Compressive Stability of High-Pile-Cap Steel Tube Piles - Experimental and Numerical Analysis

Literature Overview

This 2011 study published in Rock and Soil Mechanics by Jia Qiang and colleagues from Shandong Jianzhu University investigates the compressive stability of steel tube piles in high pile-cap configurations used for underground foundation underpinning. The research is funded by the Ministry of Housing and Urban-Rural Development Science and Technology Program. The study employs both 1:2 scale model testing and nonlinear buckling numerical analysis to determine the stress distribution and ultimate bearing capacity of steel tube piles subjected to compressive loading. The research is particularly relevant to the engineering challenge of underground foundation replacement for existing buildings where additional basements are being constructed.

Core Findings and Technical Analysis

The study addresses a critical engineering problem: when steel tube piles are used for foundation underpinning, the lower end of the pile is inserted into the existing concrete pile cap, while the upper end is connected to the new underpinning pile cap. After the surrounding soil is excavated, the pile's stability is reduced, and the ultimate compressive bearing capacity cannot be determined by current design codes. The study determines the effective length coefficient (k) for the steel tube pile under these boundary conditions through both experimental and numerical methods.

Buckling Analysis and Stability Assessment

The effective length coefficient is a critical parameter in determining the buckling load of a compression member. For a steel tube pile with the lower end fixed in the concrete pile cap and the upper end pinned at the underpinning pile cap, the theoretical effective length coefficient is 0.7. The experimental value of 0.616 and the numerical value of 0.683 are both lower than or close to this theoretical value, indicating that the actual boundary conditions provide slightly more restraint than the idealized pinned-fixed condition. This is physically reasonable because the insertion of the pile into the concrete pile cap provides some rotational restraint beyond a simple fixed support, and the connection at the top may also provide partial rotational restraint.

The buckling mode of the steel tube pile under these boundary conditions involves lateral deflection with the maximum displacement occurring at approximately 0.43L from the fixed end (where L is the free length of the pile). The stress distribution shows compressive stresses on the concave side and tensile stresses on the convex side, with the maximum stresses occurring at the fixed end where the bending moment is maximum. The nonlinear analysis captures the geometric nonlinearity of the buckling process, including the P-Δ effect (second-order moment due to lateral displacement), which is not accounted for in the linear Euler buckling theory.

Connection with Steel Pipe and Welding Engineering

The steel tube piles used in foundation underpinning applications are typically manufactured by seamless or welded processes. For the stability analysis, the geometric properties of the steel tube—outer diameter, wall thickness, and length—are the primary parameters. However, the manufacturing quality of the steel tube significantly affects the actual buckling capacity. Geometric imperfections such as out-of-roundness, ovality, and waviness reduce the buckling load compared to the idealized straight, perfectly round tube.

From a welding perspective, the connection between the steel tube pile and the underpinning pile cap is a critical detail. This connection is typically made by welding a base plate to the top of the steel tube, or by inserting the tube into a socket welded to the pile cap. The weld quality at this connection determines the effective boundary condition at the top of the pile. A full-penetration weld with a well-designed fillet weld provides a more rigid connection than a simple lap weld, and this affects the effective length coefficient.

The steel tube insertion into the existing concrete pile cap also involves welding considerations. The lower end of the steel tube may need to be welded to a reinforcement cage or a base plate that is embedded in the concrete pile cap. The welding process must be compatible with the existing concrete to avoid thermal damage to the pile cap. Preheating, controlled heat input, and post-weld inspection are essential quality control measures for this connection.

Quality Control and Inspection Requirements

The stability of steel tube piles in underpinning applications is sensitive to several quality factors that must be controlled during construction:

• Steel tube straightness: the initial out-of-straightness should not exceed L/1000, where L is the free length, to avoid premature buckling
• Wall thickness uniformity: the wall thickness should be uniform within ±10% of the nominal value; thin spots reduce the local buckling resistance
• Weld integrity at both ends: the connections at both the top and bottom of the pile should be inspected by ultrasonic testing (UT) or magnetic particle testing (MT) to ensure full fusion and absence of cracks
• Concrete pile cap integrity: the existing concrete pile cap into which the steel tube is inserted should be inspected for cracks and honeycombing that could reduce the rotational restraint at the fixed end
• Soil excavation sequence: the soil around the pile should be excavated in a controlled sequence to avoid sudden loss of lateral support that could trigger buckling before the underpinning pile cap is completed

Key Questions and Reflections

A significant question arising from this study is the effect of soil-pile interaction on the buckling behavior. The study focuses on the case where the surrounding soil has been excavated, leaving the pile in a free-standing condition. However, in practice, the soil excavation is rarely complete around the entire pile length simultaneously. Partial soil support along the pile length provides lateral restraint that increases the buckling capacity beyond what is predicted by the simplified pinned-fixed model. The study does not address this soil-pile interaction, and engineers should consider it in detailed design through finite element analysis that includes the soil-pile interface.

Another reflection concerns the dynamic effects during construction. The excavation process and the installation of the underpinning pile cap involve dynamic loading that could trigger buckling if the pile is in a critically loaded state. The study focuses on static stability, and the dynamic amplification factor should be considered in the design to ensure that the pile does not buckle during construction activities such as pile driving, concrete placement, or heavy equipment operation near the excavation.

Study Insights and Implications

The study provides a practical methodology for assessing the compressive stability of steel tube piles in underpinning applications, which is a common and important engineering problem in urban development where existing buildings need additional basements. The determination of the effective length coefficient through both experimental and numerical methods provides a robust basis for design. The recommendation to use the numerical analysis results, which are slightly conservative, is appropriate for engineering safety. The 1:2 scale model test provides valuable experimental validation that increases confidence in the numerical predictions.

Reference Value and Outlook

This research provides direct engineering guidance for the design of steel tube piles in underground foundation underpinning projects. The effective length coefficient values determined in this study can be used as a starting point for design, subject to verification through site-specific analysis. Future research should investigate the effect of partial soil support, dynamic loading during construction, and long-term settlement on the stability of steel tube piles in underpinning applications. The development of design charts or simplified calculation procedures based on the findings of this study would further facilitate their application in routine engineering practice.