Work is due to start this month on the $21.3m widening, from two lanes in each direction to three, of the 337m long Nazzano Tunnel on the A1 Milan- Naples motorway. This will be accomplished without any interruption to traffic in either direction.

At present, the tunnel has a diameter of 12.2m, to be widened to 19.7m. Maximum overburden is 45m and ground comprises sandy and silty clays. Prof. Ing. Pietro Lunardi has designed a new method for construction which is being employed by consultant Rocksoil and contractor Ferrovial & Agroman. The owner of the patent for the method is the client, Autostrade.

The process is carried out above a mobile ‘steel traffic protection shell’ under which traffic continues to flow.

The construction equipment is mounted on a semi-circular, rail-mounted gantry which straddles the traffic protection shell and allows free rotation and access to the arched section of ground to be removed. This rig has been built by STAC of Mozzate (Como) and is capable of undertaking all the operations in the widening process.

This process is performed in four main stages:

1. After any necessary ground improvement to the face is completed, a fibre reinforced shotcrete shell is created along the extrados of the future cavity using mechanical pre-cutting technology.

2. The ground is excavated in steps under the protection of the pre-cut shell until the theoretical profile of the widened tunnel is reached and demolition of the old lining is accomplished. Spoil is removed by trucks running between the exterior wall of the traffic protection shell and the new segmental tunnel lining.

3. The final lining is placed immediately very close to the face (5m maximum) by successive erection of one or more arches of prefabricated segments according to the active arch principle. Special jacks in the key section provide the necessary arch compression.

4. The tunnel invert is constructed. Depending on the type of ground and the stress-strain conditions, the system employs various technologies, including horizontal jet grouting, ground improvement using valved and injected fibre glass bolts, in addition to mechanical pre-cutting.

The project is scheduled for completion in April 2001.



Related Files
Cross Section