Prefabricated Steel Pipe Concrete Internal Support in Shield Vertical Shaft Deep Foundation Pit Engineering

Literature Overview

This paper by Guo Xueyuan et al., published in the Journal of Lanzhou University of Technology (2020, Vol. 46, Issue 6, pp. 131-136), addresses a critical engineering challenge: the design and construction of internal bracing systems for deep foundation pits associated with shield tunneling vertical shafts. The authors propose a prefabricated steel pipe concrete (PCFT) internal support system, which consists of twin-leg rectangular steel tube lightweight aggregate concrete members connected by high-strength bolts and anchoring devices. The research is supported by the National Natural Science Foundation of China (Grant No. 51538001) and the Hebei Provincial Natural Science Foundation (Grant No. E2020209086).

Core Technical Concept

Traditional internal bracing systems in shield vertical shaft pits face significant constraints: limited excavation working faces, extreme pit depths, and the need for rapid installation and removal to minimize interference with shield tunneling operations. The proposed PCFT system overcomes these limitations through a modular, bolted assembly approach that eliminates on-site welding requirements.

The structural configuration employs twin rectangular steel tubes as the primary load-bearing elements, filled with lightweight aggregate concrete to reduce self-weight while maintaining compressive capacity. The prefabricated nature of the system means that individual segments are manufactured off-site and assembled in-situ using high-strength bolted connections and mechanical anchoring hardware.

Key Design Parameters and Structural Characteristics

Engineering Practice and Monitoring Results

The case study is based on the Beijing Metro Line 17 shield vertical shaft deep foundation pit project. The authors conducted comprehensive informatized monitoring during construction, tracking four critical parameters:

1. Support axial force - monitored via strain gauges at mid-span and quarter-span locations
2. Retaining pile deformation - measured at multiple depths along the pile wall
3. Pile head displacement - tracked at ground level and below
4. Ground surface settlement - measured at multiple distances from the pit perimeter

The monitoring results demonstrated that the PCFT internal support system effectively controlled both pit stability and surrounding environmental safety. The bolted connection approach significantly reduced construction time compared to traditional welded steel struts, which is particularly advantageous in shield tunneling shafts where excavation windows are tightly constrained by tunneling schedules.

Technical Insights and Reflections

From a steel pipe manufacturing and connection technology perspective, several aspects of this work deserve attention:

• Rectangular steel tube fabrication: The twin-leg rectangular tube members require precise dimensional tolerance control during rolling or welding. For prefabricated applications, the flatness and squareness tolerances must be tightened beyond standard GB/T 706 requirements to ensure proper bolted joint engagement.
• Bolted connection integrity: The elimination of on-site welding shifts the quality control focus entirely to the bolted connections. The preload accuracy, bolt grade verification, and connection surface preparation become critical quality gates. This aligns with FMEA thinking, where the failure modes shift from weld defects (porosity, lack of fusion) to connection-related failures (bolt relaxation, bearing surface crushing).
• Lightweight concrete interaction: The use of lightweight aggregate concrete within the steel tubes introduces considerations regarding the bond interface between steel and concrete, differential thermal expansion, and long-term creep behavior. The concrete confinement effect on the steel tube is a well-established phenomenon in CFST design, but the lightweight nature may reduce the confinement efficiency compared to normal-weight concrete.
• Demountability advantage: For shield tunneling shafts, the internal bracing must be installed and removed multiple times as excavation proceeds in stages. The prefabricated bolted system offers superior demountability compared to welded alternatives, reducing material waste and construction cycle time.

Standards and Specification Considerations

The design methodology draws from multiple standard frameworks:

Study Conclusions

The PCFT internal support system represents a pragmatic solution to a well-recognized engineering constraint. The prefabricated bolted approach trades some structural redundancy for significant construction efficiency gains, which is an appropriate trade-off given the schedule-critical nature of shield tunneling shaft construction. The monitoring data confirms that the system performs reliably within its design envelope. However, further research on the long-term durability of the bolted connections under cyclic loading conditions, and on the interface behavior between the lightweight concrete and the rectangular steel tubes under sustained axial compression, would strengthen the technical basis for widespread adoption.