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Depot deposit Preparatory work is nearing completion on a huge repository to store contaminated silt dredged from the bottom of Lake Ketelmeer in the Netherlands.

DUTCH GEOTECHNICS

Rivers do not acknowledge international boundaries and so it falls to the Dutch to deal with river-borne pollutants deposited by some of the great industrial rivers of Europe as they slowly make their way towards the North Sea.

The Rhur, for example, carries contaminants from Germany's industrial northern heartland into the Ijssel tributary which leads into Lake Ketelmeer. Over many decades the pollutants have settled as a 0.5m thick layer of heavy metal contaminated silty sludge.

Today Lake Ketelmeer, some 50km to the north east of Amsterdam, is considered the most polluted body of water in the Netherlands. Contamination in Ketelmeer is also a risk to the adjacent Ijsselmeer which is an important store for drinking water in the Netherlands.

Three years ago, after many years analysing the Ketelmeer problem and assessing the available options, the decision was taken to construct a giant sludge disposal basin in the middle of the lake. The whole lake would then be cleaned by removing the contaminated sediment and placing it in the depot.

In concept the project is similar to the Slufter depot, constructed just off the Dutch coast in the 1980s, which takes sludge from Europort and the Harbour of Rotterdam.

Construction work on the huge circular Ketelmeer depot is now complete and later this summer contractors will be bidding to carry out sludge transfer to the basin.

The basin measures 1km across and is nearly 50m in depth. It was formed by dredging out the lake bed deposits and placing them in sheet piled basins which form an island around the repository. A 10m dyke around the basin's circumference increases the volume.

A key issue during construction was minimising disturbance of the heavily contaminated silt and sludge. The process of moving the sludge is environmentally a high risk activity, with an increased risk of mobilising the contaminants during the work.

To establish which was the most accurate and least disruptive dredging technique, client Rijkswaterstaat invited four different dredging companies to participate in a trial.

The methods proposed by the contract winning joint venture of HAM, Boskalis and Ballast Nedam satisfied the client's criteria. The suction disc dredger and in particular the 'wormwheel' auger suction dredger proved the cleanest for picking up the seabed sludge. The latter, developed by HAM and listed as fleet vessel HAM 291, is a cutter suction dredger fitted with an auger which is able to remove contaminated silt from the bottom with

great precision. When placed on the bed the auger loosens the polluted layer with centimetre accuracy, transports it to the suction mouth and then removes it by suction. The loosening and transport operation of the polluted material is covered by a cap thereby avoiding losing material which could be in suspension.

Used in conjunction with HAM's spraying pontoon developed for the Ijburg reclamation project (see page 15) also enabled the dredged material to be placed in thin and controllable layers on the new land being formed around the basin.

Working in small lifts meant the contractors minimised bed instability and squeezing of the sludge and so avoided the need for a basal geotextile.

The completed depot has a volume of 23M.m3. Its construction involved moving 2M.m3 of sludge within the depot area, while below the contaminated sludge the contractors dredged 15M.m3 of clean sand. Of this, half was used in the construction, 20% was sold with the remainder reserved for future use by the client.

When work starts on removing the contaminated silt from the lake bed later this year, the same care will be needed. Accuracy will also be important because overdredging by just 10mm over the entire lake area would cost an extra £1.5M.

Clean-up is expected to take eight years and cost more than G250M (£85M). Materials from Ketelmeer will fill two thirds of the total volume, leaving space for safe disposal of other contaminated sludge. There is even talk in Holland of constructing similar sludge disposal facilities, as the country continues to push the world standards for environmental clean- up.

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