Room for the River
In order to maintain the Dutch flood defense in times of climate change, the national ‘Room for the River’ program has been initiated. More than 30 projects have been defined by the Dutch government in order to contribute to the task to create a safe place to live, work, and recreate. The Nijmegen location in the river Waal was the main bottleneck in the Dutch river system, and therefore a key project in the ‘Room for the River’ program. The ultimate aim for the project was to dig a secondary channel on the river in order to lower the water level and increase the river’s capacity; therefore reducing the flood risk. This was accompanied by the relocation of an existing dike 350 meters further inland and building three new bridges and a new quay. Due to the construction of the secondary channel, the existing area transformed into an island in the center of the river that is set to host a mix of new living, recreational, cultural, and natural spaces.
The winning contract strategy
A single bypass for the highway was the key qualification for contractor combination i-Lent (Dura Vermeer and Ploegam) to win the contract in 2012. The client, the government and authorities of Nijmegen, wanted minimal impact on the city’s traffic during the building phase. Despite it being a relatively large secondary project, the single bypass (approx. 320.000 cubic meters of soil and a steel construction of 120 tonnes) also meant a cost-saving measure by eliminating the need to have multiple solutions for different phases of the work.
In addition to this, presenting a strong design that met the client’s specification for a striking part of the city that made a statement was also key to winning the contract. Whilst other competitors offered lower cost solutions, i-Lent’s balance between delivering a project on budget and on time, meeting the client’s demands for aesthetics of the structures and landscaping seemed to be the better fit.
Multidisciplinary design team
Having a great architectural team and a strong position on BIM engineering not only enabled i-Lent to offer an appealing design but also a clever, workable design in terms of construction. The contractor winning the contract was committed to subcontracting the preselected architect of each of the three bridges crossing the secondary channel.
A total of four architectural and seven engineering firms were subcontracted during the design and constructing phase. Fugro was responsible for the interface between the spatial design of the river park (coordinated by HNS
architects) and the geotechnical and hydraulic design challenges (several engineering firms).
Typically, for geotechnical design issues, the best solutions are invisible once construction is finished. Nevertheless, a well thought subsoil layout is indispensable in order to guarantee the functionality and usability of the constructions and facilities.
Figure 1: Extended Waalbrug
Building the bridges: the challenges
Two new bridges and an extension of an existing bridge were part of the project. The bridges connect the new island to the northern part of Nijmegen (the town of Lent). Furthermore, in the former floodplain, the new secondary channel crosses the existing railroad bridge, which had to be provided with fortified piers.
The Waalbrug bridge (1936), a steel arch bridge across the river Waal, had to be extended in order to cross the secondary channel. The architectural design of Zwart Jansma did not provide a second arch. With respect to the ‘Old lady of Nijmegen’, the lower side of the new bridge and piers is accented.
The new bridge extension is constructed at the exact location of the former ramp. This 15-meter-high soil construction created a certain amount of compaction of the subsoil due to 80 years of loading. In order to build the foundations of the piers, this soil ramp had to be excavated, resulting in expansion of the compressed soil layers.
Recompression, due to reloading during the pouring of the concrete, was a severe risk for the quality of the concrete. Ten-stage compression-unloading-compression tests were carried out. The PLAXIS calculations performed by Fugro proved that the replacements fell within the critical margins of the concrete construction.
Figure 2: 'Lentloper' bridge
The river park: functionality and pleasure in one design
The primary purpose of the urban river park was designed by HNS landscape architects to result in a water level reduction of 34 centimeters during a 1/1250 flood event.
From an extensive soil investigation, some typical geotechnical problems of the area were determined. Due to the high water-permeability of the subsoil, inhabitants of the nearby town of Lent would face severe seepage problems from the secondary channel during high water periods.
As a preventive measure, a 20-meter-long cement- bentonite wall was installed to block seepage flow under the dike and quay. Besides an iconic and recreational function, the 1,600-meter- long new quay includes a partially hidden eight-meter-high L-wall which also performs as a water-retaining structure.
Since every construction and element above and below the surface was designed three-dimensionally and combined in a Building Information Model (BIM), regular clash control revealed problems in an early stage of the multidisciplinary
design process. This method proved to be efficient as well as cost-reducing in the overall planning.
