Bømlafjord vs Qing-Huang: length, opening date and class compared

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Side by side

Bømlafjord is recorded on data/subsea.html and Qing-Huang on data/subsea.html, compiled to the same convention, which is what makes the two figures comparable at all.

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

How this table is compiled

Bømlafjord and Qing-Huang are compared here on the figures this archive records for each, taken from the same sources and the same measurement convention. Bømlafjord belongs to the 2000s and Qing-Huang to the 2010s, two decades in which what a tunnel had to provide by way of escape and ventilation was not the same thing.

Bømlafjord is the longer of the two at 7.93 km, ahead of Qing-Huang at 7.8 km, a difference of 0.13 km. Driven end to end the two would run 15.73 km, which is the figure to hold against the 10.8 km of the longest single subaqueous tunnel in the record.

That gap is about 2 per cent of the longer bore, which is the figure worth holding on to rather than the raw metres. Both rows are reachable from the subaqueous tunnel table they were taken from, where the note column carries the tube count, the gradient and any reconstruction Bømlafjord or Qing-Huang has had.

Across every class this archive records, Bømlafjord stands 164th by length and Qing-Huang 165th, so the pair sits inside the top 165 of 244 structures. 58 other structures in this record opened within two years of Qing-Huang, which is the cohort it was designed alongside and the fairer comparison for how it was built.

The 2000s account for 53 entries in this archive, and Bømlafjord is one of them, which is the context a single opening date does not carry on its own. Where a crossing has two bores of different lengths, the longer governs the comparison and both are recorded on the source table.

Reading the figures

Both are recorded in this archive as subaqueous tunnels, so the measurement convention is identical and the comparison is direct. 163 recorded structures are longer than Bømlafjord, the nearest of them Changxing at 8.95 km. Geology decides cost and programme far more than distance does, so a shorter bore through difficult ground can be the greater achievement.

Published lengths for the same structure differ by a few metres between sources, depending on whether cut-and-cover approaches are counted. The most useful sources are the least accessible: inspection records and commissioning documents rather than announcements. Overburden at the deepest point sets the lining, and on a long crossing it varies more than the length suggests.

Qing-Huang was designed under the modern regime, with cross-passage spacing and emergency ventilation specified from the outset. A tunnel's gradient is often the more interesting figure, particularly on subsea crossings where vehicles have to climb back out. A boring machine has to be assembled underground and is economic only past a length that varies with the rock.

Among the subaqueous tunnels this archive records, Bømlafjord stands 4th by length and Qing-Huang 5th, adjacent in the ranking. Widening an existing bore is rarely economic, so capacity is added by driving a second one alongside. Lighting is brightest at a portal in daylight, because the adapting eye is the hazard rather than the dark.

Figures are checked against one another where they differ. Class conventions differ, so like is compared with like. Pressure waves in a long rail tunnel are severe enough to require relief shafts or sealed carriages. Traffic volumes are excluded for the same reason, and because they date far faster than the structures themselves.