Within the framework of the Dutch "Room for the River" programme, modifications are being made to various waterways at more than 30 locations in the Netherlands.
One of those waterways is the River Waal. The water of the River Waal has to flow through a relatively small winter bed at the bend between Nijmegen and Lent. Here, the winter bed is only 350 m wide, while the average width elsewhere is 1 km. Due to this bottleneck, the water impounds, which is not beneficial to the discharge of the water through the Waal. To facilitate a higher river discharge, the river has to be given more room at this location.
On the instructions of Rijkswaterstaat, a by-pass channel is being constructed by Bouwteam iLent. This project is part of the "Room for the River" programme and is called "Room for the River Waal".
Between the River Waal and the excavated by-pass channel, an urban island is being constructed for recreational purposes.
Within the framwork of this project, the side channel has to be conmstructied under the existing rail bridge over the River Waal. The rail bridge is an old construction from 1879 with nine masonry pillars beneath the first spans in the northern floodplains. Due to construction of the side channel, the soil around the three pillars (foundation level +5.0 m NAP) will be dredged to +2.0m NAP and thus undermining the bearing capacity of the pillars. The bottom of the side channel will be 3 m below the current foundation level of the pillars. For this reason, Rijkswaterstaat has instructed ProRail to make the pillars of the rail bridge's first spans suitable for constructing a side channel.
In partnership with two sister companies, Elja Beton- en Waterbouw and Volker InfraDesign, VSF was contracted by ProRail's to carry out the modification of the pillars.
The work comprised of reinforcing the existed pillars by constructing a diaphragm wall around the pillars to prevent the soil beneath existing foundation from washing away.
The diaphragm wall, with a thickness of 1.5 m, reaches to 12 m -NAP, some 14 m deeper than the current foundation level. The 24-m-long panels have been calculated for a retaining height of 10 m. A total of 9,500 m³ of concrete was poured to make the wall. Once the diaphragm wall was installed, pon top of it, a further concrete construction was added around the arched pillars, which keeps the diaphragm wall together, therefore contributing to a stable containment of the pillars.
During the construction of the wall the rail bridge was in operation.
In order to promptly detect any shifting as a result of the excavation, an extensive monitoring system was installed. Calculation of the trench stability also took into account an increased safety factor relating to the highly critical setting requirements of the heavily-burdened rail bridge pillars.
Volker InfraDesign delivered the detailed engineering of both the diaphragm wall and the concrete pillar caps.
The excavation itself presented its challenges, due to the limited working height of less than 6 metres beneath the bridge. As the minimum standard craneboom was too long, the Sennebogen 6100 crane had to be modified. A special low-headroom grab was also necessary. Slurry wall grabs are generally long to ensure good guidance vertically of the trench. For this job, a shortened version had to be used. Incidentally, this had no consequences for the wall excavation achieved tolerances. The end stops and reinforcement also had to be fitted in sections beneath the bridge.
Despite these challenges, the walls were constructed without any defects or problems, after which the sister company, Elja, was able to begin the concrete work for the concrete cap constructions.
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