Introduction: The Growing Need for Tunnel Reinforcement
With the rapid expansion of subway systems in China, the number of shield tunnels requiring reinforcement has grown substantially. By the end of 2023, mainland China’s shield tunnel mileage reached approximately 8,548 kilometers, with a significant portion of these tunnels showing signs of structural impairment. Shield tunnels, primarily used in urban rail systems, face challenges due to aging, corrosion, and ground vibrations.
Shield tunnels are typically constructed using precast concrete segments, and over time, these structures are subjected to varying environmental conditions, leading to degradation. As these tunnels deteriorate, reinforcement is needed to restore their structural integrity and extend their service life. However, reinforcing shield tunnels presents challenges, especially since many tunnels operate in confined spaces and under tight construction schedules. A major concern is that traditional reinforcement methods, such as epoxy-bonded steel plates, are heavy and require large machinery for installation.
To address these challenges, a new reinforcement material, Stainless Steel Corrugated Plate (SSCP), made from duplex stainless steel S32001, offers a promising solution. This material combines high strength, corrosion resistance, and a lightweight structure, making it suitable for reinforcing tunnel structures exposed to harsh environmental conditions.
The Properties and Design of Stainless Steel Corrugated Plates (SSCP)
Stainless Steel Corrugated Plates (SSCP) are an innovative reinforcement material designed specifically to enhance the structural integrity of shield tunnels. The SSCPs used in this study are made from duplex stainless steel S32001, which is a high-performance alloy known for its remarkable mechanical and physical properties:
• High Strength: SSCPs made from S32001 exhibit a yield strength of 509 MPa at a plastic strain of 0.2%, significantly higher than conventional steel plates, which typically have a yield strength of around 235 MPa.
• Lightweight: Each SSCP weighs only about 40 kg, making it much easier to handle, transport, and install compared to heavier traditional steel plates, which often weigh several hundred kilograms.
• Corrosion Resistance: The duplex stainless steel S32001 is highly resistant to corrosion. In a 3.5% NaCl solution, S32001 exhibits a corrosion rate of only 0.1 µA/cm², which is substantially lower than conventional steel, making it ideal for tunnels exposed to moisture and other corrosive elements.
• Fatigue Strength: S32001 also demonstrates superior fatigue strength compared to traditional steel reinforcement materials, such as threaded rods and rebars, which is particularly important in tunnels subject to vibrations over time.
Additionally, SSCPs have a corrugated design with a sinusoidal wave pattern, with a crest-to-trough height of 51.2 mm. This design provides the SSCP with increased stiffness in the transverse direction, enhancing its ability to resist deformation and improve the overall structural integrity of the tunnel. SSCPs are designed with an overlap section of 76.5 mm, allowing easy bolting of adjacent plates, further simplifying installation.
Full-Scale Testing and Design Concepts for SSCP Reinforcement
To evaluate the effectiveness of SSCP in reinforcing impaired shield tunnels, a comprehensive full-scale test was conducted. The objective was to analyze the mechanical behavior and failure modes of SSCP-reinforced shield tunnels, particularly focusing on how the SSCPs interacted with the existing tunnel structure.
The full-scale testing revealed several key findings:
1. Enhanced Bearing Capacity: SSCP reinforcement significantly improved the tunnel’s bearing capacity, allowing it to better withstand external loads and internal stresses. The increased strength helped prevent the tunnel from further deterioration.
2. Increased Stiffness: The stiffness of the tunnel structure was also substantially improved. The SSCPs provided additional support to the tunnel, preventing excessive movement or deformation under operational conditions.
3. Primary Failure Mode: The primary failure mode identified during the tests was shear failure at the bonding interface between the SSCP and the tunnel’s concrete structure. This failure was found to occur when the bond between the reinforcement and the tunnel structure was not sufficiently strong to withstand the applied loads.
Proposed Design Improvements
Based on the experimental results, two key design concepts were proposed to optimize SSCP reinforcement:
1. Enhancing Interface Bonding Shear Strength: To address shear failure, it was suggested that the interface bonding between the SSCP and the tunnel structure be strengthened. This could involve improving the adhesive properties of the bonding material used between the SSCP and the concrete or using advanced surface treatments to increase adhesion.
2. Improving Connection Stiffness: The connection between adjacent SSCPs was another area of focus. The research proposed using reinforced bolted connections to improve the stiffness between plates, ensuring that the SSCPs work together as a cohesive unit to distribute the applied loads more efficiently across the tunnel.
These design improvements are intended to maximize the effectiveness of SSCP reinforcement and prevent failure under extreme conditions, such as heavy loads or seismic activity.
Practical Application of SSCP Reinforcement in Shield Tunnels
The theoretical design and experimental testing of SSCPs were followed by their practical application in a real-world shield tunnel reinforcement project. The case study tunnel, located in China, featured a typical shield tunnel structure with an outer diameter of 8.5 meters and an inner diameter of 7.7 meters. The tunnel was constructed using precast concrete segments connected by bolts at both longitudinal and circumferential joints.
The SSCP reinforcement was successfully applied in the tunnel, following the proposed design concepts. The SSCPs were bolted into place, significantly improving the tunnel’s structural capacity and stiffness. The reinforcement process was relatively quick and cost-effective, as the lightweight SSCPs eliminated the need for heavy lifting machinery, and the bolted connections made installation easier compared to traditional welded steel plates.
Challenges and Considerations for SSCP Reinforcement
While SSCP reinforcement offers numerous benefits, there are still several challenges that need to be addressed:
• Height Limitations: The 51.2 mm height of the SSCP may pose issues in tunnels with limited space or severe deformations. In such cases, the application of SSCP reinforcement might be infeasible, and alternative methods may need to be considered.
• Design Optimization: To further improve the performance of SSCPs, additional research is needed to optimize the bonding and connection methods. Ensuring that the SSCPs are firmly attached to the concrete tunnel structure is critical to preventing failure.
• Implementation in Smaller Tunnels: While SSCPs are well-suited for larger tunnels, their application in smaller-diameter tunnels may be limited due to space constraints. However, as tunnel diameters continue to grow, particularly in river-crossing and sea-crossing projects, SSCP reinforcement may become more viable for these larger structures.
Key Takeaways:
• SSCP Advantage: Stainless Steel Corrugated Plates (SSCP) made from duplex stainless steel S32001 offer enhanced strength, corrosion resistance, and fatigue resistance compared to traditional reinforcement materials.
• Full-Scale Testing: Experimental testing revealed that SSCP reinforcement significantly enhances the bearing capacity and stiffness of shield tunnels, providing a robust solution for impaired tunnel structures.
• Design Concepts: To optimize SSCP reinforcement, two key design concepts were proposed: enhancing interface bonding shear strength and improving connection stiffness between SSCPs.
• Practical Application: SSCPs were successfully applied in a real-world shield tunnel project, demonstrating their construction efficiency, cost-effectiveness, and ease of installation.
• Construction Efficiency: The lightweight nature of SSCPs reduces the need for heavy lifting equipment, making them a more practical solution for tunnel reinforcement compared to traditional methods.
• Challenges: SSCP reinforcement may face limitations in smaller tunnels with severe deformation, and further research is needed to optimize bonding and connection methods for maximum effectiveness.
By addressing these challenges and continuing to refine SSCP design and application methods, this innovative reinforcement material could become a standard solution for enhancing the safety and longevity of shield tunnel structures globally.
