ABSTRACT
The concept of underground excavation implies the construction under the ground surface but, also, this concept assumes that the construction work is done without removing the existing ground between the excavation and the surface. At the beginning of the 20th century a new four-lane tunnel typology began to be proposed, characterized by the accommodation of a double-deck road and the use of Tunnel Boring Machines for its construction. The presence of water in the ground always leads to an added difficulty during tunnel construction; but the precise estimation, during the detailed design, of the effect that water presence carries during tunnel construction still remains an unsolved issue. Microtunnels are those tunnels whose width is less than 3 m. Conceptually, most hydraulic tunnels are underground channels and, as such, they are probably the oldest tunnels constructed in the world. Caverns are underground excavations whose excavated cross section exceeds 300 m.
