Performance Study of Double Steel Tube Concrete Pile Heads

Literature Overview and Research Motivation

The study by Wang Haijun, Wei Hua, and Yang Jianmin (2005), published in the Journal of Shenyang University of Technology, Volume 27, Issue 6, investigates the mechanical performance of double steel tube concrete pile heads through axial static compression tests. The research was supported by the Liaoning Provincial Natural Science Foundation (2001102032) and the Ministry of Education Science and Technology Fund of Japan (11695057). The work addresses a critical practical problem in pile foundation engineering: the inadequate load transfer and weak deformation capacity of conventional single steel tube concrete piles at the pile head, where the concrete core is not effectively confined and the pile tip does not fully mobilize the soil resistance.

Experimental Program and Specimen Design

Twelve specimens were tested, including single steel tube piles, ribbed single steel tube piles, and double steel tube piles. The specimens were designed to evaluate the effects of stiffening ribs, pile tip shape, and outer tube protrusion length on the ultimate bearing capacity and deformation performance. The test program is comprehensive and systematic, allowing for the isolation of individual design parameters and their influence on pile head performance.

The double steel tube pile configuration consists of an outer steel tube that extends beyond the inner steel tube, with the space between the two tubes filled with concrete. The outer tube protrudes from the pile tip, creating a wedge-shaped or flat-ended configuration that enhances the bearing capacity through the closure effect, where the soil resistance is increased by the confinement provided by the outer tube at the pile tip.

Test Results and Performance Analysis

The experimental results clearly demonstrate the superiority of the double steel tube pile configuration over the conventional single steel tube pile. The single steel tube pile exhibits interface bond failure, where the concrete core separates from the steel tube wall under axial compression, resulting in low bearing capacity and weak deformation capacity. The concrete core is not effectively confined, and the steel tube does not fully mobilize its load-bearing potential.

The ribbed single steel tube pile shows improved bearing capacity due to the mechanical interlock provided by the stiffening ribs, which prevent the concrete core from sliding relative to the steel tube. However, the deformation capacity improvement is limited, as the ribs primarily enhance the interface bond rather than providing true confinement to the concrete core. The double steel tube pile, on the other hand, provides significant improvement in both ultimate bearing capacity and deformation performance. The outer tube creates a confinement effect on the concrete core that extends beyond the inner tube, effectively preventing the interface bond failure that plagues single tube piles.

The closure effect coefficient, which quantifies the enhancement of soil resistance due to the pile tip configuration, increases with the outer tube protrusion length. The authors recommend that in practical engineering applications, the outer tube protrusion length should be taken as D, the outer diameter of the pile. The 45-degree wedge shape of the outer tube end is particularly effective in increasing the bearing capacity and closure effect coefficient, as the inclined surface directs the soil resistance more efficiently into the pile shaft.

Engineering Practice Implications

The findings of this study have direct implications for the design and fabrication of steel tube concrete piles. For engineers specifying double steel tube piles, the fabrication tolerances of both the inner and outer tubes must be tightly controlled to ensure proper concentricity and the intended gap between the tubes. The welding of the inner tube to the outer tube, typically at the top of the pile, must be performed with full penetration welds and subjected to ultrasonic testing to ensure complete fusion. The concrete used to fill the space between the tubes should have a slump appropriate for placement in the confined annular space, typically 100-150 mm slump, and should contain a superplasticizer to ensure adequate workability.

The pile driving process for double steel tube piles requires special consideration. The outer tube protrusion must be protected during driving to prevent damage to the wedge-shaped tip, which is critical for the closure effect. A driving cap should be designed to distribute the hammer impact uniformly over the outer tube cross-section. The driving energy and blow count must be monitored to ensure that the pile is driven to the required depth without excessive damage to the pile structure.

Study Insights and Recommendations

This research provides compelling evidence for the adoption of double steel tube pile configurations in applications where high bearing capacity and good deformation performance are required, such as offshore platforms, bridge piers, and heavy industrial structures. The concept of the closure effect, where the pile tip configuration enhances soil resistance, is a valuable design principle that can be applied to other pile types. The recommendation to use a 45-degree wedge tip and an outer tube protrusion length equal to D provides clear, actionable guidance for engineers. However, further research on the long-term performance of double steel tube piles under cyclic loading and in corrosive environments is warranted to fully establish the technology's reliability for marine and offshore applications.