Zhuojin Pipe Fitting Co., LtdCentrifugal Model Test Study on Lateral and Axial Bearing Characteristics of Steel Tube Concrete Composite Piles
Literature Overview
The paper by Feng Zhongju et al. (2018), published in the Chinese Journal of Civil Engineering (Vol. 51, No. 1, pp. 114–123), investigates the lateral-axial bearing characteristics of steel tube concrete composite piles used in bridge foundation cofferdam structures in water environments. The research employs centrifugal model testing to study how steel casing embedment depth and soil modulus in the steel tube squeezing zone influence the lateral-axial ultimate bearing capacity, lateral soil resistance curves, H-y curves, and pile body bending moment distribution. This work is significant for bridge engineers working in aquatic environments where steel casings protect reinforced concrete piles during construction and subsequently form composite structural elements.
Core Technical Content
The study focuses on a specific engineering scenario: in water environments, steel protective casings (cylindrical steel tubes) are driven into the ground to form cofferdams around bridge pile foundations. After the reinforced concrete pile is cast within the casing, the steel tube and concrete form a composite pile. The lateral and axial bearing characteristics of this composite pile differ substantially from those of conventional reinforced concrete piles because the steel tube introduces additional stiffness, changes the soil-pile interaction mechanism, and creates a soil-squeezing zone around the casing.
Two key research variables were selected:
- Steel tube embedment depth — the depth to which the steel casing is driven below the scour line or waterline
- Soil modulus in the steel tube squeezing zone — the stiffness of soil displaced during casing installation
The centrifugal model testing method was chosen because it allows accurate simulation of in-situ stress conditions at prototype scale within laboratory dimensions, overcoming the limitations of conventional small-scale model tests that cannot replicate realistic overburden pressures.
Key Experimental Findings
| Parameter | Observation | Engineering Implication |
|---|---|---|
| Embedment depth ≤ 12 cm (model scale) | Significant increase in lateral-axial ultimate bearing capacity | Increasing embedment depth is highly effective up to this threshold |
| Embedment depth > 12 cm (model scale) | Continued increase but with diminishing returns | Beyond this point, additional embedment depth offers marginal benefit |
| Increased soil modulus in squeezing zone | Moderate improvement in lateral-axial bearing capacity | Soil improvement around the casing can enhance performance but with limited effect |
| H-y curve characteristics | Nonlinear relationship between lateral load and pile deflection | Design must account for progressive stiffness degradation |
| Bending moment distribution | Distinctive moment profile compared to conventional RC piles | Reinforcement layout must accommodate shifted moment peaks |
Technical Interpretation and Engineering Practice
From a steel pipe manufacturing and welding perspective, several important considerations emerge from this study. First, the steel casing used in these composite piles is typically a seamless or ERW steel pipe conforming to standards such as GB/T 8163 (for structural steel tubes) or SY/T 5037 (for welded steel pipes). The casing must withstand driving stresses during installation without cracking or deformation, which demands careful control of weld quality and material toughness.
The finding that embedment depth beyond a certain threshold yields diminishing returns has direct implications for construction economics. Engineers should optimize the casing length to balance lateral stability requirements against material and driving costs. In practice, this means the casing length design should target the critical embedment depth identified in this study rather than arbitrarily extending the tube.
The soil-squeezing zone concept is particularly relevant for understanding residual stresses in the installed casing. During driving, the soil is displaced radially outward, creating a zone of compacted soil with elevated stiffness. From a welding and materials standpoint, the casing itself experiences cyclic loading during driving, which can induce microstructural changes in the steel, particularly near weld seams in ERW pipes. Quality control procedures should include ultrasonic testing of the casing after driving to detect any fatigue-related defects.
The lateral soil resistance curves obtained from the centrifugal tests provide valuable input for finite element modeling of composite pile behavior. Engineers can use these curves to calibrate spring models in software such as PLAXIS or MIDAS GTS, improving the accuracy of lateral load predictions for bridge foundations in scour-prone environments.
Key Questions and Reflections
One critical question that arises is the scalability of centrifugal model test results to full-scale applications. While centrifugal testing is superior to conventional model testing in replicating stress conditions, the geometric similarity factor and material property scaling must be carefully validated. The 12 cm embedment depth threshold in the model corresponds to a specific prototype depth that depends on the centrifugal acceleration and model scale factor. Engineers applying these results should verify the scale conversion and consider site-specific geotechnical conditions.
Another consideration is the long-term durability of the steel casing within the concrete-encased composite pile. The casing is protected from external corrosion by the surrounding concrete, but the interface between steel and concrete may develop differential settlement or debonding under cyclic lateral loading. Welding joints in the casing, if present, represent potential weak points that should be inspected during construction.
The interaction between axial and lateral loads in composite piles creates a complex stress state in the steel tube. The axial compressive load from the pile dead load combined with lateral bending produces biaxial stress conditions that may exceed the yield criterion at certain cross-sections. Designers should employ interaction diagrams or limit state analysis methods that account for this combined loading, consistent with approaches used in API 5L and ASME B31.3 pipe design codes.
Study Insights and Implications
This research provides a solid theoretical foundation for the design and construction of steel tube concrete composite piles in aquatic environments. The centrifugal model testing methodology demonstrates that the lateral-axial bearing capacity is governed primarily by embedment depth up to a critical value, with soil modulus playing a secondary but positive role. For bridge engineering projects involving cofferdam-supported pile foundations, these findings enable more rational casing length optimization and improved prediction of lateral load response.
From a quality assurance perspective, the study underscores the importance of controlling casing installation quality — including driving impact energy, alignment, and post-driving integrity inspection — as these factors directly influence the soil-squeezing zone characteristics and, consequently, the composite pile performance. Engineers should incorporate casing inspection protocols into their construction quality plans, utilizing methods such as magnetic flux leakage testing for detecting wall thickness variations and ultrasonic testing for internal defect identification. The integration of these findings into design codes and construction standards would further enhance the reliability of composite pile foundations in bridge engineering applications.