CONCRETE | TECHNICAL
Above: Concept of lowering concrete elements to sea bed to construct the immersed tube tunnel PHOTO CREDIT: FEMERN A/S
Above left: Visualisation of formwork in casting hall PHOTO CREDIT: FEMERN A/S Above right: Concept of complete Fehmarn harbour with drydocks and casting factory
Planning for doing so is married to the reality that the
coast-to-coast, very long hollow concrete tube will be constructed in long sections – called ‘elements’. In total, the immersed tube will be formed from 79 elements that are ‘standard’ in cross section (more than 42m wide, and 8.9m high), plus some slightly larger ones to house the key M&E systems, placed at intervals along the tunnel. Those hollow elements are the same in cross-section,
all the way along – consisting of five cells: two are for two-lane roads; two are for single-lane rail lines; and, one is a service and escape tube. An element will not be cast in one go; it is way too long. At 217m in length, that is far, far too long to do so.
A whole standard element equates to approximately 73,500 tonnes of reinforced concrete. They will, therefore, be built in parts to be joined
together. The element will be cast in shorter slices, or what
are termed ‘segments’. While but pieces of the whole element, the segments are only small in the scale of the overall larger tunnel for, in themselves, each is significant. Each segment, or slice, is 24m long with the same cross-section throughout as the full element, of which it is a part. That calls for more than 8,000 tonnes of reinforced concrete to build such a slice, and there are nine such segments. They will then be bonded together to form a full element.
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