Three-Dimensional Interaction of Twin Tunnels: Numerical Analysis of the Waterloo International Terminal Case Study

Being able to predict with precision and certainty how existing tunnels respond to new tunnelling works in urban areas is vital for the safety of the existing tunnels and for minimizing the cost and environmental impact of the new tunnels. The three-dimensional interaction of tunnels in stiff, overconsolidated clays has mainly been restricted to field studies, with only a few generic numerical studies. Nonetheless, a large part of underground tunnel construction has happened and continues to occur in overconsolidated clays. The paper bridges this gap by using the case study of Waterloo International Terminal, where two new tunnels were excavated beneath two 70-year-old tunnels, to validate a numerical model. The validated numerical results provide new, valuable insights into the differences and similarities of the response of the existing tunnels depending on their typology (running or station tunnels) and time after the excavation of the new tunnels. Furthermore, the results reveal the significance of the stiffness reduction factors that need to be applied to account for the segmental nature of the tunnel linings, highlighting the need for further research into the operational value of the tunnel stiffness.