10 October 2019

Unique soil cells give plane trees a second life. (Photo: Adriaan van Dam)

The Coolsingel is a boulevard in Rotterdam. But the allure that the nineteenth-century promenade once had had disappeared. And Rotterdam wanted it back. One of the two double lanes for cars in Coolsingel has been removed and in its place is a comfortable space for pedestrians and cyclists. The plane trees along the boulevard also play a role in the renewed majestic appearance. But this was by no means an easy feat…


There are 160,000 trees in the municipality of Rotterdam and each of them faces a battle for every square metre available. City trees must also be able to cope with gas pipelines, road construction, brine, clinkers and asphalt. And even if it succeeds, it will nevertheless have an impact on its condition. Since the turn of this century, more attention has been paid to the quantity and quality of Rotterdam’s trees.

In 2008, the Green Year, the municipality launched a plan to make the city centre greener: ‘Binnenstad als City Lounge’ [Inner City as a City Lounge]. The approach started a year later. Remco Versteeg, project manager for Coolsingel on behalf of the engineering office of the City of Rotterdam, explained: “The main objective of the project is to reduce the number of cars in the city centre and to develop the centre as a ‘city lounge’. A number of projects were undertaken, including De Meent, Weena, Binnenwegplein, Binnenrotte, Lijnbaan and Central Station. The Coolsingel is the final piece in the overall operation.”

The specific ‘green’ work, the construction of the TreeParkers, has been outsourced by Van Gelder, the main contractor, to BTL Realisatie BV, in collaboration with BTL Bomendienst. (Photo: Adriaan van Dam)

Plane trees remain spared
It became clear that the plane trees along Coolsingel, some of which were planted in the 1960s and survived the construction of the metro, had to be spared. Rotterdam was aware of what trees can do and for this reason decided to take a radical approach. A competition was launched for the design of the new space, which was won by West 8, a design agency for urban planning and landscape architecture. The work started in April 2018.
West 8’s plan is to transform Coolsingel from a vehicle-dominated boulevard into a comfortable area for pedestrians and cyclists. Fewer cars, but without disrupting the flow of traffic in the rest of the city. The pedestrian zone was created on the west side of the tram track. The existing tram and underground infrastructure will remain in place, thereby accentuating the quality and image of the public space, with many details such as quality paving, decorative furniture, decorated metro stations, lighting and lots of added greenery, including in the form of ‘pocket parcs’.

Iconic trees
By taking advantage of the existing green space and combining it with a new high-quality public design, Coolsingel aims to provide visitors with a warm welcome, at any time of the day and in any season. Henk Wallaard and Jan van den Enk, colleagues of Versteeg, the project manager, showed their enthusiasm: “The trees are becoming iconic again. It will take some time, because you can’t cut more than a fifth or sixth from a tree. You can see that city life can be hard for a tree, especially along a busy street like Coolsingel.”
In the new situation, the plane trees are all protected by the TreeParker system, the first of which was built in 2018. The company TreeBuilders is the main supplier of these soil cells. Remco Versteeg: “Our first contacts with TreeBuilders dates back to 2015, when we were working on Binnenrotte with a more or less similar (soil cell) system. The system has since been refined and it is now fully modular and adaptable to existing conditions.” 

A unique opportunity
The latter was decisive for choosing TreeParker. “The Coolsingel project is quite unique, because it’s all about maintaining existing trees,” said Werner Hendriks, director at TreeBuilders. “The City of Rotterdam also wanted to improve their situation at the same time. This can be done with soil improvement or with nutritional elements. But because a lot of it had to be ‘open’, it gave the unique opportunity to add a considerable amount of soil volume. The choice was therefore more or less determined by the constraints: work is currently underway, so now something must also be done about the living conditions of the trees. The TreeParker modular structural soil cell system allows for everything to be integrated into the work.”
Coolsingel boasts a total of 250 trees, including 87 new trees that have been or will be planted. The new trees are completely in the new system, while the existing trees have been given ‘air’ on the street side, where possible. The existing and compact subsoil was excavated and the TreeParker system was put in its place, and in this case modules of sixty centimetres in height were linked up to a width of three metres over the entire length of where trees are standing. The system has since been filled with tree soil and is now covered. The new paving is or will be laid on top of that.
Joeri Faas, technical advisor at TreeBuilders: “An investigation showed that the trees had rooted nicely downwards. This allowed us to get close to each tree when excavating the existing soil. This gives the trees the opportunity to create plenty of new roots towards the middle of Coolsingel.”

The modular system can always be made to measure for any situation. (Photo: Adriaan van Dam)

Specialised green company
The fact that this is now happening was clearly demonstrated by the fact that a small part of the system was damaged during work earlier this year. Werner Hendriks: “The construction is only at the desired strength once the paving has been laid over the TreeParker. It is a system that needs attention until all phases of the work have been completed.” Remco Versteeg added: “The workspace is limited because traffic continues to pass behind you. It is difficult to turn and unfortunately we also have to drive over ‘our own work’. The span of control is great, and you end up doing a lot of things at once. This means that the contractor has to be present in many markets.”
That is why the specific ‘green’ work, the construction of the TreeParkers, has been outsourced by general contractor Van Gelder to BTL Realisatie BV, in collaboration with BTL Bomendienst. “Setting up a location for a tree is different from that for a rhododendron; we go further than the average landscaping company,” said Korné den Boer, local and European Tree Worker at BTL Bomendienst. Project Manager Corné den Otter (BTL Realisation) added: “These are tasks that you’d rather have done by a specialist green company, rather than by a road builder, who simply ‘adds it in’. The fact that these are existing trees makes it even more important that the construction of the TreeParkers is done by experts. It is a grandiose system, which, unlike concrete bunkers, can always be made to measure.”

Within a year, the roots find their own way… (Photo: TreeBuilders)

Roots finding their own way
When placing light poles near the soil cells, the opportunity was taken to open the tree bunkers that were placed in 2018. There was great joy when it became clear that the roots were doing well, not even a year later. “We know what this system can do for trees. But we may see the results sooner than we had hoped,” said Remco Versteeg. Half of the project has now been completed. “In October we will turn around to start work on Coolsingel.” And then by mid-2021, a part of Rotterdam’s grandeur will have been restored to its former glory.

How does the TreeParker system work?
The TreeParker system is the latest generation of structural soil cell systems for trees in ‘hard landscaping’. The TreeParker system provides large amounts of uncompressed high-quality soil and it maximises usable underground space. There is room for unhindered root growth, while supporting traffic loads at the same time. It is ideal for new and existing trees in areas such as sidewalks, parking garages, squares, green roofs and even green walls.

Author: Tom Verstegen