In the world of underground construction, shield tunnels are a marvel of modern engineering, enabling the swift and safe excavation of tunnels for metro systems, highways, and utilities. However, one persistent challenge has plagued these projects: segment flotation. This occurs when the buoyancy of the grout used to fill the annulus between the tunnel and the segments causes the segments to float, leading to potential misalignment and structural issues. A recent study published in *Yantu gongcheng xuebao* (translated as *Rock and Soil Mechanics*) by WANG Xianming and colleagues from the China Key Laboratory of Transportation Tunnel Engineering at Southwest Jiaotong University, along with China Railway 14th Bureau Group Shield Engineering Co. Ltd., sheds new light on this problem and offers a promising solution.
The researchers developed a novel testing device to study the time-dependent variation of net buoyancy in two types of grout: typical single-component and two-component grout. Their findings revealed that the net buoyancy of the grout decreases over time, initially in a linear fashion and then more rapidly in a nonlinear phase. Notably, the grout’s buoyancy dissipates well before its final setting time, which has significant implications for tunnel construction.
“Our study shows that the net buoyancy diminishes to zero while the grout is still in a fluid-plastic state,” said lead author WANG Xianming. “This is crucial because it means that the grout’s buoyancy can be managed and mitigated before it sets, providing a window of opportunity for engineers to intervene and prevent segment flotation.”
To translate these findings into practical applications, the team developed a finite element model using ABAQUS software. This model considers various factors, including grout buoyancy dissipation, segment self-weight, shield thrust, ground load, and tail restraint. The model was validated against field monitoring data, demonstrating its accuracy and reliability.
The study’s results have significant commercial implications, particularly for the energy sector, where underground construction is often required for pipelines, cables, and other infrastructure. By understanding and managing grout buoyancy, engineers can ensure the stability and integrity of these critical structures, reducing the risk of costly repairs and delays.
Moreover, the research highlights the advantages of two-component grout over single-component grout. Two-component grout exhibits a shorter buoyancy dissipation time and physical setting time, enabling rapid stabilization of segments and substantial suppression of flotation. This finding could lead to more efficient and cost-effective construction practices in the future.
As the demand for underground infrastructure continues to grow, the insights gained from this study will be invaluable. By providing a theoretical basis for flotation-resistant design, the research paves the way for safer, more reliable, and more efficient shield tunnel construction. For projects facing severe segment flotation issues, the adoption of synchronous grouting materials with fast gelling time and high early-stage strength is recommended.
In the ever-evolving field of construction engineering, this study stands as a testament to the power of innovation and the potential for technology to overcome long-standing challenges. As WANG Xianming and his team continue to push the boundaries of what is possible, the future of underground construction looks brighter than ever.