Visser & Smit Hanab recently installed two sinkers of glassfibre reinforced plastic (GRP), each with a nominal diameter of 1,500 mm (DN1500), in the floodplains of the River IJssel near Deventer. The finished project was an opportunity for not only the in-house engineering expertise of Visser & Smit Hanab to shine, but also the partnership among all of the VolkerWessels companies involved.
The VolkerWessels company Van Hattum & Blankevoort | ELJA faced the considerable undertaking of adapting two existing concrete effluent channels, also DN1500, so that the floodplains could be dredged. These dredging operations are needed as part of a Rijkswaterstaat flood defence project known as 'Room for the river'. Van Hattum & Blankevoort brought in Visser & Smit Hanab to help on this specialised job.
A variety of solutions were proposed. Ultimately, the decision was made to build sinkers using glassfibre reinforced pipes.
Working closely with the manufacturer, the engineers came up with a design for the sinker, which was then approved by the end client: the Groot Salland Water Board.
First, however, there was the water to deal with. The project work was planned for summertime, when there was the least chance of high water levels. Work on the project still had to be suspended for four weeks, however: the floodplains did indeed become inundated. And that was in June!
The sinkers were eventually assembled in the floodplains. This involved welding the pipe sections and bends together with laminated joints.
Because of groundwater pumping activities, the sinkerswere placed in a sheet-pile cofferdam filled with water. First, the cofferdam was carefully excavated, and then the existing concrete pipe was removed.
The two sinkers are each approximately 175 metres long, with a depth at the pipe's main horizontal section of 10 metres. Four 400-tonne telescopic cranes were brought in to carefully install the inverted siphons. Ballast was then added to the pipelines to prevent them floating. The hoisting and sinking of the pipes themselves involved close precision work under the supervision of the engineers assigned to the project.
After the construction pit had been partially filled with sand and gravel, the countersunk holes were pumped dry. Then the siphons were hydraulically tested and finally connected to the existing concrete pipe.
With that, Visser & Smit Hanab's job was done and Van Hattum & Blankevoort | ELJA could continue completing the rest of the work.