TECHNICAL | SOFT GROUND TUNNELLING


West portal Existing tunnel


409 ramp wall


Shaft


East portal


Existing piles


Tunnel 2 Tunnel 1


Pre-support pipes 813mm dia. 12mm thick


Above: View of structural elements at final design Top – BIM; Above – FEA


A 3D Finite Element (FE) model was developed


early in the project, simulating the entire construction process, including tunnel excavation, pipe installation, and the interaction between the new tunnels and existing infrastructure. The FE model, consisting of over 1 million elements and 459 steps, provided a detailed representation of the


12 | November 2024


site’s geotechnical conditions, including the engineering fill, the glacial till deposits, and the backfill at the sides of the existing tunnel, as shown in Figure 4. Non-linear material behaviour was defined for both soil (Figure 8) and tunnel linings (Figure 7): the Hardening Mohr Coulomb model (Doherty & Muir Wood, 2013) with the main parameters shown in Table 1 was used for the


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