Richard de Jong, senior project coordinator at the ALSÉÉN construction consortium, explains exactly what is being done at the moment. "Two new bridges are being built, one for each direction of traffic. We are building the first bridge next to the existing one, so that traffic can continue to flow as much as possible during the construction. This means that the work will take place in the immediate vicinity of the current bridge. This requires extra care and attention to safety."

It is essential to understand the effect of the foundation work. "For the foundation of the new Merwedebrug, we are using sheet piling and tubular piles, among other things," explains Richard. "With a tubular pile, for example, we do this by pile driving. As you can imagine, pile driving causes vibrations in the immediate vicinity. Usually, this is not a problem. But because the existing Merwedebrug is so close to the work site, it is important to know exactly what the effect of the foundation work will be. A small vibration in the foundations of the existing bridge could result in the bridge gate no longer closing properly, for example. We obviously want to avoid this at all costs, especially as traffic still uses the bridge and it is also used by shipping. That is why we are now conducting an impact test."

“With this influence test, we are examining how the subsoil reacts to the installation of sheet piling and tubular piles,” Richard continues. "We measure both horizontal and vertical movements in the subsoil. We also monitor vibrations that occur during the work. Based on these results, we can estimate quite accurately what effect the vibrations or shifts may have on the existing Merwedebrug."

But that is not the only thing being tested with the influence test. "Whereas we always have to drive tubular piles, we can install sheet piles in different ways," explains Richard. "For example, by vibrating them into the ground or pushing them into the ground. With this test, we are testing four different methods, so that at the end of the test we will also know which method produces vibrations and movements that are acceptable."

The location of the influence test and the measuring field has been carefully selected. "The location - the floodplains next to the Merwedebrug - is still far enough away from the existing bridge so as not to put any strain on it, but it does have the same type of subsoil," says Richard. "To measure various factors, we are using a total of more than 50 measuring devices, which are located up to 25 metres from the test foundations. This allows us to see exactly how large the 'radius' of the vibrations and shifts is."

"We will be finished by early November at the latest," Richard concludes. "Then we will remove the sheet piling and tubular piles that we installed for the test. The removal will be done by means of vibration. We will also measure the effects of this. After that, we will examine what the results of the measurements mean for our final plan of action. This will bring us one step closer to the construction of the first new Merwedebrug!"

Source: Rijkswaterstaat