Bridges and viaducts › Digswell Viaduct

Digswell Viaduct

The East Coast Main Line's viaduct across the Mimram valley near Welwyn, opened in 1850: 40 brick arches of 30 ft (9.1 m), about 1,560 ft (475 m) long and about 100 ft (30 m) high.

Opened
7 August 1850 (built 1848-50)
Line
East Coast Main Line
Kind
Brick arches
Length
About 1,560 ft (475 m)
Arches
40, each 30 ft (9.1 m)
Height
About 100 ft (30 m) from the stream to the rails
Train drawn
Thameslink Class 700

unverified Some of these figures haven’t yet been checked against an official source or a published history. What’s in doubt ↓

Digswell Viaduct, schematic side elevation at 1 px a metre, with a Thameslink Class 700 crossing
A Thameslink Class 700 crossing. Schematic side elevation at 1 px a metre, a tenth of the scale of the train drawings: proportions from the published figures, details simplified.

Four times larger

Digswell Viaduct, schematic side elevation at 4 px a metre, with a Thameslink Class 700 crossing
A Thameslink Class 700 crossing. Schematic side elevation at 4 px a metre: proportions from the published figures, details simplified. Scroll sideways to see it all.

Close up

At 10 px a metre, the scale of the train drawings, so the train is the size it is on its own page. Each is cut from the same drawing.

Digswell Viaduct, close-up: The south (London) abutment and its embankment.
The south (London) abutment and its embankment.
Digswell Viaduct, close-up: Pier 13, the deepest: 100 ft from the valley floor to the string course.
Pier 13, the deepest: 100 ft from the valley floor to the string course.
Digswell Viaduct, close-up: A Thameslink Class 700 behind the parapet and the string course.
A Thameslink Class 700 behind the parapet and the string course.

Sources

How we source figures →  ·  Spotted a mistake? Tell us → (opens in a new tab)

unverified
  • The piers' width at springing (6.4 ft) is read off one photograph (Commons, 2020), scaled by an arch's 30 ft span; within about 0.5 ft. No published figure was found.
  • Humber's batters are 1 in 30 on the piers' ends and 1 in 40 on their sides; the sides (facing the arches) are taken as the faces seen widening in this elevation, so 1 in 40 each side.
  • The abutments (56.7 ft each) are what Humber's 1,563 ft leaves after the arches and piers, split equally. OSM's bridge is 1,526 ft (465.2 m); Historic England gives 519 yd (1,557 ft), the original plans 521 yd, Grinling 497 yd.
  • The ground is the EU-DEM's 82 ft (25 m) profile along the line, hung so that the floor by the Mimram is 100 ft below the rails (Humber); the DEM smooths the valley, so within about 10 ft. Wrottesley gives 89 ft as the greatest height before electrification.
  • The arches are drawn semicircular: in the photograph each near-face opening fits a circle (crown to springing equals half the span within 1.5 px); no published rise was found.
  • The parapet (3.5 ft above the string course), the string course (2.7 ft deep), the crown's depth below it (6.4 ft) and the imposts (1.2 ft) are read off the same photograph, scaled by an arch's span.
  • The rail's height (1.5 ft below the string course's top) is from the roof of the LNER train in that photograph; within about 1.5 ft.
  • The arch rings (1.5 ft) are Humber's 'four rings' of brick at about 4.5 in each; the four back rings he describes are hidden inside.
  • The colours (palette) are read raw off the 2020 photograph, taken in warm evening sun: pier face, spandrel, string course shadow and parapet.
  • The overhead line portals (about 1980) are not drawn: their places have not been measured.
  • The line speed (115 mph (185 km/h)) is OSM's tag, not checked against Network Rail's Sectional Appendix.

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