Femern
Search

Water pressure will press the elements thightly together

When the tunnel element has been placed in the tunnel trench and lined up with the previous one, a hydraulic arm is typically used to haul the new element right up to the previous one. The arm is equipped with a hook that simply grabs hold of a large bracket on the new element and drags it to the previous one.

An additional piece of equipment is needed, however, when the tunnel elements have to be connected very tightly and securely. This means that a tunnel element from the factory is fitted with a rectangular, specially designed, huge rubber gasket at one end. The gasket encircles the entire end of the element. 
 

The hydraulic arm pulls in the newly immersed element until the tip of the rubber gasket touches the end of the previous element. This produces a watertight chamber between the steel bulkheads at the ends of the two elements. Due to the flexibility of the rubber gasket, it is now possible to use the high water pressure at the free end of the immersed element to press it tightly against the previous one. 

Tunnel element from the Oresund construction

A finished tunnel element fitted with a black and rectangular rubber gasket around the edge of the end of the element. The red plates are watertight steel bulkheads. 


IT IS POSSIBLE TO IMAGINE THE WEIGHT OF 15,000 TONNES OF WATER PRESSURE PUSHING ON THE FREE END OF THE NEW ELEMENT. THE FLEXIBLE RUBBER GASKET MAKES IT POSSIBLE TO PUSH THE ELEMENTS TIGHTLY TOGETHER.

Did you know that...

...all themechanical and electrical equipment used for maintenance and requiring space will be placed in the special elements on an extra level under the roadway. In this way, the standard elements can be made technically simpler and more suitable for batch production. 

…a new tunnel element factory will be established in Rødby for production of the tunnel elements. 


Pressure makes the difference

The water pressure works as follows: initially, there is seawater in the space between the two elements that are to be connected. After the hydraulic arm has done its job, work begins on pumping water out of the space between the elements. When the pumps remove the water from the space between the bulkheads, the pressure in the space itself is reduced to normal air pressure. In contrast, the far end of the new element is subject to a very high water pressure, which corresponds to the current depth in the Fehmarnbelt. 30 metres of water pressure, for example. 

The great pressure differential creates a tremendous force on the free end of the new element, which presses it against the existing one to secure the connection. It is possible to imagine the weight of 15,000 tonnes of water pressure pushing on the free end of the new element. The flexible rubber gasket makes it possible to push the elements tightly together. 

Cross section of two tunnel elements
Cross-section of two tunnel elements that are tightly connected by emptying the water from the space between the elements. The vertical red lines mark the elements’ watertight bulkheads. 

After the watertight sealing with the rubber gasket is complete, reinforced concrete is used to complete the joint. Finally, the watertight steel bulkheads between the elements are removed and the final installations are made.