Pouring underwater concrete is not a simple affair. The moment that concrete comes into contact with water, it disperses into the separate components of sand, cement and gravel. Underwater concrete has the same composition as concrete used on dry land. To be able to pour concrete underwater, it is vital that the concrete reaches the bottom as a single, solid mass, after which it can spread out on the bottom. In this way, the contact surface with the water is kept to a minimum, and dispersion is avoided.

Construction method
Steel sheetpiling were drilled into the ground to make a construction pit. The average construction pit was 125 metres long and almost 18 metres wide. In total, 23 construction pits were built in this way.

Dry soil was excavated from the construction pit to a depth of a couple of metres, so that the props (steel pipes) could be put in place. These props were installed to keep the sheetpiling in place. From that point, wet soil was excavated from the construction pit, to a depth of around 12 metres. Then water was pumped into the construction pit until the water level was higher than the land around it. This was done to create counterpressure against the upward pressure of the groundwater. The sheetpiling reached 1.5 to 2 metres above ground level.

For the tunnel foundation, vibro-combination piles were used. Piling was done from a bridge over the construction pit, which was still filled with water. In each construction pit, some 340 piles were driven into the ground. Instead of driving the concrete piles into the ground, they were hoisted into a metal sheath that was driven to the required depth.

In a period of 36 hours, a one metre thick layer of underwater concrete was poured . After hardening, this layer secured the construction pit against the groundwater.

The concrete was poured through a steel or plastic chute that was suspended fairly close to the bottom. Before filling the chute with concrete, a bucket full of tennis balls was emptied into the chute. Of course the tennis balls floated on the water, but when the concrete was poured into the chute, it pushed the balls down under its weight. At that moment, the balls formed a layer that separated the concrete and water, thus making sure that at the bottom of the chute, a solid flow of concrete came out and spread out over the bottom. Once they had reached the bottom, the tennis balls were released from the concrete and shot up to the surface like rockets. From that moment on, it was vital to continue pouring concrete without interruption, and that the opening of the chute kept contact with the layer of concrete. Divers and underwater cameras monitored the entire pouring process.

It was also important that the concrete would form a even layer. To make sure of this, the divers and the team on the surface used measuring equipment to see if the desired thickness had been reached or not. Concrete also hardens under water. No oxygen is needed for the cement to react, only water. The concrete also adhered well to steel objects such as reinforcement. Underwater concrete behaves in the same way as concrete that is poured in the open air. But the working conditions are much more complicated.

A one-metre layer of concrete weighs enough, to be sure, but still an underwater concrete slab may start to float upwards because of the enormous upward pressure of the groundwater. Letting the pile ends stick out a little above the bottom solved this problem. This made it possible for the concrete to adhere. The slab was secured in the foundations and could no longer be pushed upward by the groundwater. Now that the floor was anchored, the workers could build the real construction on top of this foundation: a bridge pier or a tunnel.

The underwater concrete had to harden for approximately two weeks.

Now that the construction pit had a watertight floor, it could be pumped dry and hosed clean to start the actual construction work.

The armouring has been installed. One after the other, the tunnel floor, partition wall and roof were poured in modular formwork. The pile ends were also cast in the final floor, to obtain the maximum anchoring strength (holding the floor down). After some days of hardening, the formwork could be removed.

The space between the tunnel wall and the sheetpile wall was filled with earth. During this process, the sheetpiles and props were removed. Then the pit was filled to ground level.  

A part of the (semi-)sunken construction at Bergschenhoek/Berkel en Rodenrijs (the section near the Berkelseweg road, where the tunnel trough is sunken completely) was built with sheetpile walls and underwater concrete.