The Noord/Zuidlijn in Amsterdam, will have a metro station underneath the existing central train station of Amsterdam. The train platforms of the metro station will be constructed inside an immersed tunnel element (CS-element).
Dimensions of CS-element:
Length – 136 meters
Height – 9 meters
Width – 21 meters
To make this highly special operation possible combination Strukton van Oord (CSO) is currently building support structures underneath the main hall and the central passengers way of the station. These supports will be used as a foundation of the central train station and will function as earth retaining walls. The soil between these walls will be dredged to a level of 19 meters below NAP, the excavation will be enclosed with a concrete floor. By doing this a cannel is created for the immersion of the CS-element.
The CS-element and the IJ-elements (IJ-tunnel project) are already built on the other side of the riverbank, in a cut-and-cover construction site. After construction the elements were transported to a temporary mooring location and parked for more than three years. During this years the elements are frequently inspected on leakage and alike.
When the preparations under the central station are finished, the tunnel element will be transported from its mooring location to the central station, on the river IJ. The water level on this river is higher than the maximal possible water level in cannel underneath the central station. Therefore a sheet pile lock will be build in the river IJ, to make a water drop of approximate 3 meters. After the water drop the element can be transported underneath the central train station.
For Mergor the phases underneath the central station is the trickiest part of the whole operation. During this phases the CS-element will encounter several obstacles. Such as the narrow passage between the walls, less than 30cm on both sides. The working height is less than 3 meters (from water level to floor). Several struts are placed at a level of -4.75m NAP, so this struts are under the water level and therefore ‘not visible’. The CS-element will be immersed to fit underneath the struts and will continue its transport to its final destination.
During the sand flow, underneath the CS-element the tunnel will be hang from mechanical struts/suspensions from the station floor (instead of supports underneath the element). These suspensions will secure the fixed location of the CS-element.
After the sand flow Mergor has accomplished her job and the area around the CS-element will be refilled with dirt and ground.
After finishing the connecting constructions of the metro station a connection will made between the these constructions and the primary and secondary end of the CS-element. Temporary connections will be made by using a freezing technique to secure watertight passages, to erect the permanent connections in concrete.