Fehmarnbelt Tunnel

2011/08/26

The construction of the Fehmarn tunnel

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immersed-tunnel

This blog post will pick up the underwater construction process where the last blog post left off. In the previous blog I explained how the trench was dredged, the element immersed and afterwards joined with the previously installed element; The bulkheads between the elements being removed to complete the operation.

Following these steps with the element being placed and correctly positioned in situ, the element is then locked in place using locking fill material. Finally soil and stones are filled on either side and on the top of the elements protecting the tunnel against falling or dragging anchors and sinking ships (see figure above).

A description of the immersion and connection of elements is given in the previous blog following points 1-5. The described process leaves the ballast tanks in place and also filled with water. The ballast tanks are a temporary installation used in the immersion process, but how will the ballast tanks be removed without surfacing the element? First, we need to look into what makes an object float or sink. The videos below give excellent insight into this matter. The first video gives a brief presentation; the second gives a little more thorough and scientific description.

In short, an object floats if its density is less than water and sinks if its density is higher than water. For objects with air inside (such as ships and tunnels) the effective density is the total weight divided by the volume of the container. Immersed tunnel elements with empty ballast tanks are designed to be less dense than water, thus they float with a well defined freeboard that facilitates transportation by tugboats to the immersion area. Filling the ballast tanks with water increases the density of the element until it sinks.

Once the element is immersed and joined with the previous element, the water ballast tanks are gradually replaced with ballast concrete, this ensures that the element stays in place. Sufficient ballast concrete is poured into the element to make certain that the element will always stay immersed.

After the soil, stones and ballast concrete have been placed, the shell structure is complete and the finishing and installation works can commence. The finalization works will take several years, but once done the tunnel is finished and ready to open in 2020 for both cars and trains.

 

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