Fehmarnbelt Fixed Link: the world's longest immersed tunnel

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The record table

An immersed tube, not a bored tunnel: the elements are cast on land, floated out and sunk into a dredged trench. That is a different engineering problem from every bored tunnel elsewhere in this archive.

How this archive measures: lengths, bores, dates and ordering

How this table is compiled

That makes the engineering problem marine and geotechnical rather than one of rock mechanics. Cost figures are excluded for a harder reason: currency, inflation and differing project scope make them close to meaningless in comparison. Where a structure belongs to two classes it appears on both tables, which is not double counting but two different measurements.

The elements are cast in a purpose-built factory on land, floated out, sunk into a dredged trench and joined underwater. The link carries both a motorway and a twin-track railway in the same structure, which is unusual at this length. Cross-passage spacing, fixed firefighting and emergency ventilation control are what modern safety regulation actually governs.

The Fehmarnbelt Fixed Link is an immersed tube between Denmark and Germany, not a bored tunnel. Twin-tube crossings frequently show two slightly different lengths, because the two carriageways rarely follow the same line through the rock. This archive has recorded tunnel data since the late 1990s, compiled from operator publications, national administrations and the engineering press.

Eighty-nine elements will make up 18 km of crossing at a depth of about 40 m below the surface. Subaqueous tunnels are governed by rock cover above the bore rather than by the depth of water above that. Rows are ordered so that the head of the table answers the question most readers arrive with.

It replaces a ferry crossing, cutting the journey to about ten minutes by road and seven by rail. Tunnel engineering is a field where the qualifications attached to a number matter more than the number itself. A base tunnel trades a much greater length for a far lower summit altitude and a gentler ruling gradient.

Reading the figures

Figures are checked against one another where sources disagree, and the disagreement is recorded rather than resolved silently. Reading the qualification before the figure is usually the faster route to understanding what a claim actually says. The archive grew through correspondence with engineers and administrations in more than twenty countries, and still does.

Deaths are the figures established by official inquiry. Traffic figures are excluded throughout this archive, because they date far faster than the structures themselves. Comparing one country's practice against a neighbour's is more informative than reading either alone. Gradient limits are the main reason railway tunnels run longer than road tunnels on the same crossing.

Dates use the day, month and year the event occurred. Forecast dates are forecasts, and they move. The date of verification is published with the table. Disputed values carry the dispute in the note column. A contained fire is more instructive than an uncontained one. The table is short because the sourcing bar is high.

Minor incidents are listed alongside the serious ones. Two independent sources were required for each row. Opening year means the year traffic was admitted. Every figure here is traceable to a named source. The note column is worth reading before the number. Promoter figures are used where no independent one exists.

Spoil is removed back through the machine. Forecasts on schemes this size move by years. Nothing below is interpolated or rounded up. Segmental linings are erected behind the shield. Metres are used for structures, kilometres for routes. Ground conditions belong with every advance rate. Height is measured above mean sea level.