The works consisted in a microtunnel for laying r.c. casing pipes for the connection of the OLT floating regasification terminal, off the coast of Livorno, to the entry point into the national gas transport network Snam Rete Gas.
The “trenchless” microtunnelling technology was adopted as an alternative to the traditional trench excavation, to cross a section where the presence of polluted soils was detected. Starting from the thrust shaft, located in an artificial peninsula in the Arno drainage channel, a closed shield with an external diameter of 2,5 m has been pushed forward towards the sea for about 750 m, until reaching its arrival point. Subsequently, after closing the back door of the TBM and flooding the tunnel, the TBM recovery took place with the aid of a divers team who positioned the spreader beam and slings for recovery by crane from the barge. The execution of works has posed several challenges including: the unconventional recovery of the TBM in water, the crossing of soils with poor load-bearing capacity, the management of very fine grain size spoils through a complex system of filter presses and very short contractual execution times.