Watertight pedestrian tunnels (MOSE - Venice)

Nautilus co-operated in the construction of 1,240 meters of underwater tunnels, necessary for the inspection and maintenance of the MOSE system.

MOSE (Experimental Electromechanical Module) is a project intended to protect the city of Venice and the Venetian Lagoon from flooding. The MOSE is made up of 4 barriers located at the inlets of the Venice lagoon composed of a total of 78 independent mobile gates, hinged to the seabed on one side and activated by the variation of their buoyancy through the introduction and expulsion of water and air.

The Cavallino Treporti, San Nicolò and Malamocco barriers are parts of the MOSE project, a system of mobile barriers to protect Venice and its lagoon from flooding. The inlets of Cavallino Treporti, San Nicolò and Malamocco are three of the four gaps that connect the Adriatic Sea to the Venetian lagoon. Each barrier is made up of concrete caissons immersed on the seabed at the entrance to the inlet and of mobile steel “gates” barriers connected to the caissons by hinges.

Each barrier is made up of 9 prefabricated caissons and 21, 20 and 19 mobile gates respectively.

The caissons, 2 “shoulder” caissons and 7 “threshold” caissons, were built in dry docks at a distance from 500 m to 17 km from the laying area. The caissons were transported to the laying area by floating with the use of tugs and a particular laying pontoon.

Once in the laying area, the “threshold” caissons were submerged up to the final position by using the laying-pontoon and filling the ballast compartments with water in a controlled manner.

The 2 “shoulder” caissons have been placed at the two ends of the barrier and remain partially emerged, above water, thus creating access to the tunnels, inside the “threshold” caissons, necessary for inspection and maintenance of the system. The shoulder caissons were immersed by filling, step by step, the fifteen ballast compartments with water. A guiding structure mounted on the quay wall held the caissons in the correct position.

All the caissons were placed on temporary support bearings using a lifting system with hydraulic jacks. The remaining space below the caissons was filled with concrete injection to form the foundation slab. At the end of the laying, the liquid ballast necessary for the sinking was replaced with concrete jets under pressure.

A key factor required by the project was compliance with extremely high positioning tolerances. The required precisions, respected during the installation phase, allowed the correct junction of the caissons and the construction of watertight pedestrian tunnels.

At the front, the caissons were aligned with the previous caisson using a guiding system. At the back extremity, the realignment was carried out by a system of hydraulic jacks positioned between the caisson and the wall of the containment trench. Particular attention was required for the connection joints positioned eccentrically with respect to the axis of the body.

In each barrier there are 2 watertight pedestrian tunnels with an average length of 400 mt each.

The role of Nautilus

As part of the project, Nautilus worked with teams of Technical Underwater Operators for:

Sinking assistance and verification of correct installation
Underwater operations for positioning and connecting the caissons
Removal of the connections between the caissons and the laying pontoon (M64 bolts with tightening torque of 11860 Nm)
Opening of the foundation bags
Assistance in the execution of under pressure concrete castings
Removal of all metal structures necessary for laying the caissons
Laying the formworks and making the completion castings to seal the sealing joints
Final inspections and checks
Installation of 60 metal “gates” barriers.