Application of Steel Tube Concrete Arch Frames in Subway Tunnel Construction

Literature Overview

This paper by Gu Shuancheng and Liu Haodong, published in Railway Engineering (2009, Vol. 49, No. 12), investigates the application of steel tube concrete (STC) arch frames as an alternative to traditional steel lattice girders in the initial support system of shallow-buried metro tunnels constructed using the New Austrian Tunneling Method (NATM). The study addresses a practical engineering challenge: the high steel consumption and complex fabrication of conventional lattice girders, proposing STC arch frames as a technically feasible and economically advantageous alternative. The research combines theoretical analysis using the load-structure model with practical engineering case studies.

Core Technical Findings

The primary finding is that STC arch frames are technically viable for subway tunnel initial support, with construction costs reduced to 61% of those for equivalent lattice girder systems. The analysis employs the elastic center method to calculate section internal forces, with the design calculation primarily based on large eccentric compression behavior.

Comparison of Support Systems

Structural Analysis Approach

The load-structure model was adopted for the STC arch frame analysis, treating the arch as a structural element resisting external loads from the surrounding rock mass. The elastic center method was used to determine the internal force distribution along the arch, with the critical design condition being large eccentric compression (大偏心受压). This approach considers:

1. Vertical loads from the overburden rock mass and surcharge loads.
2. Horizontal earth pressure acting on the arch sides.
3. Self-weight of the arch frame and infill concrete.
4. Construction loads during the sequential excavation and support installation process.

The large eccentric compression design condition arises because the resultant load on the arch cross-section does not pass through the centroid, creating significant bending moments in addition to axial compression. The combined stress state must be verified against the bearing capacity of the composite section.

Engineering Practice Implications

The practical implementation of STC arch frames in subway tunnel construction involves several critical considerations:

Construction Sequence and Quality Control

Material and Fabrication Requirements

For the steel tubes used in STC arch frames, the following requirements are critical:

• Tube specification: Typically Φ219 mm or Φ273 mm seamless steel tubes (Q345B or Q355B grade), with wall thickness of 8–12 mm depending on span and load conditions.
• Welding of tube segments: Field-welded joints between prefabricated tube segments must be full-penetration fillet or groove welds, inspected by magnetic particle testing (MT) for surface defects and ultrasonic testing (UT) for volumetric defects.
• Concrete specification: Pumpable concrete with a slump of 180–220 mm, compressive strength grade C30–C40, with appropriate air entrainment for workability in confined tube conditions.

FMEA Analysis of Construction Risks

Applying a Failure Mode and Effects Analysis approach to the construction process:

1. Incomplete concrete fill — voids within the tube reduce the effective composite section and may lead to premature failure. Countermeasure: use of transparent tubes for visual inspection or insertion of inspection ports at regular intervals.
2. Tube misalignment during installation — leads to eccentric loading and reduced structural capacity. Countermeasure: use of alignment templates and real-time surveying during installation.
3. Weld defects at tube segment joints — can propagate under cyclic loading and lead to catastrophic failure. Countermeasure: mandatory UT inspection of all field welds with acceptance criteria per GB/T 3323 or equivalent standards.
4. Concrete segregation during pumping — results in non-uniform concrete distribution within the tube. Countermeasure: controlled pumping rate, use of thixotropic admixtures, and sequential filling from the springing points upward.

Key Questions and Reflections

An important question concerns the long-term structural behavior of STC arch frames under sustained loading and environmental exposure. The steel tube provides confinement to the concrete, but the long-term durability of the composite section depends on the bond integrity between steel and concrete, which may be affected by corrosion of the steel tube interior surface. The study does not address corrosion protection measures for the interior surface of the tubes, which is a critical consideration in the aggressive groundwater environment typical of urban subway tunnels.

Another reflection pertains to the standardization of STC arch frame design. While the study demonstrates technical feasibility, widespread adoption requires the development of detailed design codes and construction standards specific to STC arch frames in tunnel applications. Currently, engineers must adapt existing STC column design provisions to arch geometry, which introduces uncertainty in the design process.

Study Insights and Implications

This research demonstrates a practical and economically attractive alternative to conventional steel lattice girders in NATM tunnel construction. The 39% cost reduction achieved through STC arch frames is significant for large-scale urban transit infrastructure projects where initial support systems represent a substantial portion of construction costs. The key technical insight is that the composite action between steel tubes and infill concrete provides superior load-bearing capacity with simpler fabrication and installation. For engineers involved in subway tunnel design, the study provides a validated approach that can be incorporated into preliminary design estimates and construction method selection. The recommendation is to conduct detailed structural analysis for each specific tunnel section, considering the local geology, groundwater conditions, and loading scenarios, rather than applying generalized design assumptions. The integration of STC arch frames into the overall tunnel support system requires careful consideration of the interaction with shotcrete, rock bolts, and the secondary lining.