bearing Performance

The parametric study of steel tube thickness provides valuable design guidance for selecting appropriate wall thickness for large-diameter composite piles. The following table summarizes the key findings:

The results indicate that while increasing steel tube thickness improves both axial and lateral bearing capacity, the relationship is not strictly linear. The confinement effect on concrete provides diminishing returns beyond a certain thickness, while the bending stiffness continues to increase proportionally with wall thickness. This suggests that for applications dominated by lateral loading, thicker steel tubes are beneficial, while for primarily axial loading applications, the optimal thickness should balance cost against the confinement benefit.

Engineering Practice Implications and Reflection

The self-balancing test methodology employed in this study is a significant advancement in pile testing technology. Traditional static load tests for large-diameter piles require enormous reaction masses, which are often impractical to mobilize, especially in congested construction sites or offshore locations. The self-balancing method eliminates this constraint and provides reliable capacity data, making it a preferred method for large-diameter pile verification.

The finding that the steel tube provides both confinement and bending stiffness enhancement is well-established in the literature but is valuable to confirm through rigorous testing and modeling. In my experience with steel pipe fabrication and welding, the wall thickness of the steel tube is a critical fabrication parameter that must be tightly controlled. Variations in wall thickness directly affect the confinement effectiveness and lateral stiffness, which in turn influence the pile's load-bearing capacity and serviceability.

The moment distribution finding that the maximum bending moment occurs at approximately 31 m depth is particularly significant for pile design. This depth corresponds to the transition zone between the upper soil layers and deeper strata, where the lateral soil resistance changes character. The design of the pile reinforcement and steel tube thickness should be optimized for this critical depth to ensure adequate capacity without excessive material usage.

The shear force distribution, which shows a significant reduction at the steel tube bottom and negative values below, suggests that the steel tube bottom acts as a transition point for load transfer. Below this point, the pile behavior is dominated by the concrete core and soil interaction, with the steel tube contributing less to shear resistance. This has implications for the design of pile splices and connections at the steel tube bottom, which should be designed to accommodate the change in shear demand.

Summary

This study provides comprehensive experimental and numerical evidence for the bearing characteristics of large-diameter steel tube composite piles, with particular emphasis on the effects of steel tube thickness on axial and lateral capacity. The self-balancing test methodology offers a practical and reliable approach to pile capacity verification, and the ABAQUS numerical model provides a validated tool for parametric analysis and design optimization. The findings on vertical strain distribution, moment concentration depth, and shear force transitions offer actionable design guidance for engineers. The integration of steel tube confinement with concrete core load-bearing creates a synergistic system that leverages the strengths of both materials, and the parametric study of wall thickness provides a practical framework for selecting optimal design parameters based on the specific loading conditions and soil profile of each project.