Along the waterfront in Harderwijk, an old town in the center of the Netherlands, a new residential area is being built.
The site was formerly used as industrial area, which has resulted in a polluted top layer of the soil. To avoid handling this polluted material, Boskalis used an innovative and challenging method: a DOP250 was used to mine clean sand from deeper down, and cover the polluted layer – which remained in place.
The DOP is lowered in the hole using its own weight, to a max depth of ca -8.5m. Once at this depth plenty of jet nozzles spaying sideways create a pumpable mixture. The dredged mixture is pumped to a spoil field approx. 600 m away.
The mined sand is entirely clean, good quality, coarse building sand – excellent for sealing of the polluted soil. When the projects is ready, the entire area will be covered with a 2m layer of clean top soil.
Great care is taken to control the complex dredging process. The jet water used for losing the soil and transporting the sand, is recirculated. During the recirculating process the quality of the water is monitored.
For the full duration of the process the water has been tested as clean and unpolluted. The 3m thick polluted layer of soil has proven to remain in situ. Moreover, the sand won is continually checked and found clean and unpolluted.