A green roof on the A9 Gaasperdammertunnel: what approach do we take?

“It often happens that a park has to be planted within one year. The problem is that the groundwork has to be done outside the planting season, when a lot of rain falls. This affects the soil, which means that the trees and plants in these parks usually do not live that long, ”says tree consultant Gerrit-Jan van Prooijen. To prevent this scenario, Gerrit-Jan has been involved in this extensive project by the City of Amsterdam. He is one of the fifteen specialists who must ensure that the park becomes a great success and that IXAS - a construction consortium of Ballast Nedam, Heijmans, Fluor and 3i - can deliver it with confidence in 2021/2022. “Soil is a living product,” he continues. “There is something growing on every land in the Netherlands and it must be very extreme if we cannot build a good park with this land. But we have to do that in a smart way. ”

 

An important summer

The mission is clear: to ensure that the park in the Gaasperdammertunnel remains healthy. That is why IXAS carried out as much groundwork as possible in the summer, using the available soil from the depots. That timing was essential, says Tom Peppink, project manager Soil, Road and Water at Ballast Nedam. “Soil compositions are quickly suitable, because there are always trees or plants that match the soil. But if you lay the ground during a downpour, it will become one big mess. This amount of rain influences the oxygen content, compactness and soil life. It really doesn't matter whether you work with the most fantastic soil. ”

In addition to oxygen, water and food, smart planning was therefore also required. In consultation with the client, Rijkswaterstaat, IXAS looked for the most suitable time to plant the trees. Because the soil from the depots needs time to ripen and come to life, it was decided to include an extra year in the planning for planting the trees. That decision gives IXAS the leeway to carry out groundwork in the periods with the greatest chance of good weather.

Check, check, check

How exactly does IXAS proceed to realize the park? When the consortium joined forces with the municipality of Amsterdam and Rijkswaterstaat, a plan was drawn up based on the available surveys of the soil in the depots. In addition, because surveys are only random checks, monthly inspection rounds were planned. Gerrit-Jan, Tom and the other specialists visit the depots monthly and compare the excavated soil with the initial plan. Gerrit-Jan indicates whether the soil is suitable for the desired planting, or whether the soil needs to be tilled.

There are also checks on the places where soil has been applied. The team uses measuring equipment to check whether the ground is not too compact. In the worst scenario, compact soil could hinder the drainage of rainwater, causing puddles to remain for a long time and for example grass to die. It can also affect trees and soil life. That is why Gerrit-Jan moves around the fields with the group, while he checks randomly whether things are going in the right direction.

Finally, the group tests about two months before planting whether the oxygen content in the soil is up to standard. Has the soil life not yet penetrated deep enough and is there too little oxygen? Then the specialists know that the planting has little chance of success.

Thanks to these preparations, IXAS can demonstrate the quality. Gerrit-Jan: “We are now building in enormous certainty because we can see exactly whether the soil is doing what is intended. As a result, we know that the planting cannot really fail after these preparations.”

 

Without park visitors seeing it

To guarantee quality, the soil on the park is applied in three layers. The bottom layer consists of moderately coarse sand with a drainage system. This layer ensures that excess water is drained and roots do not go under. On top of that is a layer of top soil with moisture and nutrients that make plants and trees grow. The surface layer is a top layer that is less nutrient-rich, but is very important for strong meadows and lawns.

An important process is now taking place in these layers. Deep-rooting herbs rapidly dig themselves to great depths and bring oxygen and life far underground. And that benefits the ripening process of the soil, says Gerrit-Jan: “If the soil has no structure, it collapses. The roots of the herbs invite soil life and stimulate this further and further.”

From plain to park

When the soil is up to standard, the drainage systems can drain the water and the herbs have dug their way through the soil, Ballast Nedam Road Specialties can start planting the park. She entered into an agreement with an arborist early in the process. This allowed young trees and plants of different types and sizes to grow up in the nursery. When planting, Ballast Nedam Road Specialties will rely on its own scenario with logistics routes, so that the soil does not settle unnecessarily and the tunnel deck is not overloaded. The complexity of the park does not end when the ground is ready.

From Ballast Nedam, Tom Peppink is looking at a great collaboration with Rijkswaterstaat and the municipality of Amsterdam: “We work on the basis of trust. The municipality and Rijkswaterstaat have even formally established that confidence by giving Gerrit-Jan and myself the freedom to jointly determine the best working methods. We do not take any unnecessary risks in this, because we also benefit ourselves from making this park a success."

Gerrit-Jan recognizes this: “Ultimately, we all want a good result, so we are dealing with win-win situations here. If we do this well, there will be music for everyone.”

Improving the accessibility of the northern Randstad and increasing the quality of life in Amsterdam Southeast: these are the goals of the A9 Gaasperdammerweg extension. The project is part of the Schiphol-Amsterdam-Almere road expansion. Commissioned by Rijkswaterstaat, IXAS designs, builds, finances and maintains the tunnel.