2. Tunnel construction methods

2.1 Two main categories of construction methods

Tunnel construction can be summed up in three stages:

• digging up the soil: this is the felling function;

• evacuate excavated soil: this is the marinating function;

• support the ground if it is not naturally stable: this is the function of support (temporary) and/or lining (permanent).

Ever since the first underground structures were built, the principle has been to excavate over a limited length so that the structure remains stable in the short term, i.e. for the time it takes to set up any retaining structures: this is sequential work, which means that the various functions have to be carried...