The Netherlands is one of the most densely populated countries in Europe but nowhere is space at more of a premium than in the country's largest city, Amsterdam.
Work is currently under way on an ambitious reclamation project which will see 250ha of land formed from the Ijsselmeer, the giant inland sea that lies to the east of the city. Five conncected islands in the south west corner of the sea will house a new suburb, Ijburg, with 18,000 homes. Work also includes new road and rail links.
Typically about 2m of very soft normally consolidated clay sits on the seabed, but the engineering is complicated by two buried river channels which, since the Middle Ages, have become infilled with up to 8m of soft silt and clay.
In the past the soft bed sediments has been left in place and new islands built using sand won from an adjacent part of the Ijsselmeer.
A trial reclamation was carried out to investigate whether a geotextile would be required, the influence of embankment height on settlement rates and the optimum spacing of drains. The aim was to achieve rapid settlement so that construction could start at the earliest opportunity.
Results showed that geotextile was needed only where the buried channels underlaid the reclamation and at the edges of the reclamation where the difference in loading could cause squeezing in the underlying clay. The trial also established that a lightweight geotextile would be sufficient, with plastic vertical geodrains inserted to speed settlement.
The trial work was carried out by HAM, which was then awarded the main dredging contract working in joint venture with Boskalis, Ballast Nedam and Fernhout. An important factor in the contract was use of HAM's sand spraying suction dredger which was used to bring reclamation level up to the surface. This allowed the material to be placed in relatively thin horizontal layers with greater precision than has been previously feasible. This prevents uneven stresses developing below the reclamation.
The below water reclamation was formed in three 500mm to 700mm layers. To achieve the necessary accuracy the dredger's movement was controlled by six constant tension winches each attached to an anchor point, the position of which determined the reach of the dredger.
During dredging, the sand content of the dredged material is measured on the suction dredger and again on the spraying pontoon. The volume of sand in the discharging material is the factor used to calculate the velocity of spraying required to ensure even distribution of the sand. The velocity of spraying increases in proportion the sand content in the dredged discharge. The operation is computer controlled, with movement controlled by the winches.
The sand and water mix runs off a fan-shaped chute mounted on the front of the spraying pontoon. Once the reclamation is above water level, rainbow dredging completes the work.
The project is being developed in two phases, with the first due for completion by April, and the second five years later.
In total the first phase of the project involves winning 6M.m3 of sand from an adjacent part of the Ijsselmeer, after first removing 7M.m3 of clay. The new islands are expected to settle by 2m, with 80% scheduled to be completed before construction of the new town begins.