After two years of work, the largest tunnel boring machine (TBM) ever manufactured by Herrenknecht has achieved final breakthrough on the Sparvo Tunnel project in Italy.

The 15.5m diameter earth pressure balance shield TBM – known as Martina – produced the shell of the 2.4km long dual-tube road tunnel, which runs between Bologna and Florence. The project is designed to reduce the traffic on the the Autostrada A1 in the future.

Contractor Toto Costruzioni Generali began tunnelling in August 2011, reaching breakthrough on the first tunnel in July 2012. The TBM then turned 180o to start the second bore in December, completing the task in eight months.

Herrenknecht said Martina – which it claims is the largest TBM in the world to complete a project successfully – demonstrated top performances of 24m a day and 126m a week of driven and lined tunnel.

A sophisticated safety system was developed for Martina to allow it to tackle pockets of gas it encountered along the way. The belt conveyor on the TBM was fitted with double-walled casing and pressurised so that excavated material containing methane could be removed from the tunnel safely.

In addition, fresh air was permanently fed into all areas of the TBM using a ventilation system. Both the tightness of this casing and the gas concentration were monitored continuously.

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