Land reclamation will create a new Amsterdam suburb with 18,000 homes.
The Netherlands is one of the most densely populated countries in Europe and space is at no 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 lake that lies just a few kilometres to the east of the city's Central Station. This land will be used by the City Authority to create a new suburb, Ijburg, with 18,000 homes.
The project involves constructing five connected islands in the south west corner of the Ijsselmeer, the largest area of inland water in the Netherlands which was formed by closing off the Zuiderzea in the 1930s. Work also includes new road and rail links.
Typically about 2m of very soft normally consolidated clay sits on the lake bed, but the engineering is complicated because the site is underlain by two buried river channels which since the Middle Ages have become infilled with up to 8m of soft silt and clay.
The general construction approach has been to leave the soft bed sediments in place and build the new islands using sand won from an adjacent part of the lake.
A trial reclamation was carried out in advance to investigate the need for a geotextile, the influence of embankment height on settlement rates and the optimum spacing of drains. The aim was to achieve rapid settlement to enable construction at the earliest opportunity.
Results from the trial area showed that geotextile was only needed where reclamation was over the buried river channels, and at the edges of the reclamation where the difference in loading could cause squeezing in the underlying clay. The trial also established that where geotextile was needed a light weight material was sufficient, while it was concluded that plastic vertical geodrains were needed to speed up the settlement.
Trial work was carried out by HAM, which then picked up the main dredging contract 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 allows the reclamation material to be placed in thin horizontal layers with much 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 determine the potential reach of the dredger.
During dredging, the sand content is measured both at the suction dredger and again at the spraying pontoon. The sand content of the discharging material is used to calculate the necessary velocity of the spraying vessel to ensure even distribution of the sand (ie the velocity of the spraying vessel speeds up if the sand content in the dredged discharge increases). The whole operation is computer controlled, with movement control through the winches.
The sand and water discharge runs off a fan-shaped chute mounted on the front of the spraying pontoon. Once the reclamation is above water level, rainbow dredging completed the work.
The project is being developed in two phases, with the first due for completion by April 2000, 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 lake, a process first involves removal of 7M.m3 of clay. The new islands are expected to settle by 2m, with 80% scheduled to be complete before construction of the new town begins.