After the 1953 and 1976 North Sea floods a flood defence plan was conceived to protect the Flemish part of the Scheldt estuary against future storm surges, called the Sigma Plan.
It is governed by Waterwegen en Zeekanaal NV (W&Z), a Flemish government agency, and since aims 2005 to combine flood defence with strengthening the river’s ecology through the creation of flood control areas.
W&Z set up a pilot project in Dendermonde, involving the construction of a compartment dyke of a flood control area using dredged material from the river Scheldt without intermediate storage between the dredging and the construction process.
As in the case in many rivers, the material to be dredged lacks the necessary geotechnical characteristics for direct construction purposes due to its fine granular nature. Therefore, Jan De Nul Group and its environmental subsidiary Envisan set out an extensive testing campaign to develop an engineered sediment for use in the construction phase.
This involved the testing of over 250 combinations of specific additives on the geotechnical characteristics of the dredged material. Based on the results of the geotechnical dyke model and an economical appraisal, a specific combination was identified for large scale use.
After being mechanically dredged and transported by barges to a location near the flood defence works, the sediment material was pumped ashore to the stabilisation plant. Normally this pumping would involve fluidising the sediment with water, in this case the dredged material was delivered to a barge fitted out with filters and a set of piston pumps (normally used for pumping concrete) that were able to push the relatively chy sediment approximately 600m to the stabilisation plant.
The custom built stabilisation unit constantly monitors the incoming sediment flow and automatically doses the additives to obtain a stable material for dyke construction.
This project has demonstrated that sediments from maintenance dredging normally considered unsuitable for beneficial reuse can be turned into a valuable construction materials for dyke construction in a cost effective manner.
Dredging Today Staff, June 10, 2013