Design of the Motorway A31 Valdastico North (Italy)

The currently operating A31 motorway connects the A4 motorway (which is part of the European Lisbon-Kiev corridor V) with the northern part of the Province of Vicenza (length approximately 39,1 km). The project on subject is aiming to continue on north direction (about 40 additional km) to connect to the A22 and ease transportation of goods and people in the northern part of Italy. 

The contract signed by consortium RAETIA was for a preliminary and detailed design of about 40 km of Motorway. Once the preliminary design was finalized, the Client asked to focus for the main design just on the first part of the stretch (20km). 

The orographic characteristics of the territory crossed by this motorway section, and the geometry of the motorway, combined with the requirement to respect the environment have necessitated lengthy sections of the alignment underground. The preferred solution (selected among a range of five alternative options, duly technically and economically analyzed) foresees with eight natural tunnels of lengths ranging between 700 m and 15,100 m, all double tube one-way tunnels with a roadway of the same dimensions and characteristics as in the open: 11.25 m paved width with two 3.75 m wide lanes, a 0.75 m wide left embankment and a 3.00 m hard shoulder. All underground works exceeding 1,000 m in length have pedestrian and vehicular cross connections at 300 meter and 900 m distances respectively. The geometric shape of the tube has, in the event of traditional excavation with polycentric cross-section, an intrados radius equal to 6.55 m for the cap and the piers and a 13.50 m radius for the invert. In the event of mechanical excavation with a perfectly circular cross-section there is obviously only one intrados radius which is equal to 6.50 m.

The pass tunnel is undoubtedly the most important work both for the complexity of its structural and plant works and for its challenging cost and construction times. This structure, once completed, will in fact be the longest motorway tunnel in Europe (15.1 km). For the purpose of reducing construction times, given its great length, it has been decided to have it, unlike all the other tunnels, excavated mechanically with a full cross-section size cutter. Construction envisages the use of two cutters, starting from the Trent side towards the Veneto side, to excavate the tunnels on the rise.

The tunnel design from the safety standpoint complied with Legislative Decree 264/2006 “Implementing directive 2004/54/EC regarding the safety of tunnels in the trans-European motorway network”.