Netherlands: The world's largest and most technologically advanced environmental dredging project is underway: cleaning the floor of the Ketelmeer, a lake at the mouth of the IJssel river, where industrial pollutants from factories along the Rhine and IJssel have settled to form a thick layer of contaminated sludge.
The dredgings are being stored in a man-made island depot in the middle of the Ketelmeer. One kilometer across and fifty meters deep, the depot is designed to hold 23 million cubic meters of sludge from the Ketelmeer and other areas. In the photo above, the depot is the circular structure in the background. The circular structure in the foreground is a temporary depot for storing the sludge excavated for the permanent depot.
The island has processing facilities to separate pollutants from the dredged sand and peat. The cleansed sand will be used for construction at other locations; the peat will be used to construct a nearby marshland area; and the remaining sludge will be pumped into the depot. To prevent leakage, the floor of the depot is sealed with clay, the dike is lined with foil, and the water level is held below that of the lake. When the depot is full, it will be sealed with layers of clay and sand, and the island and marshland will be used for recreation and nature reserves.
Eighty VW Push-In Piezometers , supplied by Boart Longyear BV, were installed to monitor the circular dike as it was constructed. The piezometers were connected to five solar-powered data acquisition systems. Readings were retrieved weekly by GSM modems and transmitted to the project office for interpretation. Now completed, the dike rises some ten meters above the level of the lake.
Additional monitoring will be required during the next fifteen years to monitor the consolidation of the sludge within the depot. For this, engineers had to design a monitoring system that could accomodate settlements of ten to fifteen meters.
The system consists of "floating" instrument racks that slide freely along a large diameter steel pile. The racks provide a means of orienting Interfels VW total pressure cells while keeping them a suitable distance from the pile so that readings are not affected. Instrumentation also includes VW piezometers.
Currently, there are four racks in use. Two are located at fixed depths near the bottom of the depot to provide baseline data, and two others are floating. Engineers can determine the depth of the racks by the length of cable that is pulled down as settlement occurs. Another four racks may be launched as the project continues.
Thanks very much to Jos Nutbroek of Boart Longyear for providing this story.
Photo credits: Ketelmeer depot - courtesy of the Dutch Ministry of Transport, Public Works and Water Management. See the Ministry's very interesting web site documenting the project. Data acquisition system - courtesy of Geodelft . Instrument rack - courtesy of Fugro Ingenieursbureau .