In the heart of Chongqing, a city known for its dramatic karst landscapes and bustling urban life, a groundbreaking method for constructing large-span metro tunnels is being pioneered. The superimposed arch-wall support (SAWS) method, developed by Hu Qifan of China Railway Design Corporation in Tianjin, is set to revolutionize the way we build underground infrastructure in environmentally sensitive areas.
The challenge with traditional sequential excavation methods, such as central diaphragm walls and double-side heading techniques, is that they often require temporary supports that can hinder the use of large-scale mechanized operations. “In areas where low-vibration, mechanized construction is crucial, these methods can fall short,” explains Hu. The SAWS method aims to address this issue, offering a more efficient and environmentally friendly approach.
To validate the SAWS method, Hu and his team conducted a comparative study using FLAC 3D software to simulate and analyze the geotechnical and structural mechanical behaviors during the entire station excavation process. They compared the conventional double-side heading method with the SAWS method, focusing on land subsidence, plastic zones in surrounding rock, deformation and stress in the initial support structure, and axial force in system anchors.
The results were promising. The SAWS method resulted in a maximum land subsidence increase of approximately 12.4%, a maximum arch vault settlement of 8.5%, and a maximum horizontal convergence within the tunnel along the direction perpendicular to the tunnel line of 26.0%. While these increases might seem significant, they all remained within safe margins. Moreover, the internal force at the arch foot of the initial support was reduced by about 42.9%, and the axial force of system anchors in the side wall middle and lower parts decreased by approximately 20.0% to 30.0%.
“This method demonstrates sufficient safety and reasonable stress distribution in the initial support and anchoring structures,” Hu notes. “It can meet the requirements for large-scale mechanized construction and is feasible for implementation.”
The implications for the energy sector are substantial. As cities continue to expand and the demand for efficient public transportation grows, the need for large-span metro tunnels in urban areas will only increase. The SAWS method offers a viable solution for constructing these tunnels in a way that minimizes environmental impact and maximizes efficiency.
Published in the journal ‘Chengshi guidao jiaotong yanjiu’ (translated to English as ‘Urban Rail Transit Research’), this study highlights the potential of the SAWS method to shape the future of underground construction. As Hu’s research demonstrates, innovation in this field is not just about pushing technological boundaries but also about finding sustainable and practical solutions that can be implemented on a large scale.
In the coming years, we can expect to see the SAWS method adopted in more projects, particularly in areas where traditional methods have proven challenging. This development is a testament to the power of innovation and the potential for technology to transform the way we build and live in our cities.