Characterization of Granite and the Underground

ABSTRACT: The characterization of the granitic mass of Porto for the design and construction of the Metro works of the city was based on weathering grades and structural features which were used for the derivation of the design parameters. The highly variable nature of the deeply weathered Oporto granite posed significant challenges in the driving of the 2.3 km long C line and the 4 km long S line of the project. Two 8.7 m diameter Herrenknecht EPB TBMs were used to excavate these tunnels but the nature of the rock mass made it extremely difficult to differentiate between the qualities of the mass and apply an open or a closed mode operation of the TBM accordingly. Thus early problems were encountered due to over excavation and face collapse. The matter was finally resolved by the introduction of an Active Support System, which involves the injection of pressurized bentonite slurry to compensate for deficiencies in the face support pressure when driving in mixed face conditions. Both the C and S lines have now been completed with minimal surface subsidence and no face instability