During the tunnel element production in the drydock, all temporary structures necessary for the immersion process are been implemented.

Bulkhead structures temporarily closes the tubes to make the tunnel element afloat. Large ballast water tanks are necessary for creating a negative buoyancy during the immersion process as well as they are used for trimming the element. Cast-in items are part of the concrete works where later bollards and lifting lugs can be mounted on.

Also temporary support structures are part of the immersion equipment. Just before immersion survey towers are connected of which one of them also functions as an access shaft.

After finishing the tunnel element production and all preparations the drydock is flooded. Before flooding the ballast tanks have been filled so that the tunnel elements remain on the bottom of the drydock. Just before immersion, a tunnel element is made afloat by pumping out the ballast water. At the mooring location the tunnel element is been prepared for the immersion operation.

From the mooring location the tunnel element is transported to the immersion location. For this floating transport are several options using tugboats or large winches or a combination of both.

The immersion process follows a script that describes the operation in very detail with pre-defined steps and decision points. Also a thorough go/nog decision process is followed before and during the immersion process. With immersion pontoons attached to the lifting lugs and six winching wires keeping the element in its position the immersion process can start. By filling the water ballast tanks, the tunnel element reaches its negative buoyancy and will go through the waterline.

Following small steps, the immense mass of the tunnel element is brought to its final position on the riverbed within very small tolerances of around 30 millimetres. A rubber GINA gasket creates the watertight connection between the tunnel elements. Subsequently a closure joint for the remaining gap will be constructed. 

A second OMEGA seal will be placed at the joints between the tunnel elements after which the temporary bulkheads can be removed and the finishing works including the safety systems in the tunnel.

After acceptance of the final position of the tunnel element the sandflow process for the final foundation can start. A water/sand mixture is pumped through a series of pipes in the bottom slab of the tunnel element. The outlets are configured in a specific pattern that circle shape “pancakes” are created under the tunnel element. After finishing the sandflow bed the tunnel element is released from its temporary foundation structures to the sandflow foundation bed.

Read more: Sandflow tunnel foundations