When tunnelling through challenging ground conditions, particularly those characterized by high deformability like cohesive-frictional materials, specific deformations can impact tunnelling processes. Recent research conducted by authors P. Thomas and G. Anagnostou at the Swiss Federal Institute of Technology Zurich focuses on the phenomenon of extrusion-induced excavation volume during mechanised tunnelling and its important ramifications for tunnel construction.
During the tunnelling process, significant ground displacement occurs at the tunnel face, known as extrusion, which results in additional excavation volume beyond what is accounted for by the tunnel’s cross-section. Understanding this volume is pivotal as it could substantially influence the logistics of tunnel construction and the structural integrity of the tunnel face itself.
The researchers utilized numerical models incorporating re-meshing techniques to continuously evaluate the re-profiling of the tunnel face during advance. Their study explored various parameters including the Young’s modulus (Young’s modulus values ranging from 0.15 to 2 GPa), face support pressure (ranging from zero to 2 MPa), and the characteristics of the ground through which tunnelling occurs.
One of the study’s notable achievements was the development of what the authors termed as "a simple closed-form expression". This equation allows for rapid and accurate assessment of extrusion-induced excavation volumes. It renders complex calculations more accessible, potentially streamlining the design process for TBM (Tunnel Boring Machine) operations.
Importantly, the study revealed the extent of additional excavation: typically falling within the range of 4–20%, with averages around 9% for operations like those seen at the Sedrun section of the Gotthard Base Tunnel. This finding indicates how successful management of face extrusion can mitigate adverse outcomes, such as logistic disruptions and jamming of the TBM cutterhead.
"If the additional excavation volume due to face extrusion is not taken... it may pose logistical and disposal challenges and lead to erroneous conclusions about the stability of the tunnel face," wrote the authors of the article. This highlights the necessity for engineers and project managers to incorporate these findings when planning tunnelling projects, particularly when dealing with ground conditions likely to result in severe squeezing.
While the focus was primarily on cohesive-frictional materials, the research also drew comparisons to purely cohesive materials, finding they exhibit more unfavourable response under similar conditions. "The behaviour of purely cohesive ground is consistently more unfavourable than... cohesive-frictional ground," the authors noted, emphasizing the practical benefits of recognizing varied material responses.
Such insights are deeply relevant for current and future tunnelling operations, especially as geographical challenges increase with urban development and infrastructure demands.
The analytical findings provide engineers with valuable tools for potential operational adjustments and improvements. For example, if extrusion volumes exceed predictions, reducing the rate of advance or adjusting support mechanisms could stabilize the tunnel face and mitigate risks.
From this research, it becomes clear: effectively managing face extrusion is not just about calculations; it's about ensuring safety, efficiency, and sustainability within tunnelling practices. The work provides groundwork for applying multifaceted TBM designs, with relevance extending far beyond the confines of current projects.
Acknowledging the role of ground conditions allows for advances not just in tunnelling but paves the way for innovations aimed at tackling the complex challenges of underground construction.