The Bridge over the Hollandsch Diep waterway is 2 kilometres long, with 1200 metres above water. The bridge is 24 metres high at its highest point and rises above the old rail bridge adjacent to it. It is the longest bridge on the HSL route Amsterdam-Paris. Crossing the Hollandsch Diep in a high speed train takes a mere 14 seconds. The bridge is supported by 11 piers and 2 land abutments. It was decided to build a so-called composite bridge, a bridge made of steel and concrete, because this type of bridge is extremely rigid. This is necessary because passing high speed trains cause vibrations, which the bridge must be able to withstand and absorb, so that passenger comfort is guaranteed. The construction method is also special because there was no need to drive traditional construction pit walls for the bridge piers.
The construction of the bridge started with driving steel tubes into the riverbed of the Hollandsch Diep. The hollow piles are the foundation for the bridge's piers. These tubes are enormous, hollow, steel tubes with a diameter of around 3 metres. The tubes are 19 to 33.5 metres long. The wall thickness of the tube varies between 35 and 45 millimetres. From a floating pontoon, a pile-driving machine drove the tubes into the riverbed. For most piers, 4 tubes were driven in a square. In five places, a fifth pile was driven in the centre of the square (and later under the centre of the pier) to make these piers extra strong. When the driving was finished, the top of the piles were 4.5 metres below the water.
After piling, a concrete tank of 10 x 25 x 2.65 metres was placed above the piles: a so-called caisson. The bottom of a caisson has holes in it, which have been sealed with steel plates. The holes must exactly match the position of the tubes that were driven into the riverbed. Because of the required precision, the locations of the top of the tubes are measured, and only then the caissons are built to fit.
The caissons are sunken down in such a way that the holes ended up on top of the piles. This was a meticulous and time-consuming job. After sinking down, the caissons rested on the tubes, under water. Divers with underwater cutting torches cut the steel plates from the concrete bottom. During this work, the caissons remained on the piles.
In the resulting holes the reinforcement, a steel mesh with a length of 12.5 metres, was placed. Then the tubes and the lower part of the caissons were filled with underwater concrete. This required dozens of full concrete trucks that were transported over the water on a pontoon. When the concrete hardened, the caissons were immovably connected with the steel tubes.
During production, watertight partitions were placed on the walls of the caissons. When the caissons had been sunk, these partitions protruded far above the water surface. In this way, construction pits were created. These were drained for further work.
In each construction pit, the base of a pier was poured first. Afterwards, the piers were poured. In all piers, the reinforcement runs into the tubes underneath. This quarantees an extremely robust construction. In the top of all the piers, studs (bolts without a head) were fitted for the rubber supports.
As soon as the pier was ready, a floating sheerlegs crane, the Smit Tak 1, placed a so-called splayed support section on top of each pier. This section is a V-shaped, steel segment weighing 540 tons. After mounting, concrete was poured on the segments. Between pier and segment four rubber supports were fitted. This rubber will be able to absorb the vibrations from the high speed trains and the expansions and contractions of the bridge. Ten of the eleven splayed support sections rest freely on the supports, albeit that they are confined in the centre by a sliding construction to absorb changes in temperature.
Between the splayed support sections, so-called cross members were placed. These are steel boxes whit the concrete deck already in place. These cross members weigh 1200 tons (including concrete deck), and are around 60 metres long, 14 metres wide and 5 metres high. In this manner, a closed roadway was created. All joints between the splayed support sections and the cross members were poured on site. After this, the railway builders could start laying the tracks and placing the overhead wire masts.