Concrete-Filled Steel Tube Arches in Metro Tunnel Applications

Literature Overview

The paper by Gu Shuancheng and Liu Haodong, published in Railway Engineering (2009, Vol. 49, No. 12, pp. 56-60), investigates the application of concrete-filled steel tube (CFST) arches as initial support in metro interval tunnels constructed using the shallow cover cut-and-cover method. Conducted at the School of Architecture and Civil Engineering, Xi'an University of Science and Technology, this research addresses a practical engineering challenge: replacing conventional lattice girder arches with CFST arches to reduce cost and accelerate construction.

Core Technical Content and Methodology

The study uses a load-structure model approach to analyze the internal forces in CFST arches, employing the elastic center method for calculating section forces. The design calculation is primarily based on large eccentric compression conditions, which is characteristic of arch members subjected to combined axial load and bending moment from surrounding rock pressure.

The elastic center method is a classical approach for analyzing statically indeterminate arch structures. The arch is treated as a curved beam, and the elastic center (centroid of the flexibility properties) is determined to simplify the calculation of redundant forces. This method is particularly suitable for the preliminary design stage and provides conservative estimates of internal forces.

Key Findings and Technical Interpretation

The primary finding is that CFST arches are technically feasible as replacements for lattice girder arches in metro tunnel initial support, with a support cost of only 61% of the conventional lattice girder system. This represents a 39% cost reduction, which is significant for large-scale metro tunnel projects where thousands of arch rings may be required.

From a steel pipe engineering perspective, the CFST arch concept leverages the well-established composite action between steel tubes and concrete. The steel tube provides immediate load-bearing capacity during installation (before concrete reaches design strength), while the concrete fill enhances the ultimate capacity, reduces the steel tube wall thickness requirement, and provides fire protection. The large eccentric compression condition indicates that the arch section experiences significant bending in addition to axial compression, which requires adequate section modulus and moment capacity.

The feasibility of this approach depends on several factors related to steel tube fabrication and assembly:

1. Tube bending and forming: CFST arches require steel tubes to be bent into the arch profile. This can be achieved through cold bending (for smaller diameters and thicker walls) or hot bending (for larger diameters). The bending process must be controlled to avoid excessive thinning at the outer fiber, which would compromise the structural capacity. The minimum bend radius should comply with the relevant standard (typically D ≥ 1.5 × tube diameter for cold bending).
2. Welded connections between arch segments: The arch is typically assembled from multiple prefabricated segments connected by bolted or welded joints. The joint design must accommodate the full design loads, including the large eccentric compression forces. Welded joints require qualified welding procedures and non-destructive testing (NDT) to ensure full-strength connections.
3. Concrete filling technique: The concrete must be placed within the steel tube arch after installation. For arch-shaped tubes, the filling process must ensure complete concrete placement without voids, particularly at the crown and haunch regions where concrete can accumulate. The filling pressure and concrete slump must be carefully controlled.

Integration with Engineering Practice

The application of CFST arches in metro tunnels has several practical advantages:

• Rapid installation: Single-tube arches can be installed more quickly than multi-member lattice girders, reducing the time that the excavation face is exposed to ground pressure.
• Reduced material handling: Fewer components mean less site handling, welding, and assembly work.
• Improved safety: The immediate load-bearing capacity of the steel tube provides early support to the surrounding ground, reducing the risk of ground settlement and collapse.

However, several challenges must be addressed:

• Concrete filling quality: Ensuring void-free concrete filling in a curved tube requires careful engineering. The filling should be done from the springing line upward, with appropriate vibration and access for inspection.
• Fire protection: While the concrete fill provides inherent fire protection, the steel tube must be designed to maintain structural integrity at elevated temperatures during a fire event, per the relevant fire design code.
• Inspection and maintenance: The internal concrete condition cannot be easily inspected after filling, which may be a concern for long-term maintenance and monitoring.

Study Insights and Implications

This research demonstrates a practical and economically attractive alternative to conventional lattice girder arches for metro tunnel initial support. The 39% cost reduction is compelling, and the technical feasibility has been validated through both analysis and engineering practice. For steel pipe manufacturers, this application opens a market opportunity for supplying bent steel tube arch segments for underground construction projects. The key success factors are the quality of tube bending, the integrity of welded joints, and the reliability of concrete filling procedures. Engineers should carefully evaluate the project-specific conditions, including ground type, depth, and adjacent structure proximity, before adopting CFST arches as the primary initial support system.