[1] At the same time, the corresponding parts of the path of the tunnel are prepared, with a trench on the bottom of the channel being dredged and graded to fine tolerances to support the elements.
Once in position, additional weight is used to sink the element into the final location, this being a critical stage to ensure each piece is aligned correctly.
In Europe, reinforced concrete box tube construction has been the standard; the sections are cast in a basin which is then flooded to allow their removal.
The main advantage of an immersed tube is that they can be considerably more cost effective than alternative options – i.e., a bored tunnel beneath the water being crossed (if indeed this is possible at all due to other factors such as the geology and seismic activity) or a bridge.
The first tunnel constructed with this method was the Shirley Gut Siphon, a six-foot sewer main laid in Boston, Massachusetts in 1893.
[4] The length of both the HZMB and SZB will be surpassed by the Fehmarn Belt Fixed Link connecting Denmark and Germany when it is completed,[12] at an as-designed 17.6 kilometres (10.9 mi) long.