Zhuojin Pipe Fitting Co., LtdCentrifugal Model Testing of Vertical Bearing Characteristics of Steel Tube Concrete Composite Piles
Literature Overview
This paper by Feng Zhongju, Wang Fuchun, Zhang Qilang, Zhen Dongxiao, Xi Chengxin, Su Hangzhou, Yin Honghua, Tian Jianglei, and Jin Ziliang, published in the Journal of Chang'an University (Natural Science Edition) in 2018 (Vol. 38, No. 2, pp. 42-49), presents centrifugal model test results on the vertical bearing behavior of steel tube concrete composite piles in aquatic environments. The research was funded by the National Natural Science Foundation of China (41272285) and involved collaboration between Chang'an University, Guangdong Nanyue Transportation Investment Construction Co., Ltd., Guangzhongjiang Expressway Project Management Office, China Power Construction Group Northwest Survey and Design Institute, and Guangdong Transportation Planning and Design Research Institute.
Research Background and Significance
Steel tube concrete composite piles represent a hybrid foundation system that combines the advantages of steel tube piles (rapid installation, high driving capacity, corrosion protection through steel casing) with the load-bearing efficiency of concrete-filled piles. In aquatic environments such as estuaries, rivers, and coastal waters, these piles offer particular advantages because the steel tube provides immediate structural integrity during driving and prevents soil ingress into the concrete core during construction.
The centrifugal model test technique is particularly well-suited for pile foundation studies because it enables the simulation of prototype-scale stress conditions in small-scale models by applying gravitational acceleration proportional to the model scale factor. For a model scale of N, the centrifugal acceleration required is Ng, where g is the standard gravitational acceleration.
Experimental Design and Parameters
Variable Selection
The study identified two critical variables for parametric investigation:
- Steel tube embedment depth: 8 cm, 12 cm, 16 cm, and 20 cm
- Soil modulus in the soil-displacement zone: 1.1E, 1.2E, 1.3E, 1.4E, and 1.5E (where E is the soil modulus in the non-displacement zone)
The concept of a "soil-displacement zone" refers to the region of soil that experiences disturbance and stress redistribution during the driving of the steel tube pile. The modulus of this zone differs from the undisturbed soil due to densification (in cohesionless soils) or disturbance effects (in cohesive soils).
Measured Parameters
The centrifugal model tests measured:
- Load (P) versus displacement (S) curves
- Axial force distribution along the pile shaft
- Shaft friction resistance distribution along the pile length
Key Results and Analysis
Effect of Steel Tube Embedment Depth
The results demonstrate a clear trend in the influence of steel tube embedment depth on the ultimate vertical bearing capacity:
| Steel Tube Embedment Depth | Ultimate Bearing Capacity Influence Factor |
|---|---|
| 8 cm | 3.1% |
| 12 cm | Significant improvement threshold |
| 16 cm | Continued improvement but diminishing returns |
| 20 cm | 6.7% |
The critical observation is that increasing the steel tube embedment depth from 8 cm to 12 cm produces a marked improvement in ultimate bearing capacity, but beyond 12 cm, the rate of improvement diminishes significantly. This suggests an optimal embedment depth around 12 cm for the model scale, beyond which additional embedment provides marginal benefit relative to the increased construction cost and driving effort.
Effect of Soil-Displacement Zone Modulus
| Soil-Displacement Zone Modulus | Ultimate Bearing Capacity Influence Factor |
|---|---|
| 1.1E | 3.1% |
| 1.2E | Intermediate |
| 1.3E | Intermediate |
| 1.4E | Intermediate |
| 1.5E | 5.1% |
Increasing the modulus of the soil-displacement zone from 1.1E to 1.5E increases the ultimate bearing capacity influence factor from 3.1% to 5.1%. This result confirms that the densification and stress state of the soil immediately surrounding the steel tube casing plays a significant role in the overall pile capacity, particularly through enhanced shaft friction resistance in the steel tube segment.
Technical Discussion
Load Transfer Mechanism in Composite Piles
The steel tube concrete composite pile operates through a dual load transfer mechanism. The steel tube segment transfers load to the surrounding soil through shaft friction and end bearing, while the concrete core below the steel tube transfers load through its own shaft friction and end bearing. The interface between the steel tube and concrete core represents a critical load transfer zone where stress continuity must be maintained.
The centrifugal model test results on axial force distribution along the pile shaft provide direct evidence of this load transfer behavior. The measured axial force profiles reveal the point of transition from steel tube-dominated load transfer to concrete-dominated load transfer, which is influenced by the embedment depth and the relative stiffness of the two segments.
Soil-Displacement Zone Considerations
The concept of differentiating between the soil-displacement zone and the non-displacement zone is a sophisticated approach to pile-soil interaction modeling. During steel tube pile driving, the soil immediately surrounding the pile experiences significant disturbance, including densification in sandy soils, smearing in clay soils, and changes in pore water pressure. The resulting soil modulus in this zone differs from the undisturbed soil, and accounting for this difference is essential for accurate bearing capacity prediction.
Centrifugal Model Test Methodology
The centrifugal model test technique offers several advantages for pile foundation research:
- Geometric similarity: Model dimensions are scaled down by a factor N while maintaining prototype stress conditions
- Time scaling: Time effects such as consolidation and creep are accelerated by the same factor N
- Soil modeling: Real soil materials can be used with appropriate preparation procedures to simulate prototype soil properties
The key challenge in centrifugal pile testing is the preparation of soil models with the desired properties, including the differentiation between displacement and non-displacement zones. This requires careful control of soil density, moisture content, and compaction procedures.
Engineering Recommendations
Based on the test results, the following engineering recommendations can be derived:
- Optimal embedment depth: For composite piles in aquatic environments, a steel tube embedment depth of approximately 12 cm (model scale) provides the best balance between bearing capacity improvement and construction economy. In prototype scale, this translates to a depth-dependent value that should be determined through site-specific analysis.
- Soil condition assessment: The modulus of the soil-displacement zone should be evaluated through in-situ testing such as CPT or SPT, and the results should be used to calibrate the bearing capacity calculation.
- Bearing capacity formula: The proposed vertical bearing capacity calculation formula for steel tube concrete composite piles should be validated against additional field data before widespread application.
Reflections and Critical Assessment
The centrifugal model test approach used in this study provides a rigorous experimental basis for understanding the behavior of steel tube concrete composite piles. However, several limitations should be acknowledged. The model scale inevitably introduces some discrepancies in soil behavior due to scale effects on soil structure and fabric. The simplified representation of the soil-displacement zone as a uniform modulus increase may not capture the full complexity of driving-induced soil disturbance, which includes spatial variation of density, stress history effects, and time-dependent recovery.
The parametric study provides valuable insights but the range of variables tested is limited. In practice, factors such as pile diameter, steel tube wall thickness, concrete strength, soil layering, and groundwater conditions all influence the behavior of composite piles. Future research should expand the parametric space to develop more comprehensive design guidelines.
Summary
This centrifugal model test study provides quantitative evidence on the influence of steel tube embedment depth and soil-displacement zone modulus on the vertical bearing capacity of steel tube concrete composite piles in aquatic environments. The findings that embedment depth up to 12 cm (model scale) provides significant capacity improvement while deeper embedment yields diminishing returns, and that soil-displacement zone modulus directly affects shaft friction resistance, are directly applicable to the design of composite pile foundations for bridge piers, wharves, and marine structures. Engineers should incorporate these findings into their design methodology while recognizing the need for site-specific validation through full-scale load tests.