Sunk tunnels consist of separate tunnel components. On both the river banks, the builders construct part of the tunnel, including the access ramp. Trenches are dug in the location where the tunnel is projected. 

The construction process of a sunk tunnel starts in a construction dock in the town of Barendrecht, where from the 1960s various tunnels were made, including the Heinenoord Tunnel, the Drecht Tunnel, Willem Rail Tunnel and Second Benelux Tunnel. The construction dock resembles a polder, situated lower than the Oude Maas river flowing beside it, and measures 10 hectares (roughly 20 football pitches).

Each tunnel has 7 tunnel components. Each tunnel component consists of 6 sections with a length of 25 metres, so a complete tunnel component spans 150 metres. A tunnel component consists of two rectangular pipes, each with a 7.35-metre width. The pipes, or tunnel halves, are separated by an 80-cm concrete wall. There are also tunnel components that are 134 metres long. One component weighs around 23,000 tons. In the Barendrecht dock 14 tunnel components are built for the HSL.

On the bottom of the dock, a layer of gravel is spread out, on top of which the reinforcement for the floors is laid and the concrete for the tunnel floor is poured. After this the reinforcement and formwork for the walls and the roof can be put in place.

The walls and roof of a tunnel component are 90 cm thick. Walls and roof are poured in a single operation on top of the tunnel floor, with reinforcement and formwork still there for walls and roof. A spiralling pipe is placed in the walls. When the concrete hardens, water is pumped through this pipe to convey the heat that is produced by hardening. Without cooling, the wall would harden too quickly and the concrete would be of much poorer quality.

The concrete floor of a tunnel component is 1.1 metres thick and contains reinforcement steel to strengthen it. The floor is fitted with a so-called earthing network, which will convey the leakage currents when the train passes over the tunnel floor. The earthing network prevents the reinforcing bars from rusting too quickly. In addition, pretensioning cable ducts are put into the concrete floor.

When the component is ready, cables are put through it, which are used to pull the individual components tightly together (like beads on a chain), resulting in a rigid construction. Finally, each element is fitted with a pipe with a discharge hole at the bottom and a filling hole at the side. Once the element is in place, a mixture of water and sand is pushed through this pipe to fill the gap below the element.

The separate tunnel segments are connected together in a watertight construction, joined by rubber expansion seams. These are partly poured into the concrete. The segments are hoisted together with the pretensioning cables. The joint between the elements is later sealed with a so-called Gina profile.

When all the tunnel components are ready, various sinking facilities are put into place: partitions, which can seal of the tunnel component from both sides, and ballast tanks filled with water, to make the components float or sink down. In the dock, the tanks are first filled to so such an extent that they lie on the bottom. Now the construction dock is filled with water to make the water level the same as the adjoining river. After this, part of the dike between the Oude Maas and the dock is dredged open, creating an opening to ship the tunnel components to the river. At this stage, the tanks are drained, so that they start to float and can be transported.

Four tugboats, a pusher tug and some support ships take the tunnel components to their destination. The distance from the dock to the Oude Maas Tunnel is 7 km; the transport time was 2 to 3 hours. For the Dordtsche Kil Tunnel, 14 km had to be covered, which took twice as long.

Once they have reached the location, the connected components are sunk down into a dredged trench. A great part of these trenches are dug in both the embankments.

Each component has at its ends rectangular dovetails, which helps the components interlock, so that there is practically no vertical movement. Finally, the temporary partitions are removed from the tunnel components, creating a long tunnel duct where the finishing work can be carried out.

The connection from the sunk tunnel to the fixed tunnel segments that are on dry land always deserves special attention. The sunk section of a tunnel might subside a little in the riverbed. But the section on land is founded with piles and therefore extra stable.

At the connection point, the difference in height, measured in millimetres, may not become too great. In particular if high speed trains will be crossing this point. The segments, elements and the final joint (which is made after sinking the tunnel down, in special formwork) are therefore provided with interlocking teeth. By using such a connection, individual vertical movement of two separate tunnel components is no longer possible. The tunnel floor remains level, so that train passengers can pass through a tunnel in the desired comfort.

On the island of Dordrecht and in the Hoeksche Waard, the walls of the construction pit must retain the river water, because the construction requires that the river dikes are partly dredged away. As an extra safety measure against flooding, the construction pits in question are enclosed by a second dike. Similarly, at the Oude Maas Tunnel, the walls of the construction pit, and afterwards the tunnel walls, take the place of dikes. As an extra safety against flooding in the place where the HSL cuts through the Molen Dike, pointed floodgates have been built. 

As a result, this secondary dike can be closed off if water from the Oude Maas would flow into the polder, thus preventing that the water would flood further into the Hoekse Waard Area via the HSL. These measures were taken to ensure that the residents keep their feet dry.

A layer of sand on the roof and ballast in the tunnel components ensure that the tunnel stays in place under water. The result is a watertight tunnel.