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  Thessaloniki Metro Extension Technical Challenges during Interstation Tunnelling

  The present case study refers to the design of the twin tunnels bored by TBM/EPB along the first Interstation of Thessaloniki Metro Extension to Kalamaria district. Four major challenges were encountered during tunnelling this section. The alternatives examined and the final solutions adopted by Project Owner ATTIKO METRO S.A. and the Contractor are herein presented. Firstly, the twin tunnels were to pass close to both sides of Kritis Shaft (a cut & cover structure) already excavated and with its temporary retaining support in place. In this respect, the TBM/EPB support pressure and the ground column between the shaft and each TBM tunnel should ensure the structures stability, considering the shaft’s future connection to the tunnels and the adverse existence of tall buildings at the surrounding area. Secondly, the twintunnels would have to cross an existing active fault (Pylaia) where big stresses and displacements may develop due to potential fault movements. Therefore, special segments with seismic connectors between the rings were implemented. Additionally, the tunnels alignment passed above one of the already constructed tunnel of the Main Line. Since all three tunnels were closely spaced, it was critical to investigate their interaction and secure safe construction. Finally, entering of the TBM machines into Patrikios Station, where high settlements had been recorded, was a difficult enterprise.

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