6. Considerations for the choice of construction methods

6.1 Selection criteria

• Numerous criteria will guide the choice of construction method for an underground structure.

  • Length of structure

    This is a major criterion when it comes to choosing a mechanized method (TBM): the supply of a TBM is a significant investment, and only a sufficiently long tunnel length can make this investment profitable.

    A minimum tunnel length of the order of one kilometer can be assumed to justify the use of a TBM, with exceptions of course (e.g. unfeasibility of building the structure using conventional methods, availability of a machine suited to the geometric and geotechnical context of the structure).

  • Cross-section...