Centrifugal Model Testing of Vertical Bearing Characteristics for Steel Tube-Concrete Composite Piles

Literature Overview and Research Background

This 2018 paper published in the Journal of Chang'an University (Natural Science Edition) by Feng Zhongju and colleagues from Chang'an University, along with collaborators from Guangdong Nanyue Transportation Investment, Guangzhongjiang Expressway Project Management, and other institutions, presents centrifugal model test results for steel tube-concrete composite piles in marine environments. The research was funded by the National Natural Science Foundation of China (Grant 41272285).

Steel tube-concrete composite piles represent an innovative foundation solution that combines the corrosion resistance and construction speed of steel tubes with the load-bearing capacity of concrete-filled sections. In marine environments, where steel piles suffer from chloride-induced corrosion and concrete piles face durability challenges, composite piles offer a hybrid advantage. However, the vertical bearing behavior of such piles, particularly the interaction between the steel tube segment and the concrete-filled segment embedded in marine soils, requires careful investigation.

Experimental Methodology and Test Matrix

The centrifugal model test approach allows simulation of prototype-scale soil-pile interaction by subjecting scaled models to elevated gravitational acceleration. The researchers selected two key variables: steel tube embedment depth and soil modulus in the displacement zone. The test matrix is summarized below.

The centrifugal test method provides several advantages over conventional model tests: it preserves the stress-strain state of the soil, accurately simulates the soil-pile interface behavior, and allows measurement of internal forces within the pile. The scaled model was designed to maintain geometric similarity and stress similarity according to centrifugal scaling laws.

Key Findings and Bearing Capacity Analysis

The study reports that increasing the steel tube embedment depth from 8 cm to 12 cm significantly improves the ultimate vertical bearing capacity of the composite pile. Beyond 12 cm, further increases in embedment depth continue to improve capacity but with diminishing returns. The influence degree of embedment depth on ultimate bearing capacity increases from 3.1% (at 8 cm) to 6.7% (at 20 cm).

Similarly, increasing the soil modulus in the displacement zone from 1.1E to 1.5E progressively increases the lateral friction resistance along the steel tube segment. The influence degree of displacement zone soil modulus on ultimate bearing capacity ranges from 3.1% (at 1.1E) to 5.1% (at 1.5E).

The P-S curves obtained from the centrifugal tests show a characteristic three-stage behavior: an initial linear elastic stage, a transition stage with increasing nonlinearity, and a post-peak stage where capacity plateaus or slightly decreases. The axial force distribution along the pile reveals that the load transfer from the pile cap to the soil occurs progressively, with the steel tube segment contributing significantly to the total capacity through increased lateral friction.

Engineering Recommendations and Design Formulas

Based on the test results, the authors propose a vertical bearing capacity calculation formula applicable to steel tube-concrete composite piles. The formula accounts for the contributions of both the steel tube segment (through lateral friction and end-bearing on the concrete plug) and the concrete-filled segment (through lateral friction and end-bearing on the soil).

From an engineering practice standpoint, the following recommendations emerge:

1. Optimal embedment depth: An embedment depth of approximately 12 cm (in scaled terms) provides the most cost-effective improvement in bearing capacity. Deeper embedment increases construction complexity and cost with marginal returns.
2. Soil improvement: Enhancing the soil modulus in the displacement zone around the steel tube segment is an effective strategy for improving pile capacity. This can be achieved through compaction, grouting, or vibro-compaction.
3. Construction sequence: The steel tube should be driven to the design depth before concrete is poured, ensuring proper soil displacement and lateral confinement of the concrete.

Welding and Fabrication Considerations

From a steel pipe and welding engineering perspective, the composite pile fabrication process involves several critical quality control points:

• Steel tube manufacturing: The steel tube segment must be manufactured to meet the applicable standards (e.g., GB/T 9711 for line pipe or ASTM A53 for structural applications), with particular attention to wall thickness uniformity and weld quality if the tube is longitudinally welded.
• Tube-to-concrete interface: The bond between the steel tube inner surface and the concrete is critical for load transfer. Surface preparation of the tube interior, including removal of mill scale and application of mechanical or chemical bonding agents, should be specified.
• Connection details: The transition between the steel tube segment and the concrete-filled segment typically involves a welded connection or mechanical coupling. This connection must be designed and inspected to ensure continuity of load transfer.

Key Questions and Reflections

The study raises several questions for further investigation. First, the long-term corrosion behavior of the steel tube segment in marine soil is not addressed, yet corrosion would directly affect the lateral friction resistance over time. Second, the interaction between the steel tube and the surrounding soil during cyclic loading (e.g., from wave action or traffic) is not examined in this study. Third, the proposed bearing capacity formula should be validated against full-scale field tests before widespread adoption in design codes.

Additionally, the influence of driving damage on the steel tube is a concern. If the steel tube is driven into the soil, the impact of hammering on the tube wall thickness, weld integrity, and residual stress state must be evaluated through post-driving inspection using ultrasonic testing and visual examination.

Summary and Implications

This centrifugal model test study provides systematic experimental data on the vertical bearing characteristics of steel tube-concrete composite piles, demonstrating that both embedment depth and displacement zone soil modulus significantly influence capacity. The proposed design formula and engineering recommendations offer practical guidance for the design of marine foundation systems using composite piles. For steel pipe and welding engineers, the study highlights the importance of ensuring high-quality steel tube fabrication and proper tube-to-soil interaction through appropriate surface treatment and construction sequencing.